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Discovery of Astatine and Francium element

Astatine and Francium

In July 1925 the British scientist W. Friend went to Palestine but not as a pilgrim. Moreover, he was neither an archeologist nor a tourist visiting exotic lands. He was just a chemist and his luggage contained mostly ordinary empty bottles which he hoped to fill with samples of water from the Dead sea. Which has the highest concentration of dissolved salts on Earth. Fish cannot live in it and a man can swim in it without any danger of drowning–so high is the density of water in it.

The somber Biblical landscapes failed to dampen Friend’s hopes for success. His goal was to find in the water of the Dead Sea eka–iodine and eka–cesium which chemists had sought in vain. Sea water contains many dissolved salts of alkali metals and halogens and their concentration in the Dead Sea water must be exceptionally high. The higher the probability that they hide among them the unknown elements, namely the heaviest halogen and the heaviest alkali metal, even if in trace amounts.

Of course, Friend was not entirely original in choosing the direction of his search. As early as the end of the 19th century a chemist would not hesitate to answer the question where to look for eka–iodine and eka–cesium on Earth. The obvious answer was where natural compounds of alkali metals, in sea and ocean water, in various minerals, in deep well water, in some sea algae, and soon. In other words, the field of search was quite wide.

But all the attempts to find eka–iodine and eka–cesium failed and efforts of Friend were no exception. Now let us turn back to the last decades of the 19th century. When Mendeleev developed the periodic system of elements it contained many empty slots corresponding to unknown elements between bismuth and uranium. These empty slots were rapidly filled after the discovery of radioactivity. Polonium, radium, radon, actinium and finally protactinium took their places between uranium and thorium. Only eka–iodine and eka–cesium were late. This fact, however, did not particularly trouble scientists. These unknown elements had to be radioactive since there was not even a hint of doubt that radioactivity was the common feature of elements heavier than bismuth. Therefore, sooner or later radiometric methods would demonstrate the existence of elements 85 and 87.

The natural isotopes of uranium and thorium in long series of successive radioactive transformations give rise to secondary chemical elements. In the first decade of the 20th century scientists had in their disposal about forty radioactive isotopes of the elements at the end of the periodic system, that is, from bismuth to uranium. These radioelements comprised three radioactive families headed by thorium–232, uranium–235, and uranium–238. Each radioactive element sent its representatives to these families with the only exception of eka–iodine and eka–cesium. None of the three series had links that would correspond to the isotopes of element 85 or 87. This suggested an unexpected idea that eka–iodine and eka–cesium were not radioactive.But why? Nobody dared to answer this question. Under this assumption it was meaningless to look for these element in the ores of urnium and thorium which contained all the radioactive elements without exception.     

The assumption about stability of eka–iodine and eka–cesium was not confirmed. But all efforts to find isotopes of these elements in the radioactive families met with failure. But there remained one path of investigation which seemed promising. Does a given radioactive isotope have only one or two decay mechanisms? For instance, it emits both alpha and beta particles. If so the products of decay of this isotope are isotopes of two different elements and the series of radioactive transformations at the place of this isotope experiences branching. This problem was discussed for a long time and for some isotopes this effect seemed to take place.

In 1913 the British scientist A. Cranston worked with the radioelement MsTh–II (an isotope of actinium–228). This isotope emits beta particles and converts into thorium–228. But Cranston thought that he detected a very weak alpha decay, too. If that was true the product of the decay had to be the long–expected eka–cesium. Indeed, the process is described by

\[_{89}^{228}Ac{{\xrightarrow{\alpha }}^{224}}87\

But Cranston just reported his observation and did not follow the lead.

Just a year later three radiochemists from Vienna–S. Meyer, G. Hess, and F. Paneth–studied actinium–227, an isotope belonging to the family of uranium–235. They repeated their experiments and at last their sensitive instruments detected alpha particles of unknown origin. Alpha particles emitted by various isotopes have specific mean paths in air (of the order of a few centimetres). The mean path of the alpha particles in the experiments of the Austrian scientists was 3.5 cm. No known alpha–active isotope had such mean path of alpha particles. The scientists from the Vienna Radium Institute concluded that these particles were the product of alpha decay of the typically beta–active actinium–227. A product of this decay had to be an isotope of element 87.

The discovery had to be confirmed in new experiments. The Austrians were ready for this but soon the World War I started. They indeed observed alpha radiation of actinium–227 and this meant that atoms of element 87 were produced in their presence. But this fact had to be proved. It was easier to refute their conclusions. Sceptics said that the observed alpha activity was too weak and the results were probably erroneous. Others said that an isotope of the neighbouring element, protactinium, also emitted alpha particles with mean path close to 3.5 cm. Perhaps, an error was caused by an admixture of protactinium.

Elements 85 and 87 were discovered several times and given such names as dacinum and moldavium, alcalinium and helvetium, or leptinum and anglohelvetium. But all of them were mistakes. The fine–sounding names covered emptiness.

The mass numbers of all isotopes in the family of thorium–232 are divided by four. Therefore, the thorium family is sometimes referred to as the 4n family. After division by four of the mass numbers of the isotopes in the two uranium families we get a remainder of two or three. Re–spectively, the uranium–238 family is known as the (4n + 2) family and the uranium–235 family as the (4n + 3) family.

But where is the (4n + 1) family? Perhaps it is precisely in this unknown fourth series of radioactive transformations that the isotopes of eka–iodine and eka–cesium can be found. The idea was not unreasonable but not a single known radioactive isotope could fit into this hypothetical family by its mass number.

Sceptics declared, not without reason, that indeed there had been the fourth radioactive series at the early stages of Earth’s existence. But all the isotopes that comprised it including the originator of the series had too short half–lives and hence disappeared from the face of Earth long ago. The fourth radioactive tree had withered away long before mankind appeared.

In the twenties theorists attempted to reconstruct this family, to visualize its composition if it had existed. This imaginary structure had positions for the isotopes of elements 85 and 87 (but not for the radon isotopes). But this direction of search did not bring results, too. Perhaps the elusive elements did not exist at all?

But the goal was not that far. But before we start the tale about the realization of the scientists’ dreams let us turn back to the first synthesized element, namely, technetium.

Why was technetium the first? Primarily, because the choice of the target and the bombarding particles was obvious. The target was molybdenum, which could be made sufficiently pure at the time. The bombarding particles were neutrons and deutrons and accelerators were available for accelerating deutrons. This is why the discovery of technetium manifested the dawn of the age of synthesized elements. The work on promethium proved more complicated because in belonged to the rare–earth family and the main difficulties were met in determining its chemical nature.

But the task for elements 85 and 87 looked much more formidable. In their attempts to produce eka–iodine the scientists could only have one material for the target, namely, bismuth, element 83. The bombarding particles were a case of Hobson’s choice, too–only alpha particles could be used. Polonium, which precedes eka–iodine, could not be used as the material for the target. The elements with lower numbers than bismuth could not be used as targets because the scientists lacked appropriate bombarding particles to reach number 85.

Eka–cesium looked totally inaccessible for artificial synthesis. No suitable targets and bombarding particles existed in the thirties. But such is the irony of history that it was precisely element 87 that became the second after technetium reliably discovered element out of the four missing elements within the old boundaries of the periodic system. At this point in history the line of eka–iodine and eka–cesium, which had travelled parallel for such a long time, started to diverge and therefore we shall consider their discoveries separately.

Element 85 was synthesized by D. Corson, C. Mackenzie, and E. Segre who worked at Berkley (USA). The Italian physicist Segre by that time had settled in the USA and was the only one in the group who had an experience in artificial synthesis of a new element (technetium). On July 16, 1940, these scientists submitted to the prestigious physical journal Physical Review a large paper entitled “Artificial radioactive element 85”. They reported how they had bombarded a bismuth target with alpha particles accelerated in a cyclotron and obtained a radioactive product of the nuclear reaction . The product, most probably, was an isotope of eka–iodine with a half–life of 7.5 hours and a mass number of 211. Segre and his coworkers performed fine chemical experiments with the new element produced in negligible amounts and found that it was similar to iodine and exhibited weakly metallic properties.

The results seemed convincing enough. But the new element remained nameless for the time being. Further work on eka–iodine had to be delayed as the war started. It was resumed only in 1947 and the same group announced synthesis of another isotope with a mass number of 210. Its half–life was somewhat longer but still only 8.3 hours. Later it was found to be the longest–lived isotope of element 85. It was produced with a similar technique as the first isotope though the energy of the bombarding alpha particles was somewhat higher. As a result the intermediate composite nucleus (209Bi + α) emitted three rather than two neutrons and hence, the mass number of the isotope was lower by 1. Only now the new element was given the name astatine from the Greek for “unstable” (the symbol At).

But in the interval between the syntheses of the isotopes 211At and 210At a remarkable event occurred. The scientists from the Vienna Radium Institute B. Karlik and T. Bernert managed to find natural astatine. This was an extremely skillful study straining to the utmost the capacity of radiometry. The work was crowned with success and element 85 was born for the second time. As in the cases of technetium and promethium, we can name two dates in the history of astatine, namely, the year of its synthesis (1940) and the year of its discovery in nature (1943).

But when the Segre and his coworkers were preparing for irradiating a bismuth target with alpha particles the scientific community had known about the discovery of eka–cesium for more than a year. Transactions of the Paris Academy of Science published a paper headed “Element 87: AcK formed from actinium” and dated January 9, 1939. Its author was M. Perey, the assistant of the eminent radiochemist Debierne who had announced his discovery of actinium forty years earlier.

Marguerite Perey did not invent any fundamentally new methods and did not indulge in any vague and complicated speculations about possible sources of natural eka–cesium. In 1938 she came upon a paper published in 1914. The paper was signed by the Austrian chemists Meyer, Hess and Peneth. Perey attempted to prove their ideas. She obtained a carefully purified specimen of actinium–227. This isotope has a high beta–activity but sometimes it emits alpha particles, too. The mean path of such particles in air is 3.5 cm. This alpha radiation is by no means due to protactinium as the actinium specimen was sufficiently purified. Since alpha particles are emitted the eka–cesium isotope with a mass number of 223 must continuously be accumulated in the specimen. A series of experiments definitely demonstrated that, indeed, some substance with a half–life of 21 min is accumulated in the actinium specimen. Now it is the turn of chemical analysis to prove that this substance is a new element. Its properties proved to be similar to those of cesium. Perey named the new element francium in honour of her country. Only for a short period it was called actinium K (AcK) in accordance with the old nomenclature of radioelements.

The first description given by Perey to the newborn element was extremely brief: the element is formed with alpha decay of actinium –227 in the reaction

\[_{89}^{227}Ac{{\xrightarrow{\alpha }}^{223}}85\

and it is alpha–active with a half–life of 21 min. Then she spent several months studying its chemical properties and demonstrated convincingly that francium is similar to cesium in all its characteristics.

None of the natural radioactive elements had such a short half–life, even the artificially synthesized element 85 had a half–life measured in hours. There were hopes to find other natural isotopes of francium with longer half–lives. But in fact francium–223 proved to be the only francium isotope found on Earth.

The only remaining path to success was synthesis but it proved very difficult. More than ten years passed after the discovery of Perey when francium isotopes were artificially synthesized. The nuclear reaction giving rise to the francium isotope with a mass number of 212 can be written in short as

\[_{92}^{232}U(p,\,\,6p21n)\,\,_{87}^{212}Fr\

This reaction is the fission of uranium nucleus by protons accelerated to very high energies. When such a fast proton hits uranium nucleus it produces something like an explosion with ejection of a multitude of particles, namely, six protons and 21 neutrons. Of course, the reaction is not due to a blind chance but is based on careful theoretical predictions. Uranium may be replaced with thorium. The reaction product, francium–212, for some time was considered to be the longest–lived isotope (a half–life of 23 min) but later the half–life was found to be only 19 min.

Artificial synthesis of francium is much more difficult and less reliable method than extraction of francium as a product of decay of natural actinium. But natural actinium is rare. What to do? A current method is to irradiate the main isotope of radium with a mass number of 226 (its half–life is 1622 years) with fast neutrons. Radium–226 absorbs a neutron and converts into radium–227 with a half–life of about 40 min. Its decay gives rise to pure actinium–227 whose alpha decay in its turn produces francium–223. The symbols At and Fr were permanently installed in boxes 85 and 87 of the periodic table and their properties proved to be exactly the same as predicted from the table. But in comparison with their unstable mates born by nuclear physics, technetium and promethium, their position is clearly unfavourable.

According to estimates, the 20-km thickness of the Earth crust contains approximately 520 g of francium and 30 g of astatine (this is an overestimation in some respects). These amounts are of the same order as the terrestrial “resources” (quotation marks are more than suitable here) of technetium and promethium. We are probably making a mistake when we talk condescendingly about astatine and francium? Not at all. Technetium and promethium are produced in large amounts, kilograms and kilograms of them. The fact is that technetium and promethium have much longer half–lives and can therefore be accumulated in larger amounts. But accumulation of astatine and francium is just unfeasible. In fact, each time their properties have to be studied they have to be produced a new.

In the radioactive families the isotopes of astatine and francium are placed not on the principle pathways of radioactive transformations but at the side branches. Here is the branch on which natural francium is born:

         

The isotope Ac in 99 cases out of 100 emits beta particles and only in one case it undergoes alpha decay.

The situation is even less easy in the case of the branches responsible for the formation of astatine:

What may be said about these branches? The producers of natural astatine (the polonium isotopes) are by themselves extremely rare. For them alpha decay is not just predominant but practically the only radioactivity mechanism. Beta decays for them seem something like a mishap as can be clearly seen from the following data.

There is only one beta decay event per 5 000 alpha decays of polonium–218. Things are even sadder for polonium–216 (1 per 7 000) and polonium–215 (1 per 200 000). The situation speaks for itself. The amount of natural francium on Earth is larger. It is produced by the longest–lived actinium isotope 227Ac (a half–life of 21 years) and its content is, of course, much higher than that of the extremely rare polonium isotopes capable of producing astatine.

Discovery of element : Promethium

Promethium

The history of one rare–earth element is so unusual that it merits individual discussion. Promethium, as it is known now, is practically non–existent in nature (we write practically but not absolutely and the reason for that will be clear later). Event which can only be described as amazing preceded the discovery of element 61 by means of nuclear synthesis.

The work of Moseley made clear the existence of an unknown element between neodymium and samarium. But the situation proved to be not so clear and dramatic events rapidly followed in the history of element 61.

The New World was unlucky in discoveries of new elements. All the elements known by the twenties of this century (not counting the elements known from ancient times) had in fact been discovered by the European scientists. This is why the American scientific community was particularly happy to learn about the discovery of element 61 by the chemists from Chicago B. Hopkins, L. Intema, and J. Harris in 1926.

Starting from 1913 scientist from various countries had been searching intensely for the elusice rare–earth element and it seemed strange that they had not found it earlier. Indeed the elements of the first half of the rare–earth family known as the cerium elements (from lanthanum to gadolinium) had been shown by geochemists to be more abundant in nature than the yttrium elements of the second half of the family (from terbium to lutecium). But all the yttrium elements had been found while an empty box had remained in the cerium group between neodymium and samarium.

The straightforward explanation was that element 61 was not just rare but rarest element. Its abundance was assumed to be much lower than that of other rare–earth elements, and the available analytical techniques were not sensitive enough to identify its traces in the terrestrial minerals. New more sensitive methods were needed for the purpose.

The American chemists employed X–ray and optical spectral techniques to study the minerals where they hoped to find element 61. These well versed in the history of range earth elements could say that the path the Americans took was a troublesome one as spectral analysis not infrequently had acted as an evil genius of rare–earth studies despite all the benefits it had brought to them. But in the twenties the feet spectroscopy stood on were not so unsteady as a few decades earlier and the Moseley law could be used for predicting the X–ray spectra of any element.

The American chemists worked hard, analysed numerous specimens of various minerals and in april 1926 reported the discovery of element 61. But they did not extract even a grain of the new element and its existence was inferred from the X–ray and optical spectral data.

The discoverers University named the element illinium in honour of the Illinois University where they worked and the symbol Il took its place in box 61 of the periodic system but just a half–year later a new claimant of box 61 came into the limelight. It had been discovered by two Italian scientists L. Rolla and L. Fernandes who had named it florencium (Fl).  Allegedly, they had discovered element 61 two years earlier than the Americans but failed to report the discovery owing to some undisclosed reasons. They had sealed the report of their discovery into an envelope and left it for safe–keeping in the Florence Academy.

If different people obtain the same result with different means that would seem to prove that the result is genuine. Americans and Italians could be only too happy. As for the question of priority it was nothing new to science. But no one of the alleged discoverers of element 61 could imagine that their argument about periodic would soon become superfluous and both symbols, Il and Fl, would be shown to be illegal squatters in box 61 of the periodic table.

To trace the events now we have to go not further but some time back to the facts that were simply unknown at the time. The report of the discoverers of element 61 started with the words: “There had been absolutely no grounds for assuming the existence of an element between neodymium and samarium until it was demonstrated through the Mosely law”. Typical dry style of a scientific report, everything would seem to be correct. But….

The following remarkable conclusion in German (please, do not look it up in a dictionary yet) appeared in the margin of a hard–written manuscript of the element table found in the papers of certain scientist (we shall supply the name a little later): “NB. 61 ist das von mir 1902 vorhergesagte fehlende Elemente”.

The real history of element 61 should prominently feature the name we have already met on these pages. It is the Czech scientist Boguslav Brauner, Mendeleev’s friend and an eminent expert in the chemistry of rare–earth elements.

Illinium had been discovered, the discoverers accept congratulations and learn about the second, third, fourth confirmation of the discovery from the scientists of other countries. The pedigree of element 61 could be started thus: “Moseley had predicted and American chemists discovered”. But a discordant not unexpectedly sounded in November 1926 from the pages of Nature. It was none other than Brauner. He congratulated him American colleagues but voiced his disagreement with the above–cited beginning of their report. He argued that it was really not important who first discovered element 61 –American or Italians; in the twenties scientists became increasingly aware that the discovery by itself was a purely technical matter. The important issue is who predicted the new element. Was it Moseley? No, declared the Czech scientist. Who then? Of course, he himself, Boguslav Brauner…..

But nothing could be further from the truth if we thought that he was immodest. His claim was based on his vast experience of work with rare earths, on his profound understanding of the spirit of the periodic system, on his superb appreciation of slight changes of properties in the series of extremely similar rare–earth elements, and, finally, on his intuition of a dedicated researcher.

But these words of praise must be substantiated with facts. Let us turn back to 1882. The old didymium of K. Mosander is close to its death. P. Lecoq de Boisbaudran had already extracted a new element, samarium, from it. B. Brauner carefully analyses the residue and employing extremely complicated chemical procedures separates it into three fractions with different atomic masses. Owing to a number of reasons he has to discontinue his work and in 1885 K. Auer von Welsbach overtakes the Czech scientist. The old didymium is dead but praseodymium and neodymium have appeared, the first and the third fractions of Brauner. But what about the intermediate second fraction? No, its tine has not come. The chemistry of rare–earth elements is in a turmoil. The muddy stream of erroneous discoveries of new elements overflows with doubts the very periodic system. But life goes on. The chaos in rare earths gradually diminishes and the known rare–earth elements form an ordered series. Now Brauner notices that the difference between the atomic masses of neodymium and samarium is rather large; it is larger than the respective difference between any two neighbouring rare–earth elements. His brilliant knowledge of rare earths suggests to Brauner that there is a discontinuity in the variations of their properties in the part of the series between neodymium and samarium. At last, he recalls his work of 1882. The clues fit into a pattern leading to premonition and even certainty that an unknown element can be found between neodymium and samarium. But as his friend, Mendeleev, Brauner was never too hasty in his conclusions. It was only in 1901 that he placed an empty box between neodymium and samarium when he put forward his views on the place of the rare–earth elements in the periodic system.

Now we can give a translation of the note he wrote in margin of his hand–written table of elements. It reads: “61st element is the missing element predicted by me in 1902”.

His short letter to Nature was an attempt by Brauner to put the record straight. This would seem to simplify the task of science historians in writing the history of element 61. But a history is meaningful only if it treats a subject which really exists. As for illinium the element proved to be still–born.

While the hotheads kept trying to squeeze the symbol Il into box 61 of the periodic table meticulous critics tried to verify the discovery. The careful experiments by the first of them, Prandtl, could be doubted by nobody. But his results did not even hint at the existence of element 61.

In 1926 the Noddacks who had just announced their discovery of masurium and rhenium (Nos. 43 and 75) started their tests. They used all available techniques to analyse fifteen various minerals suspected of containing illinium. The processed 100 kilograms of rare–earth materials and could not detect a new element. The Noddacks claimed that if the American’s results had been correct they, the Noddacks, would undoubtedly extracted the new element. Even if the element were 10 million times rarer than niodymium or samarium they would still find it… There are two possible explanations: either element 61 is so rare that the existing experimental techniques are not fine enough to find it or wrong mineral specimens were taken.

Geochemists were against the first explanation. The abundances of rare–earth elements are more or less similar. There are no reasons to think that illinium is an exception. They suggested looking for illinium in minerals of calcium and strontium. All rare–earth elements are typically trivalent but some of them can exhibit a valence of two or four. For instance, europium rather easily gives rise to cations with a charge of two. Their size is closer to those of calcium and strontium cations and they can replace the letter in the respective alkaline–earth minerals. Perhaps, illinium has a similar more pronounced capacity and can be found in some rare natural compound of strontium. One hypothesis replaced another, one assumption stemmed from another, unsubstantiated one. Just in case, the Noddacks analysed several alkaline–earth minerals. Alas, they failed once more.

The search for illinium seemed to come to a dead end; though it still went on the reported results were little believed. Chemists failed in looking for element 61 in the terrestrial minerals it was theoretical physics whose fate it was to open up the “envelope” where nature had “sealed” element 61. But when the envelope was open the scientist (not for the first time!) were disappointed. The envelope was empty.

At this point the fate of element 61 directly involves the fate the element 43, that is, technetium. According to the law formulated by the German theoretical physicist Mattauch, technetium in principal cannot have stable isotopes. This law also forbids existence of stable isotopes of element 61. Illinium is dead but element 61 must survive.

But what if it really does not exist? I. Noddack put forward a daring idea that illinium (we shall use this name for the time being) had existed on Earth in early geological periods. But it had been a highly radioactive element with a short half–life and it had decayed fairly soon and disappeared from the face of Earth. If we agree with this idea we have to make two extremely unlikely assumptions. First, illinium which is at the centre of the periodic table has no stable isotopes. Second, the half–lives of its isotopes a e much shorter than the age of Earth.

Indeed, illinium neighbours in the periodic system (neodymium and samarium) have many (seven each) natural isotopes with a wide range of mass numbers–from 142 to 154. Any feasible isotopes of element 61 would have its mass number in this range. Thus, any illinium isotopes proves to be unstable in this range of mass numbers. The Mattauch law seem to bury for good the hopes to find element 61 on Earth. But then a gleam of hope appeared. All right, the illinium isotopes are all radioactive. But to what extent? Perhaps the half–lives of some of them are very long. At that time the theory had not learned how to predict half–lives of isotopes. The search for element 61 had to continue in the dark. Physicists believed that only nuclear synthesis could solve the riddle of element 61 the more so as the case of technetium was fresh in their minds.

As if trying to restore the honour of American science after its setback in 1926 two physicists from the University of Ohio conducted the first experiment of artificial synthesis of element 61 in 1938. They bombarded a neodymium target with fast deuterons (the nuclei of heavy hydrogen). They believed that the resulting nuclear reaction Nd + d → → 61 + n gave rise to an isotopes of element 61. Their results were inconclusive but nevertheless they thought that they obtained an isotope of the new element with the mass number of 144 and the half–life of 12.5 hours.

Again sceptics said that these results were erroneous and not without a reason since nobody could be sure that the neodymium target was ideally pure. The method of identification could hardly be considered reliable, too. Even uncomplicated optical and X–ray spectra evidenced the presence of element 61 as in the study of 1926; the conclusion was made from the radiometric data.

In fact, chemistry was not involved in this work and the chemical nature of the mysterious radioactive product was not determined. Therefore, one may ask whether 1938 can be regarded as the actual data of discovery of element 61. It can rather be said that only the consistent efforts to synthesize it started at the time.

As time passed the range of bombarding particles was extending, targets of other rare–earth elements were used, and the techniques of activity measurements were improved. Reports on other illinium isotopes started to appear in scientific journals. Element 61 was becoming a reality albeit an artificially created one. Its name was changed to cyclonium in commemoration of the fact that it was produced in a cyclotron but the symbol Cy did not remain for long in box 61 of the periodic table.

Researchers had detected the radioactive “signal” of cyclonium but nobody had seen even a grain of the new element and its spectra had not been recorded. Only indirect evidence of the existence of cyclonium had been obtained.

The history of science of the 20th century knows of many great discoveries and one of the greatest is the discovery of uranium fission under the effect of slow neutrons. The nuclei of uranium–235 isotopes are split into two fragments, each of which is an isotope of one of the elements at the centre of the periodic table. Isotopes of thirty odd elements from zinc to gadolinium can be produced in this way. The yield of the isotopes of element 61 has been calculated to be fairly high–approximately 3 per cent of the total amount of the fission products.

But the task of extracting the 3 per cent amount proved to be very difficult. The American chemists J. Marinsky, L. Glendenin, and Ch. Coryell applied a new chemical technique of ion–exchange chromatography for separation of the uranium fission fragments.

Special high–molecular compounds known as the ion–exchange resins are employed in this technique for separating elements. The resins act as a sieve sorting up elements in an order of the increasing strength of the bonds between the respective elements and the resin. At the bottom of the sieve the scientists found a real treasure–two isotopes of element 61 with the mass numbers 147 and 149.

At last, element 61 known as illinium, florencium, and cyclonium could be given its final name. According to recollections of the discoverers, the search for a new name was no less difficult than the search for the element itself. The wife of one of them, M. Coryell, resolved the difficulty when she suggested the name promethium for the element. In an ancient Greek myth Prometheus stole fire from heaven, gave it to man and was consequently put to extreme torture by Zeus. The name is not only a symbol of the dramatic way of obtaining the new element in noticeable amounts owing to the harnessing of nuclear fission by man but also a warning against the impeding danger that mankind will be tortured by the hawk of war, wrote the scientists.

Promethium was obtained in 1945 but the first report was published in 1947. On June 28, 1948, the participants at a symposium of the American Chemical Society in Syracuse had a lucky chance to see the first specimens of promethium compounds (yellow chloride and pink nitrate) each weighing 3 mg. These specimens were no less significant than the first pure radium salt prepared by Marie Curie. Promethium was born by the great creative power of science. The amounts of promethium prepared now weigh tens of grams and most of its properties have been studied.

The Mattauch law denied the existence of terrestrial promethium but this denial was not absolute. The search for promethium in terrestrial ores and minerals would be quite in order if promethium had long–lived isotopes with half–lives of the order of the age of Earth.

But in this respect nuclear physics proved to be a foe of natural promethium. With each newly synthesized promethium isotope a possible scope for search became increasingly narrow. The promethium isotopes were found to be short–lived. Among the fifteen promethium isotopes known today the longest–lived one had a half–life of only 30 years. In other words, when Earth had just formed as a planet not a trace of promethium could exist on it. But what we mean here is the primary promethium formed in the primordial process of origination of elements. What was discussed was the search for the secondary promethium which is still being formed on Earth in various natural nuclear reactions.

Technetium was finally found on Earth among the fragments of spontaneous fission of uranium. These fission products could contain promethium isotopes. According to estimates, the amount of promethium that can be produced owing to spontaneous fission of uranium in the Earth’s crust is about 780 g, that is, practically, nothing. To look for natural promethium would be tantamount to dissolving a barrel of salt in the lake Baikal and then trying to find individual salt molecules.

But this titanic task was fulfilled in 1968. A group of American scientists including the discoverer of natural technetium P. Kuroda managed to find the natural promethium isotope with a mass number of 147 in a specimen of uranium ore (pitchblende). This was the final step in the fascinating history of the discovery of element 61.

As in the case of technetium, we can name two dates of discovery of promethium. The first date is the date of its synthesis, that is, 1945. But under the circumstances synthesis was unconventional (it could be called fission synthesis). The first two promethium isotopes were extracted from the fragments of fission of uranium irradiated with slow neutrons rather than in a direct way as was the case with technetium, which was produced in a direct nuclear reaction. This makes promethium a unique case among all over synthesized elements.

The second date is the date of the discovery of natural promethium, that is, 1968. This achievement is of independent significance as it stretched to the utmost the capabilities of the physical and chemical methods of analysis. Of course, the achievement is of a purely theoretical significance since nobody can hope to extract natural promethium for practical uses.

The Boiling Point of Water

Water always boils at 100˚C, right? Wrong! Though it’s one of the basic facts you probably learnt pretty early on back in school science lessons, your elevation relative to sea level can affect the temperature at which water boils, due to differences in air pressure. Here, we take a look at the boiling points of water at a variety of locations, as well as the detailed reasons for the variances.

From the highest land point above sea level, Mount Everest, to the lowest, the Dead Sea, water’s boiling point can vary from just below 70 ˚C to over 101 ˚C. The reason for this variation comes down to the differences in atmospheric pressure at different elevations.

Atmospheric pressure the pressure exerted by the weight of the Earth’s atmosphere, which at sea level is simply defined as 1 atmosphere, or 101,325 pascals. Even at the same level, there are natural fluctuations in air pressure; regions of high and low pressure are commonly shown as parts of weather forecast, but these variances are slight compared to the changes as we go higher up into the atmosphere. As your elevation (height above sea level) increases, the weight of the atmosphere above you decreases (since you’re now above some of it), and so pressure also decreases.

In order to understand how this affects water’s boiling point, we first need to understand what’s going on when water boils. For that, we’ll need to talk about something called ‘vapour pressure’. This can be thought of as the tendency of molecules in a liquid to escape into the gas phase above the liquid. Vapour pressure increases with increasing temperature, as molecules move faster, and more of them have the energy to escape the liquid. When the vapour pressure reaches an equivalent value to the surrounding air pressure, the liquid will boil.

At sea level, vapour pressure is equal to the atmospheric pressure at 100 ˚C, and so this is the temperature at which water boils. As we move higher into the atmosphere and the atmospheric pressure drops, so too does the amount of vapour pressure required for a liquid to boil. Due to this, the temperature required to reach the necessary vapour becomes lower and lower as we get higher above sea level, and the liquid will therefore boil at a lower temperature.

This is, of course, a fact that’s true for all liquids, not just water. And it’s also not just atmospheric pressure that can affect water’s boiling point. Most of us are probably aware that adding salt to water during cooking increases water’s boiling point, and this is also related to vapour pressure. In fact, adding any solute to water will increase the boiling temperature, as it reduces the vapour pressure, meaning a slightly higher temperature is required in order for the vapour pressure to become equal to atmospheric pressure and boil the water.

Another factor that can affect the boiling temperature of water is the material that the vessel it’s being boiled in is made of. Experiments have shown that, at the same pressure, water will boil at different temperatures in metal and glass vessels. It’s theorised that this is because water boils at a higher temperature in vessels which its molecules adhere to more strongly – there’s much more detail on this phenomenon here.

So, water’s boiling point is anything but absolute, and it can be affected by a whole range of factors. Useful information if you ever find yourself wanting to make a cup of tea on Everest – the lower boiling point would mean the cup you end up with is rather weak and unpleasant

 

Discovery of element : Technetium

Technetium

The upper part of the periodic system down to the sixth period (where the rare–earth elements are located) always seemed relatively quiet, particularly after the discovery of the group of noble gases which harmoniously closed the right–hand side of the system. It was quiet in the sense that one could hardly expect any sensational discoveries there. The debates concerned only a possible existence of elements that were lighter than hydrogen and elements lying between hydrogen and helium. On the whole, we can say in the parlance of mathematicians that this part of the periodic system was an ordered set of chemical elements.

Therefore, the more awkward and confusing seemed to be the mysterious blank slot No. 43 in the fifth period and seventh group.

Mandeleev named this element eka–manganese and tried to predict its main properties. A few times the element seemed to have been discovered but soon it proved to be an error. This was the case with ilmenium allegedly discovered by the Russian chemist R. Hermann, back in 1846. For some time even Mendeleev tended to believe that ilmenium was eka–manganese. Some scientists suggested placing devium  between molybdenum and ruthenium. The German chemist A. Rang even put the symbol Dv into this box of periodic table. In 1896 there flashed and burned like a meteor lucium supposedly discovered by P. Barriere.

Mandeleev did not live to see the happy moment when eka–manganese was really found. A year after his death, in 1908, the Japanese scientist M.  Ogawa reported that he found the long–awaited element in the rare mineral, molybdenite and named it nipponium (in honour of the ancient name of Japan). Alas, Asia once more failed to contribute a new element to the periodic system. Ogawa, most probably, dealt with hafnium (which was also discovered later).

Chemists grew accustomed to a few chemical elements being discovered every year and they were at a loss in the case of eka–manganese. They began to think that Mendeleev could make a mistake and no manganese analogues existed. 

H. Moseley decisively refuted this skepticism in 1913. He clearly demonstrated that these analogues have their own place among the elements. In a paper dated September 5, 1925, W. Noddack, I. Tacke, O. Berg announced that they had discovered, together with element No. 75 (rhenium), its lighter analogue in the seventh group of the periodic system, namely, masurium whose number was 43. Two new symbols, Ma and Re, appeared in the periodic table, in chemical textbooks, and numerous scientific publications. The discoverers saw nothing odd in the fact that masurium and rhenium had not been discovered earlier. These elements were thought to be not too rare, however. The lateness of their discovery was attributed to another cause. A large group of trace elements in known to geochemistry. The trace elements are classified as those elements which have no or almost no own minerals but are spread in various amounts over minerals of other elements as if the nature has sprayed them with a giant atomizer. This is why the traces of masurium and rhenium were so hard to identify. Only the powerful eye of X–ray spectral analysis could distinguish them against the formidable background of other elements. There is an ancient saying that if two people do the same things this does not mean that the results will be identical. Two biographies started under the same conditions typically follow different paths. The same can be said about the fates of elements 43 and 75; one of them went a long way and found its proper place while the other’s way soon led it to a forest of errors, misunderstandings, and controversies. This was the path of masurium.

W. Prandtl got interested in the empty slots in the seventh group of the periodic table. He had his own outlook and put forward original ideas on the structure of the periodic system. He did not compile a new version of the table, though. He suggested placing the rare–earth elements each to a group though by that time most chemists had put down such an arrangement. But in Prandtl’s version of the table the seventh group happens to reveal as many as four empty slots below manganese corresponding to yet undiscovered elements (this was in 1924) whose numbers were 43, 61, 75, and 93. Prandtl believed this to do no chance occurrence but a result of a common cause that had prevented four elements from having been discovered. The German scientist, however, made his table structure too elaborate and artificial to be accepted. The final discovery of rhenium was the first indication of his errors, and his ideas on the first transuranium element (No. 93) were little thought of at the time. But he was intuitively right in thinking of a close common link between elements 43 and 61.

The belief in masurium’s existence gradually diminished. Only the original discoverers were firm. As late as the beginning of the thirties I. Noddack continued to say that in time element 43 would be commercially available as it happened with rhenium. But as the time passed and chemists again and again failed to find masurium in whatever minerals they analysed they came to believe that I. Noddack was right only by half, that is, only about rhenium. Rarest mineral specimens were tested for masurium. Some people even went as far as to claim that masurium minerals had yet to be found and would possess unheard of properties. Naturally, geochemists were quite sceptical. The imagination of some people went even further and masurium was suggested to be radioactive. That was too much, others said. But it was precisely this shot that did not go wild.

Let us talk about some concepts of nuclear physics. We have discussed isotopes. Now we meet another term–isobars–elements having the same atomic weight or mass numbers but different atomic numbers (from the Greek for “heavy”). Isobars, in other words, are isotopes of different chemical elements with different nuclear charges but identical mass numbers. Take, for instance, potassium–40 and argon–40 which have different nuclear charges (respectively, 19 and 20). Their mass numbers are identical because their nuclei contain different numbers of protons and neutrons but their total numbers are the same; potassium nucleus contains 19 protons 21 neutrons while argon nucleus has 20 protons and 20 neutrons.

Thus, the concept of isobars turned out to be the magic key that opened the door to the mystery of masurium.

When the majority of stable chemical elements were found to have isotopes–up to ten isotopes per element–the scientists started to study the laws of isotopism. The German theoretical physicist J. Mattauch formulated one of such laws at the beginning of the thirties (the basic premise of this law was noted back in 1924 by the Soviet chemist S. Shchukarev). The law states that if the difference between the nuclear charges of two isobars is unity one of them must be radioactive. For instance, in the 40K–40Ar isobar pair the first is naturally weakly radioactive and transforms into the second owing to the so–called process of K–capture. Then Mattauch compared with each other the mass numbers of the isotopes of the neighbours of masurium, that is, molybdenum (Z = 42) and ruthenium (Z = 44):

            Mo isotopes      94         95         96         97         98         –          100       –          –

            Ru isotopes         –           –          96         –          98         99         100       101       102

What did he deduce from this comparison? The fact that the wide range of mass numbers from 94 to 102 was forbidden for the isotopes of element 43 or, in other words, that no stable masurium isotopes could exist.

If that was really so that meant a peculiar anomaly linked to the number 43 in the periodic system. All the atom species with Z = 43 had to be radioactive as if this number was a small island of instability amidst a sea of stable elements. This, of course, would be unfeasible to predict within the framework of purely chemical theory. When Mendeleev predicted his eka–manganese he could never imagine that this member of the seventh group of the periodic system could not exist on Earth. Of course, in those times (the early thirties) Mattauch’s law was no more than a hypothesis, though one that looked like quite capable of becoming a law. And it became just that. The physicist’s idea opened the eyes of chemists who lost all hope of finding element 43 and they saw the source of their errors. However, the symbol Ma remained in box 43 of the periodic system for a few more years. And not without a reason. All right, all masurium isotopes are radioactive. But we know radioactive isotopes existing of Earth–uranium–238, thorium–232, potassium–40. They are still found on Earth because their half–lives are very long. Masurium isotopes are, perhaps, long–lived, too? If so, one should not be too hasty in dismissing the chances of successful search for element 43 in nature.

The old problem remained open. Who knows which way the biography of masurium would take if not for the dawn of a new age–that of artificial synthesis of elements.

Nuclear synthesis became feasible after invention of the cyclotron and the discoveries of neutrons and artificial radioactivity. In early thirties a few artificial radioisotopes of known elements were synthesized. Syntheses of heavier–than–uranium elements were even reported. But physicists just did not dare to take the challenge of the empty boxes at the very heart of the periodic system. It was explained by a variety of reasons but the major one was enormous technical complexity of nuclear synthesis. A chance helped. At the end of 1936 the young Italian physicist E. Segre went for a post–graduate work at Berkley (USA) where one of the first cyclotrons in the world was successfully put into operation. A small component was instrumental in cyclotron operation. It directed a beam of charged accelerated particles to a target. Absorption of a part of the beam led to intense heating of the component so that it had to be made from a refractory material, for instance, molybdenum.

The charged particles absorbed by molybdenum gave rise to nuclear reactions in it and molybdenum nuclei could be transformed into nuclei of other elements. Molybdenum is a neighbour of element 43 in the periodic system. A beam of accelerated deutrons could, in principle, produce masurium nuclei from molybdenum nuclei.

That was just Segre’s thought. He was a competent radiochemist and understood that if masurium really were produced its amount would be literally negligible and its separation from molybdenum would present an enormously intricate task. Therefore, he took an irradiated molybdenum specimen with him back to the University of Palermo where he was assisted in his work by the chemist C. Perrier.

They had had to work for nearly half a year before they could present their tentative conclusions in a short letter to the London journal nature. Briefly, the letter reported the first in history artificial synthesis of a new chemical element. This was element 43 the futile search for which on Earth wasted so much efforts of scientists from many countries. Professor E. Lawrence from the University of California at Berkley gave the authors a molybdenum plate irradiated with deutrons in the Berkley cyclotron. The plate exhibited a high radioactivity level which could hardly be due to any single substance. The half–life was such that the substances could not be radioactive isotopes of zirconium, niobium, molybdenum, and ruthenium. Most probably they were isotopes of element 43.

Though the chemical properties of this element were practically unknown Segre and Perrier attempted to analyse them radiochemically. The element proved to be closely similar to rhenium and exhibited the same analytical reactions as rhenium. However, it could be separated from rhenium with technique used for separating molybdenum and rhenium. The letter was written in Palermo and dated June 13, 1937. It was by no means a sensation. The scientific community regarded it as just the authors going on record. The reported data were too patchy while what was needed to be convincing was precisely the details and clear results of radiochemical analysis.

Only later Segre and Perrier were recognized as heroes; indeed, they extracted from the irradiated molybdenum just 10–10g of the new element–an amount formerly undetectable Never before radiochemists worked with such negligible amounts of material. The discoverers suggested naming the new element technetium from the Greek for “artificial”. Thus, the name of the first synthesized element reflected its origin. The name, though, became generally accepted only ten years later.

Perrier and Segre received new specimens of irradiated molybdenum and continued their studies. Their discovery was confirmed by other scientists. By 1939 it was understood that bombardment of molybdenum with deutrons or neutrons produces at least five technetium isotopes. Half–lives of some of them were sufficiently long to make possible substantial chemical studies of the new element. It no longer sounded fantastic to speak about “the chemistry of element 43”. But all attempts to measure accurately the half–lives of the technetium isotopes failed. The available estimates were disheartening since none of them exceeded 90 days and this put a stop to all hopes of finding the element on Earth.

So what was technetium in the late thirties and early forties? Nothing more than an expensive toy for curious scientists. Any prospects of accumulating it in a noticeable amount were, apparently, non–existent. The fate of technetium (and not only of it) was reversed when nuclear physics discovered an amazing phenomenon–fission of uranium by slow neutrons.

When a slow neutron hits a nucleus of uranium–235 it in effect breaks the nucleus down into two fragments. Each of the fragments is a nucleus of an element from the central part of the periodic table, including technetium isotopes. It is not without a reason that a fission reactor (a large–scale nuclear energy producer) is known as a factory of isotopes. Cyclotron made possible the first ever synthesis of technetium and fission reactor allowed the chemists to produce kilograms of technetium. But even before the first fission reactor started operating Segre in 1940 found the technetium isotope with a mass number of 99 in uranium fission products in his laboratory. Having found its new birthplace in a fission reactor technetium started to turn into an everyday (paradoxical as it may be) element. indeed, fission of 1 g of uranium–235 gives rise to 26 mg of technetium–99.

As soon as technetium ceased to be a rare bird scientists found the answers to many questions that had puzzled them, and first of all about its half–lives. In the early fifties it became clear that three of technetium isotopes are exceptionally long–lived in comparison with not only its other isotopes but also many other natural isotopes of radioactive elements. The half–life of technetium–99 is 212 000 years, that of technetium–98 is one and a half million years, while that of technetium–97 is even more, namely, 2 600 000 years. The half–lives are long but not long enough for primary technetium to be conserved on Earth since its origin. The primary technetium would survive on Earth if its half–life were not shorter than one hundred fifty million years. This makes obvious the hopelessness of all search for technetium of Earth.

But technetium can still be produced in the course of natural nuclear reactions, for instance, when molybdenum is bombarded by neutrons. How can free neutrons appear on Earth? They can be produced in spontaneous fission of uranium. The process occurs as described above, only spontaneously, and gives rise to a few neutrons, apart from two large fragments, i.e. nuclei of lighter elements.

The search for technetium in molybdenum ores failed and scientists turned their attention to another possibility. If technetium isotopes are produced in fission reactors why cannot they be born in natural processes of spontaneous uranium fission?

Using as a basis the Earth uranium resources (taking the figure for the mean abundance of uranium in the 20–km thickness of the Earth crust) and assuming the same proportion of produced technetium as in the case of reactor fission we can calculate that there are just 1.5 kg of technetium on Earth. Such a small amount (though it is larger than for other synthesized elements) could hardly be taken seriously. Nevertheless, scientists attempted to extract natural technetium from uranium minerals. This was done in 1961 by the American chemist B. Kenna and P. Kuroda. Thus, technetium acquired another birthday–the day when it was discovered in nature. If the methods of artificial synthesis of technetium had failed to materialize, even then it would, sooner or later, be brought to light from the bowels of the Earth.

But ten years earlier, in 1951, sensational news about element 43 was heard. The American Astronomer S. Moore found characteristic lines of technetium in the solar spectrum. The spectrum of technetium had been recorded immediately when it had become feasible, that is, when a sufficient amount of the element had been synthesized. The spectral data had been compared with those reported by the Noddacks and Berg for masurium. The spectra had proved to be quite different making ultimately clear the mistake of the discoverers of masurium. The spectrum of the solar technetium was identical to that of the terrestrial technetium. An analogy with helium was apparent–both elements sent messages from the Sun before to be found on Earth. True, astronomers questioned the data on the solar technetium.  But in 1952 the cosmic technetium once more sent a message when the British astrophysicist P. Merril found technetium lines in the spectra of two stars with the poetic names of R Andromedae and Mira Ceti. The intensities of these lines evidenced that the content of technetium in these stars was close to that of its neighbours in the periodic system, namely, niobium, zirconium, molybdenum, ruthenium, rhodium and palladium. But these elements are stable while technetium is radioactive. Though its half–life is relatively long it is still negligible on cosmic scale. Therefore, the existence of technetium on stars can mean only that it is still born there in various nuclear reactions. Chemical elements continue to be produced in stars on a gigantic scale. A witty astrophysicist named technetium the acid test of cosmogonic theories. Any theory of the origin of elements must elucidate the sequence of nuclear reactions in stars giving rise to technetium.

Reason for the formation of large number of organic compounds : Catenation

Reason for the formation of large number of organic compounds

 What makes the carbon so special ?

What is it that sets carbon apart from all other elements in the periodic table ?

Why are there so many organic compounds ?

The answer lies in carbon’s position in the periodic table. Carbon is in the centre of second row elements

Li > Be >  B > C > N > O > F

First think why molecules are formed from atoms ? It is because of the reason that atom combines with same or with other atoms to form molecule so as to complete its octet and attain lower energy stable and hence become stable. That is the reason why noble gases are considered as inert gases, they generally do not combine with itself or with other atoms because they have complete octet. But what about other atoms ? They have incomplete octet, so they must combines with same or other atoms to form molecule for better stability.

Elements on the left hand side of carbon have less than 4 electrons in the valence shell (Li-1, Be-2, B-3) so they have more tendencies to loose electron to attain noble gas configuration for stability. That’s why they generally forms compounds with Li+, Be2+, B3+ by losing 1, 2, 3 electrons respectively. Elements present downside in the same group too have similar tendency as that of Li, Be and B, hence form compounds in the following states; Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Al3+, Ga3+, etc.

Elements on the right hand side of carbon have more than 4 electrons in the valence shell (N-5, O-6, F-7). To complete their octet, valance electron must be subtracted from 8 that’s why the valency of N is (8-5) i.e. 3, O is (8-6) i.e. and that of F is (8-7) i.e. 1. It is much easier to gain 3, 2, 1 electrons to complete their octet as compared to loosing 5, 6, 7 electrons to complete their octet. So these elements have more tendency to gain electrons and form compounds in the following states; N3, P3, As3, Sb3, Bi3, O2, S2, Se2, Te2, Po2, F, Cl, Br, I.

As elements present on the left hand side of carbon loose electrons to form compounds and elements of right hand gain electrons to form compounds so compounds formed are ionic in nature.

But think about carbon and the elements present down side, which are present in the middle of each period and have equal tendency to loose or gain electrons as they have 4 electrons in their octet. This led carbon and other elements of this group (Si, Ge, Sn & Pb) to share electrons with itself and other elements of periodic table to complete octet. As these compounds are formed by sharing of electrons so they are considered to be covalently bonded.

Carbon by sharing its electrons with other carbon atoms leads to formation of long chain carbon compounds which may be single, double or triple bonded, cyclic or acyclic, linear or branched. This self-linking property of carbon is called catenation. All the atoms of 14th group show the property of catenation but it decreases down the group because of weak overlapping due to large size and follows order :

C >> Si >> Ge > Sn > Pb

Carbon may also form multiple bonds with N, P, O, S etc. forming large number of functional group, which we will discuss later.

This is not the end of compound formation, carbon forms many abnormal compounds with elements of s, p & d blocks. So for sake of simplicity we are constructing an organic chemist’s periodic table with the most important elements emphasized.

Elements, which are in dark box, are generally involved in making organic compounds along with deuterium (D), which is an isotope of hydrogen (H).

As there are large number of atoms in periodic table which have valence electrons, atomic orbital of carbon may overlap with them and share its electron to form large number of compounds. But for that many other factors such as size, activation energy, electronegativity, electron affinity, catenation etc. are responsible which all come under one word “Position” i.e. position for carbon in the periodic table. This word “position” include everything related with molecule formation therefore the main reason behind large number of organic compounds is the position of carbon in the periodic table.

 

Baeyer’s strain theory

Baeyer’s strain theory : To compare stability of cycloalkanes 

 When we carefully look over the cyclic saturated compounds, we find that each atom is sp3  hybridized.  The ideal bond angle 109028’ but in cycloalkanes this angle is mathematically 180-(360/n) where n is the number of atoms making ring. 

for example Cyclopropane, angle is 600; in Cyclobutane it is 900 and so on.

Angle Strain : This difference in ideal bond angle and real bond angle, is called angle strain and it causes strain in bond which affects the stability of molecule. 

Greater is the deviation from the theoretical angle, greater is the Angle strain ; lesser the stability. 

To calculate the distortion or angle strain in cycloalkane we assume the atoms of ring in a plane, such as in cyclopropane, all the 3 carbon atoms occupy one corner of an equilateral triangle with bond angle 60o. As two corners bent themselves to form bond so strain too is divided equally. So strain in cyclopropane will be ½ (109o28’ – 600) = 24044’.

Deviation of bond angle in cyclopropane from normal tetrahedral angle

Distortion or strain = ½ (109028’ bond angle of ring). So angle strains in some cycloalkanes are listed in the table below.

Compound

No. of C in the ring

Angle between the C atoms

Distortion or strain

Cyclopropane

3

600

24o44’

Cyclobutane

4

900

9o44’

Cyclopentane

5

108o

0o44’

Cyclohexane

6

120o

-5o16’

Cycloheptane

7

128o34’

-9o33’

Cyclooctane

8

135o

-12o62’

 

From the table it is clear that cyclopropane has the maximum distortion, so it is highly strained molecule and consequently more reactive than any of one monocylic alkanes, which is clear from the reaction that ring can be opened very easily to relieve strain on reaction with Br2, HBr or H2/Ni at high temperature.

In contrast, cyclopentane & cyclohexane have least strain so they are found more readily and are very stable as compared to cyclopropane.

Baeyer strain theory satisfactorily explains the typical reactivity and stability of smaller rings (from C3 to C5) i.e. Stability order follows : Cyclopropane < Cyclobutane < Cyclopentane

But not valid for cyclohexane onwards because the strain again increases with the increase in number of carbon atom but actually large rings are more stable. So molecular orbital theory is also considered according to which covalent bond is formed by coaxial overlapping of atomic orbitals. The greater is the extent of overlap the stronger is the bond formed. In case of sp3 carbon, C – C bond will have maximum strength if the C-C-C bond have the angle 109o28’. If cyclopropane is an equilateral triangle then the bond angle of each C-C-C bond would be 60o. Therefore it was proposed by Couson that in cyclopropane the sp3 hybridized orbitals are not present exactly in one straight line due to mutual repulsion of orbital of these bonds resulting thereby loss of overlap. This loss of overlap weakens the bond and is responsible for its instability and strain in molecule. Similarly, in case of cyclobutane, there is also loss of overlap but the loss is less than in cyclopropane, so cyclobutane is more stable than cyclopropane. Overlapping of orbitals in large ring compound (5 more carbon atoms) is however much better which accounts for the greater stability of such compound.

It is natural that when a molecule has strain within it, it will affect the stability of molecule. The stability of molecules can be calculated easily by measuring heat of combustion which will give the measure of total strain and thermochemical stability which can be calculated mathematically.

Total strain = (No of C atom is the ring × observed heat of combustion/CH2) observed heat of combustion/CH2 for n alkane.

 

Experimental data of total strain for different cycloalkanes*

No. of C in the ring

Heat of combustion kJ per CH2 group

Total strain in kJ

3

697

115

4

686

109

5

664

27

6

659

0

7

662

27

8

663.8

42

664.6

54

* data from Organic chemistry solomons & Fryhle

From the data above it is clear that strain decreases from C3 to C6i.e. stability increases, but stability again deteriorates from C7 to C9 ring system. 

 According to this theory, the carbon atoms in 5 membered and smaller rings can lie in one plane as explained by Baeyer but Sachse suggested that in six membered and higher rings the carbon atoms are non planar . In this way the ideal angle 109028’ is retained and the ring is free from angle strain. Thus Sachse proposed that cyclohexane exist in two puckered forms as boat and chair form. These forms are readily inter-convertible through half chair and twist boat forms simply by rotation about the single bonds.  

 

Discovery of Actinium element

Actinium

Was it just a chance that polonium and radium were the first to be discovered among radioactive elements? The answer is apparently no. Owing to its long half–life radium can be accumulated in uranium ores. Polonium has a short half–life (138 days) but it emits characteristic high–intensity alpha radiation. Though the discovery of polonium gave rise to a controversy it soon died off.

The third success of the young science of radioactivity was the discovery of actinium. Soon after they had discovered radium the Curies suggested that uranium ore could contain other, still unknown radioactive elements. They entrusted their collaborator A. Debierne with verification of this idea.

Debierne started his work with a few hundred kilograms of uranium ore extracting the “active principle” from it. After he had extracted uranium, radium, and polonium he was left with a small amount of a substance whose activity was much higher than the activity of uranium (approximately, by a factor of 100 000). At first, Debierne assumed that this highly radioactive substance was similar to titanium in its chemical properties. Then he corrected himself and suggested a similarity with thorium. Later, in spring of 1899 he announced the discovery of a new element and called it actinium (from the Greek for radiation).

Any textbook, reference book or encyclopedia gives 1899 as the date of the discovery of actinium. But in fact, to say that in 1899 Debierne discovered a new radioactive element–actinium–means to ignore very significant evidence to the contrary.

The real actinium has little in common with thorium but we did not mean this chemical difference as evidence against the discovery of actinium by Debierne. The main argument is as follows. Debierne believed that actinium was alpha–active and its activity was 100 000 times that of uranium. Now we know that actinium is a mild beta–emitter, that is, it emits beta rays of a fairly low energy which are hot that easy to detect. Of course, the primitive radiometric apparatus of Debierne was not capable of doing it.

Then what did Debierne discover? It was a complex mixture of radioactive substance including actinium. But the weak beta radioactive of actinium was quite indistinguishable against the background of the alpha rays emitted by the products of actinium decay. It took several years to extract the real actinium from this mixture of radioactive products.

In 1911 the outstanding British radiochemist F. Soddy published a book entitled chemistry of Radioactive Elements where he described actinium as an almost unknown element. He wrote that its atomic weight was unknown, the mean life time was also unknown, it did not emit rays (this shows how difficult it was to detect the beta radiation of actinium), and its parent substance was unknown. In a word, much about actinium was still vague.

The evidence presented by Debierne for his discovery of actinium did not seem convincing to his contemporaries. It is no wonder that soon another scientist–the German chemist F. Giesel–claimed a discovery of a new radioactive element. He also extracted a certain radioactive substance whose properties were similar to those of the rare–earth element. This fact is closer to the truth in the light of our current knowledge. Giesel named the new element emanium because it evolved a radioactive gas–emanation–which made a zinc sulphide screen to glow. Along with the radiotellurium vs. polonium controversy there appeared a similar controversy between the supporters of actinium and emanium. The first controversy ended by establishing identity between the elements in question. The second controversy proved to be more complicated and could not be speedily resolved since the behaviour of the third new radioactive element was too wayward. The name of Debierne went into the historical records as the name of the discoverer of actinium. However, the substance extracted by Giesel contained a significant proportion of pure actinium as was shown later. Giesel also succeeded in observing the spectrum of emanium. Many scientists believed that they proved identity of actinium and emanium. Gradually, the controversy lost its edge.

The British radiochemist A. Cameron was the first (1909) to place the symbol Ac into the third group of the periodic system (actually, he was the first to put forward the name radiochemistry for the relevant science). But only in 1913 was the position of actinium in the periodic system established reliably. As increasingly pure actinium preparations were obtained the scientist encountered an amazing situation–the radiation emitted by actinium proved to be so weak that some scientists even doubted if it emits at all. It has even been suggested that actinium undergoes an entirely new, radiation less, transformation. It was only in 1935 that beta rays emitted by actinium were reliably detected. The half–life of actinium was found to be 21.6 years.

For a long time extraction of metallic actinium was just out of question. Indeed, one ton of pitchblende contains only 0.15 mg of actinium while the content of radium is as high as 400 mg. A few milligrams of metallic actinium were obtained only in 1953 after reduction of AcCl3 with potassium vapour.

NTSE Scholarship : Path made simple and clear

Are you a student of Class X and looking for a platform to identify your hidden talent and evaluate your preparedness for further studies? The right answer and the platform catering to your needs is National Talent Search Examination (NTSE). NTSE identify and recognize students with high intellect and academic talent at the high-school level.

NTSE is a two-tier exam- Stage I (State Level)  is  conducted by  the States/ Union  Territories  and Stage-II(National Level) is conducted by NCERT. The objective of the two-tier  examination is to identify the talented students who have a special aptitude for sciences, maths, social sciences and questions based on analytical reasoning.

Stage I is for all interested students studying in class X to participate at state level While stage II can be written by stage I qualified students. The examination acts as unique platform for the students to check their capability and potential. It also points out the weaknesses and short comings in the domains covered by the examination. Every year, lakhs of students appear for this scholarship exam, out of which One Thousand scholarships are awarded. The scholarship is open only to the students of Indian nationality whether they are studying in India or a broad in class X or equivalent.

Scholastic Aptitude Test (SAT) SAT comprises of 100 multiple choice questions, where one alternative is correct.There are 40 questions from Science, 40 from Social Science and 20 from Mathematics. The idea is to assess the subject knowledge, reasoning ability and logical thinking of the candidates.

Mental Ability Test (MAT) : 

This also comprises of 100 multiple choice questions with only one out of four options correct. Each question carries 1 mark. There is no negative marking.  It has a variety of questions about analogies, classification, series, pattern perception, hidden figures, coding-decoding, block assembly, problem-solving, etc. Here, the goal is to gauge the power of reasoning, ability to judge, evaluate, visualize in space, spatial orientation, etc. of the candidates.

The second level test takes place every year in the month of May. As per the new pattern of Stage II announced by NCERT, each correctly answered question earns the candidate one mark with no negative marks. This level of exam tests the students’ potential concerning their mental ability and scholastic aptitude. income, government school, domicile, etc. However, no scholarships shall be available for studies abroad for any course. The rates of scholarship sat different stages of study are as under.

PREPARATION STRATEGY :It’s necessary to know your strengths and weaknesses. For example, if you are strong in math and science but average in MAT and weak in social scien­ces, then you can work on your weaknesses and turn them into strengths for an over all very good or excellent score.

Follow NCERT books : Follow NCERT books of classes IX and X for NTSE preparation. For some concepts in biology, refer NCERT Class XI Biology textbook (only portions that are relevant to Class IX and X curriculum). It is imperative to study the Social Studies NCERT book for NTSE thoroughly.

Strengthen your MAT section : MAT  and   SAT  carry  equal weight in NTSE. Therefore, it is critical to give due time to both the   papers  while   preparing. Some of us tend to overdo prepa­ration of 3 subjects but  in the process  tend to ignore MAT. However,   it is important to real­ize  that   MAT  carries  more weight than any of the subjects. The MAT section tests your men­tal ability.

Solving sample papers : Practice makes a man perfect is the mantra to success. So,if you are aspiring excellence, it is important that you solve lot of sample papers and mock tests at least thrice a week.  You should aim to solve these papers within the stipulated time so that you can improve your speed and know the sections that consume more time.

Analyse your performance : Make sure you minutely assess what you could do and what you had a hard time with. Was it the subject understanding   you lacked? Or did you miss out on the scores because of silly mistakes?

Proper guidance : Find a good mentor. There are times when in quest of achieving something, you tend to go off the track. To ensure you are on the right track, it is important  to have a mentor to guide you and show you the right path in your success journey.

Self-study : For clearing any exam,a decent amount of self-study is important. It is one of the most basic and important of all the tips to become an NTSE scholar. One should devote at least 3-4 hours to self-learning to crack this competitive exam.

Keep Practicing : Solve as many questions of men­tal ability as possible so that you are not shocked on the exam day. With practice, you’ll also get more confident about  your speed, accuracy and subject knowledge. Just remember that you don’t have to beak now-it-all to compete and excel. You can be just an ordinary candidate and follow these tips to become an NTSE scholar.





Discovery of Radon Elements

Radon

Radon Rn is the 86th element of the periodic system. It is the heaviest of the noble gases. It is highly radioactive and its natural abundance is so low that it could not be identified when W. Ramsay and M. Travers discovered other inert elements. Only application of the radiometric method made possible the discovery of radon.

What we know as radon at present is the combined name for the three natural isotopes of the element No. 86, which were discovered one by one and called emanations. Their appearance heralded a new stage in the studies of radioactivity as they were the first gaseous radioactive substances.

At the beginning of 1899 E. Rutherford (who lived at the time in Canada) and his collaborator R. Owens studied the activity of thorium compounds. Once Owens accidentally threw open the door to the laboratory where a routine experimenter was performed. There was a drought and the experimenters noticed that the intensity of radiation of the thorium preparations suddenly dropped. At first they ignored this event but later they observed that a slight movement of air seemed to remove a larger part of the activity of thorium. Rutherford and Owens decided that thorium continuously emitted a gaseous radioactive substance, which they called the emanation (from the Latin to flow) of thorium, or Theron.

By way of analogy, it was suggested that other radioactive elements could also evolve emanations. In 1900 the German physicist E.  Dorn discovered the emanation of radium and three years later Debierne observed the emanation of actinium. Thus, two new radioactive elements were found, namely, radon and action. An important observation was that all the three emanations differed only in their half–lives–51.5 s for thoron, 3.8 days for radon, and 3.02 s for action. The longest–lived element is radon and therefore it was used in all studies of the nature of emanations. All the other properties of emanations were identical. All of them lacked chemical manifestations, that is, they were inert gases (analogues of argon and other noble gases). Later they were found to have different atomic masses. But there was just a single slot for these three elements in the periodic system, immediately below xenon.

Such exclusive situation soon became a rule. Therefore, we shall have to discuss briefly some important events in the history of radioactivity studies. Now we must finish the story of radon. This name remained because radon is the longest–lived element among the radioactive inert gases. Ramsay suggested to name it niton (from the Latin for glowing) but this name did not take root.

Discovery of Radium Element

Radium

When the Curies and G. Bemont analysed pitchblende they noticed a higher radioactivity of one more fraction, apart from the bismuth fraction. After they had succeeded in extracting polonium they started to analyse the second fraction thinking that they could find yet another unknown radioactive element.

The new element was named radium from the Latin radius meaning ray. The birthday of radium was December 26, 1898. When the members of the Paris Academy of Sciences heard a report entitled “On a new highly radioactive substance contained in pitchblende”. The authors reported that they had managed to extract from the uranium ore tailings a substance containing a new element whose properties are very similar to those of barium. The amount of radium contained in barium chloride proved to be sufficient for recording its spectrum. This was done by the well–known French spectral analyst E. Demarcay who found a new line in the spectrum of the extracted substance. Thus, two methods–radiometry and spectroscopy–almost simultaneously substantiated the existence of a new radioactive element.

The position of radium among the natural radioactive elements (of course, excluding thorium uranium) almost immediately proved to be the most favourable one owing to many reasons. The half–life of radium was soon found to be fairly long, namely, 1 600 years. The content of radium in the uranium ores was much higher than that of polonium (4 300 times higher); this contributed to natural accumulation of radium. Furthermore, the intensity of alpha radiation of radium was sufficiently high to allow an easy monitoring of its behaviour in various chemical procedures. Finally, a distinguishing feature of radium was that it evolved a radioactive gas known as emanation (see p. 183). Radium was a convenient subject for studies owing to a favourable combination of its properties and therefore it became the first radioactive element (again, with the exception of uranium and thorium) to find its permanent place in the periodic system without long delay. Firstly, chemical and spectral studies of radium demonstrated that in all respects it belongs to the subgroup of alkaline earth metals; secondly, its relative atomic mass could be determined accurately enough. To do be obtained. The Curies worked ceaselessly for 45 months in their ill–equipped laboratory processing uranium ore tailings from Bohemian mines. They performed fractional crystallization about 10 000 times and finally obtained a priceless prize–0.1 g of radium chloride. The history of science knows no more noble examples of enthusiastic work. This amount was sufficient for measurements and on March 28, 1902, Marie Curie reported that the relative atomic mass of radium was 225.9 (which does not differ much from the current figure of 226.02). This value just suited the suggested position of radium in the periodic system.

The discovery of radium was the best substantiated one among the many alleged discoveries of radioactive elements, which soon followed. Every year more new discoveries were reported. Radium was also the first radioactive element obtained in the metallic form.

Marie Curie and her collaborator A. Debierne electrolyzed a solution containing 0.106 g of radium chloride. Metallic radium deposited on the mercury cathode forming amalgam. The amalgam was put into an iron vessel and heated under a hydrogen flow to remove mercury. Then grains of silvery whitish metal glistened at the bottom of the vessel.

The discovery of radium was one of the major triumphs of science. The studies of radium contributed to fundamental changes in our knowledge of the properties and structure of matter and gave rise to the concept of atomic energy. Finally, radium was also the first radioactive element to be practically used (for instance, in medicine).

Discovery of Polonium Element

Polonium

Polonium was the first natural radioactive element discovered with the radiometric technique. Back in 1870 the main properties of polonium were predicted by D. I. Mendeleev. He wrote: “Among heavy metals we can expect to find an element similar to tellurium whose atomic weigh is greater than that of bismuth. It should possess metallic properties, and give rise to an acid whose composition and properties should be similar to those of sulphuric acid and whose oxidizing power is higher than that of telluric acid…

The oxide RO2 cannot be expected to have acidic properties which tellurous acid still has. This element will form organometallic compounds but not hydrogen compounds…”

Nineteen years had passed and Mendeleev made a significant addition to his description of dvi–tellurium (as he called the unknown element). He predicted the following properties: relative atomic mass 212; forms oxide DtO3; in a free state the element is a crystalline low–melting non–volatile metal of grey colour with a density of 9.8; the metal is easily oxidized to DtO2; the oxide will have weak acidic and basic properties: a hydride of the element, if it exists at all, must be unstable; the element must form alloys with other metals.

Below readers will see for themselves how accurate were Mendeleev’s predictions of the properties of a heavy analogue of tellurium. But these predictions had only an indirect effect on the history of polonium, if any. The discovery of polonium (and then radium) proved to be a significant milestone in the science of radioactivity and gave an impetus to its development.

As one can see from the laboratory log–book of Marie and Pierre Curie they started to study the Becquerel rays, or uranium rays, on December 16, 1897. First the work was conducted by Marie alone and then Pierre joined her on February 5, 1898. He performed measurements and processed the results. They mainly measured the radiation intensities of various uranium minerals and salts as well as metallic uranium. The results of extensive experiments suggested that uranium compounds had the lowest radioactivity, the metallic uranium exhibited a higher radioactivity, and the uranium ore known as pitchblende had the highest radioactivity. These results indicated that pitchblende, probably, contained an element whose activity was much higher than that of uranium.

As early as April 12, 1898 the Curies reported this hypothesis in the proceedings of the Paris Academy of Science. On April 14 the Curies started their search for the unknown element with the assistance of the chemist G. Bemont. By the middle of July they finished the analysis of pitchblende. They carefully measured the activity of each product successively isolated from the ore. Their attention was focussed on the fraction containing bismuth salts. The intensity of the rays emitted by this fraction was 400 times that of metallic uranium. If the unknown element really did exist it had to be present in this fraction.

Finally, on July 18 Marie and Pierre Curie delivered a report to a session of the Paris Academy of Science entitled “On a new radioactive substance contained in pitchblende”. They reported that they had managed to extract from pitchblende a very active Sulphur compound of a metal that had previously been unknown. According to its analytical properties it was a neighbour of bismuth. The Curies suggested, if the discovery could be proved, to name the new element in honour of the country where Marie had been born and brought up, that is, polonium after Poland.

The scientists emphasized that the element had been discovered with a new research method (the term “radioactivity”, which later became conventional, was first introduced in this report).

The introduction of spectral analysis made it possible to reveal the existence in natural objects of elements that could not be seen, felt or weighed. Now the history repeated itself but the role of indicator was played by radioactive radiation, which could be measured with a radiometric technique. However, the results of the Curies were not faultless. They were wrong in suggesting a chemical similarity between polonium and bismuth. Even a brief look at the periodic system shows that the existence of a heavy analogue of bismuth is hardly possible. But one must not forget that the Curies did not extract pure metal, could not determine its relative atomic mass, and, finally, did not see differences in the spectra of polonium and bismuth. This is why they actually ignored a possible analogy between polonium and tellurium.

Thus, we may regard 18 July, 1898, as the date of just a preliminary discovery of polonium as substantiation of the discovery took quite a long time. The high intensity of radiation from polonium made difficult its study. The radiation was found to consist of only alpha rays with no beta or gamma rays. A strange finding was that the activity of polonium decreased with time and the decrease was rather noticeable; neither thorium nor uranium exhibited such behaviour. This is why some scientists doubted whether polonium existed at all. The sceptics said it was just normal bismuth with traces of radioactive substances.

But in 1902 the German chemist W. Marckwald extracted the bismuth fraction from two tons of uranium ore. He put a bismuth rod into a bismuth chloride solution and observed precipitation of a highly radioactive substance on it which he took for a new element and named radiotellurium. Later he recalled: “I named this substance radiotellurium just for the time being since all its chemical properties suggested placing it into the sixth group into the still unoccupied box for the element with a somewhat higher atomic weight than that of bismuth…. The element was more electronegative than bismuth but more electropositive than tellurium; its oxide should also have basic rather than acidic properties.

All this corresponded to radiotellurium…. The expected atomic weight for this substance was about 210”. Later he said that he had got his idea for extracting polonium when analysing the periodic system.

As for the polonium discovered earlier Marckwald promptly declared it a mixture of several radioactive elements. This led to a stormy discussion of the real nature of polonium and radiotellurium. Most scientists supported the Curies. A. later comparison of the two elements revealed their identity. The discovery was credited to the Curies and the name “polonium” was retained.

Though polonium was the first of the new natural radioactive elements its symbol Po did not appear in the appropriate box in the periodic system. The atomic mass of the element was very difficult to measure. The lines of the polonium spectrum were reliably identified in 1910. It was only in 1912 that the symbol Po occupied its place in the periodic table.

For almost half a century scientists had to be satisfied to work only with polonium compounds (usually in rather small amounts). The pure metal was prepared only in 1946. High density layers of metallic polonium prepared by vacuum sublimation have a silvery colour. Polonium is a pliable low–melting metal (melting point 254oC, boiling point 962oC), its density is about 9.3 g/cm3. When polonium is heated in the air it readily forms a stable oxide; its basic and acidic properties are weakly manifested. Polonium hydride is unstable. Polonium forms organometallic compounds and alloys with many metals (Pb, Hg, Ca, Zn, Na, Pt, Ag, Ni, Be). When we compare Mendeleev’s predictions with these properties we see how close they are to the truth.

Discovery of Rhenium element

Rhenium

As regards history, rhenium had an undoubted advantage over hafnium: nobody had ever questioned the fact that element No. 75 had to be an analogue of manganese, or tri-manganese in Mendeleev’s terminology. However, in all other respects there was no certainty.

Let us perform an experiment. If we select at random a few monographs and textbooks where rhenium is discussed we shall see that the authors agree on some things while sharply disagreeing on others. They all agree that rhenium was discovered in 1925 but when it comes to the source from which rhenium was extracted, they disagree. Among minerals mentioned as sources of rhenium are columbite and platinum ore, native platinum and tantalite, niobite and wolframite, alvite and gadolinite. Even an experienced geochemist will be at a difficulty finding his way among so varied a group of minerals.

After these introductory remarks, we may name the discoverers of rhenium: V. Noddack, I. Takke (who later married V. Noddack), and the spectroscopist O. Berg. Their authorship was never contested by anybody. This may be the only case when engineers became interested in the yet undiscovered element. They were aware of the uses of the periodic system. Since tungsten was widely used in electrical engineering, there was every reason to believe that element No. 75 would possess properties even more valuable for this industry. It is highly probable that the first attempts of the Noddacks to find this element were prompted by practical needs.

In 1922, after thorough preparations they set to work. First of all, they collected all reports on the discovery of manganese analogues. Since these discoveries remained unconfirmed, it was tempting to check them. The scientists drew up an extensive program of research: they were going to look for two elements at once since unknown manganese analogues included not only element No. 75 but also its lighter predecessor–element No. 43 with an unusual fate (see p. 200). The periodic table made it possible to predict many of their properties. We can now compare the Noddacks’ predictions on rhenium with the actual properties of the element.

            Prediction                                            Modern data

Atomic mass 187-188                                          186.2

Density 21                                                              20.5

Melting point 3300 K                                           3 323 K

The higher oxide formula X27                                  Re2O7

Melting point of the higher

Oxide 400-500oC                                                 220oC

The agreement is, indeed, excellent. Only the melting pint of the oxide proved to be much lower that

the expected one whereas on the whole Mendeleev’s classical method of prediction was fully confirmed. In other words the Noddacks had a perfectly good idea about what element No. 75 (and element No. 43) was going to be. Thus, the history of rhenium was closely related to the history of its light analogue.

But where to search for these element? Predicting the geochemical behavior of rhenium the Noddacks used to the full the capacity of theoretical geochemistry of that time; They even knew that it had to be a very rare element. They could not know, however, that it was a trace element and that, therefore, what seemed unquestionable to them was in effect open to doubt.

The scientists planned to investigate two groups of minerals: platinum ores and so called columbites (tantalites). Four years (from 1921–1925) were spent in searching for the wanted elements but in vain. Then a communication appeared about the discovery of hafnium whose existence in nature was proved by X-ray spectroscopy. Undoubtedly, this event gave the Noddacks the idea to use the same method in order to prove the existence of manganese analogues and they turned for help to O. Berg, a specialist in X-ray spectroscopy.

In June 1925, V. Noddacks, I. Takke, and O. Berg published an article about the discovery of two missing elements, Masurium (No. 43) and rhenium (No. 75). They were found in columbite and in the Uralian platinum and named after two German provinces. The elements X-ray spectra provided the main confirmation of their existence; but there was no question of extracting the elements and the reasoning of the German scientists was, in general, too involved. However, the article attracted attention and other scientists tried to reproduce the results.

However, no such reproduction followed. A year passed and the Soviet scientist O. E. Zvyagintsev and his colleagues proved irrefutably that the Uralian platinum ore contained no new elements. After that the German scientists continued to study columbites which varied considerably in composition but, according to the predictions, had to contain mysterious manganese analogues. They subjective the minerals to complex chemical treatment in order to concentrate the unknown elements and performed X-ray spectral analysis. The data obtained were reassuring but definite conclusions would have been premature: the scientists could not obtain any noticeable amounts of elements No. 43 and No. 75 and experimentally determine their properties.

Nobody could reproduce the results obtained by the Noddacks. Their compatriot W. Prandtl even sent his assistant. A Grimm to the Noddacks’ laboratory to watch them prepare manganese analogues Back home, A. Grimm reproduced the entire procedure, perfected it and…, we do not know the extent of his distress about the wasted time. The English scientists F. Loring and the Czechs Ya. Geirovskii and Y. Druce also doubted the Noddacks’ results. Later, Loring, Geirovskii, and druce claimed the priority of discovering element No. 75 by other methods and from other sources. History has retained their names but not as discoverers of rhenium.

The two German scientists believed to have also isolated element No. 43 (known later as technetium). Now we know that they by no means could detect the presence of technetium at the time but, nevertheless, the Noddacks were more sure of its discovery than of the discovery of rhenium (the fact which is hardly a feather in their cap). As time passed, the Noddacks became convinced that the range of the minerals for analysis had to be considerably enlarged. The previous geochemical prediction did not, apparently, come true. In the summer of 1926 and in 1927 the Noddacks went to Norway to collect minerals among which were: tantalite, gadolinite, alvite, fergusonite, and molybdenite. In the early 1928 the scientists, analysing the minerals, isolated about 120 mg of rhenium mainly from molybdenite (molybdenum sulphide). Earlier it had never been considered as a possible source of manganese analogues.

Thus, rhenium became, at last, a reality. An end was put to doubts and the symbol Re occupied forever box No. 75 in the periodic table; masurium, however, remained an enigma for a long time.

Hence, 1928 is the date of the reliable discovery of rhenium, the final step in the long process of search. As regards the widely accepted date, 1925, it is only a landmark in the prehistory of the element.

Having planned the directions of research, the Noddacks assembled all publications of supposed discoveries of eka-manganese. Their notes were lost during the second World War but, undoubtedly, the name of the Russian Scientists S. F Kern and the name of the element “devium” were mentioned in them. This may be the most reliable discovery of a new element of all unreliable discoveries. And it is equally possible that the history of element No.75 could have begun 50 years earlier.

The events were as follows. In 1877 reports appeared about the discovery of a new metal “devium” named after H. Davy. The reports aroused great interest and Mendeleev suggested inviting S. F.Kern to report to a session of the Russian Chemical Society. The scientists of Bunsen’s laboratory in Heidelberg decided to check Kern’s result carefully. Later his results were confirmed by two or three other scientists the most interesting fact was that some chemical reactions proved to be identical to those found later for rhenium. Does not it point to the identity of devium and rhenium?

For some reason or other S. F Kern lost interest in his discovery and never returned to the problem after 1878. He had extracted the element from platinum ores, which seems impossible from modern point of view (recall Zvyagintsev’s work in 1926). The fact is, however, that platinum ores have a complex and varied composition. The Uralian ore does not contain rhenium but its presence as traces in ores of other deposits has been proven.

  1. F. Kern studied a very rare sample of platinum ore from Borneo where by that time mines had already been abandoned. At the beginning of the 20th century the Russian chemist G. Chernik worked on the island. Analyzing platinum ores he found a constant mass loss in all samples and tried to explain it by the presence of an unknown element. This element- could well be Kern’s “devium”.

In 1950 Y. Druce devoted a large article to devium. He wrote that if rhenium would be discovered in platinum minerals, this would confirm Kern’s discovery. Samples of platinum ores from Borneo can be found now only in a few mineralogical museums of the world. It would be of interest to analyse them thoroughly. This is a case when the history of a chemical element could be partially changed.

Discovery of Hafnium element

Hafnium

The Institute of Theoretical Physics of the Copenhagen University in Denmark was the birthplace of a new element with Z = 72; the date of birth was the end of December, 1992, although the article about the discovery appeared in a scientific journal only in January, 1923. The Dutch spectroscopist D. Coster and the Hungarian radiochemist G. Hevesy named the element after the ancient name of CopenhagenHafnia. N. Bohr, whose role in the discovery of hafnium was decisive, stood at the cradle of the element.

The source of element No. 72 was zircon, a rather common mineral, consisting mainly of zirconium oxide. And it was Bohr who suggested the mineral as a subject of investigation. Why was the Dutch physicist so confident of success? Let us go back to the 1870’s when Mendeleev was drawing up his periodic system. He reserved the box under zirconium for an unknown element with the atomic mass about 180. Using Mendeleev’s terminology, we could name it eka-zirconium. After Mendeleev’s predictions of gallium, scandium, and germanium had come true, the confidence in the existence of eka-zirconium became stronger. The question, however, remained about the properties of this hypothetical element. Mendeleev refrained from definite assessments. Generally speaking, there were two possibilities: either eka-zirconium was part of the IV B-subgroup of the periodic table, i.e. an analogue of zirconium, or it belonged to the rare-earth family as its heaviest element. Now the time has come to recall the name “celtium” (see p. 138).

Having split ytterbium and separated lutetium, the last of the REEs existing in nature, G. Urbain continued the difficult work of separating heavy rare earths. Finally, he succeeded in collecting the fraction whose optical spectrum contained new lines. This event took place in 1911 but at the time did not attract the attention of the scientific community. Perhaps Urbain himself, having suggested the name for it, was not quite sure that he had really discovered a new element. At any rate, he thought it wise to send samples of celtium to Oxford where Moseley worked. Moseley studied the samples by X-ray spectroscopy but the X-ray photographs turned out to be of a poor quality. Nevertheless, in August 1914, Moseley published a communication in which he firmly stated that celium was a mixture of known rare earths. The communication remained practically unnoticed. In a word, the discovery of celtium for a very long time considered to be doubtful, although the symbol Ct sometimes appeared in scientific journals.

Meanwhile N. Bohr was working on the theory of electron shells in atoms which also became the corner-stone of the periodic system theory and, at last, explained the periodic changes in the properties of chemical elements. Bohr also solved the problem which had interested chemists of many years: he found the exact number of rare-earth elements. There had to be fifteen of them from lanthanum to lutetium. Only one REE between neodymium and samarium (later known as promethium, see p.208) remained unknown. Bohr came to this conclusion on the basis of the laws found by him which governed the formation of electron shells of atoms with increasing Z.

Thus, if celtium were indeed a rare-earth element, Bohr’s theory would eliminate it completely. Why couldn’t it be eka-zirconium? Having proved that lutetium completed the REE series, Bohr firmly established that element No. 72 had to be a zirconium analogue and could be nothing else. Bohr advised D. Coster and G. Hevesy to look for the missing element in zirconium minerals. Now all this seems to us quite logical and clear but at that time many things were at stake: if element No. 72 could not be proved to be a complete analogue of zirconium, the whole of Bohr’s periodic system theory would have been questioned. Having separated hafnium from zirconium Coster and Hevesy confirmed this theory experimentally just as the discovery of gallium had been a confirmation of Mendeleev’s periodic system than half a century before.

When Urbain read the communication about the discovery of hafnium, he understood that this was the end of celtium. Not everybody can take the bitterness of defect with dignity. Urbain was reluctant to part with celtium and continued his attempts to identify it with element No.72. The French spectroscopist A. Dauvillier came to help; he tried to prove the originality of celtium spectra thus making the “element” one of the rare earths.

Moreover, Urbain and Dauvillier declared that Coster and Hevesy had only rediscovered celtium but nothing much came of it, since hafnium soon came into its own. It was prepared in pure form and new spectral investigations showed that there was nothing in common between hafnium and celtium. What an irony of history! Urbain had everything to be the first to discover hafnium. At the beginning of 1992 he and his colleague C. Boulange analysed thortveitite, a very rare mineral from Madagascar. The mineral contained 8 per cent of zirconium oxide and the content of hafnium oxide was even higher. It is the only case when hafnium is contained in the mineral in amounts greater than those of zirconium and, nevertheless, Urbain and Boulange failed to uncover element No. 72. The reason for this lies in the great chemical similarity between zirconium and hafnium.

Prediction of Unknown Chemical Elements

Prediction of Unknown Chemical Elements

The history of gallium, scandium and germanium shows that their discoveries were practically unaffected by the periodic law and periodic system. However, the properties predicted by D.I Mendeleev for eka-aluminium, eka-boron and eka-silicon coincided with those of gallium, scandium, and germanium. Mendeleev had determined the main features of these elements long before they were discovered in nature. Is not this fact a striking evidence of the periodic system’s power of prediction?

The discovery of gallium and its identity with eka-aluminium became milestones in the history of the periodic law and in the history of discovery of elements. After 1875 even those scientists who had disregarded the periodic system had to recognize its value. And among them there were top researchers, such as R. Bunsen, the creator of spectral analysis (he once said that to classify elements is the same thing as to search for regularities in the stock-exchange quotations) or P. Cleve who had never mentioned the periodic system in his lectures. The discovery of scandium and germanium meant further triumph of Mendeleev’s theory of periodicity.

In addition to the classic triad Mendeleev predicted the existence other unknown elements. On the whole, as early as 1870 Mendeleev saw about ten vacant places in his table. He saw them, for instance, in the seventh group where there were neither manganese analogues nor a heavy iodine’s analogue (the heaviest halogen which had to possess metallic properties).

In Mendeleev’s papers we find mention of eka-, dvu-, and tri-manganese and eka-iodine. The scientist firmly believed in their existence. And here we encounter a very interesting fact in the predictions. Eka-manganese (known subsequently as technetium) and eka–iodine (astatine) were synthesized later. Mendeleev, naturally, could not know that they did not exist in nature and firmly believed in their existence since these elements filled in the gaps in the periodic system and made it more logical.

The prediction consists of two stages: prediction of the existence of an element and prediction of its main properties. The first stage was in many respects guess-work for Mendeleev. As yet unknown was the phenomenon of radioactivity making some elements so short-lived that their earthly existence is impossible at all or they exist only because they are products of radioactive transformations of long-lived elements (thorium and uranium).

The second stage was completely within Mendeleev’s power and depended on his confidence. Sometimes Mendeleev predicted boldly and resolutely. This was the case with eka-aluminum, eka-boron, and eka-silicon: these elements had to be placed in that part of the periodic table where many well-known and well-studied elements had already been located–the region of reliable prediction. Sometimes Mendeleev predicted the properties of unknown elements with the extreme caution. Among them were analogues of manganese, iodine, and tellurium as well as the missing elements of the beginning of the seventh period: eka-cesium, eka-barium, eka-lanthanum, and eka-tantalum. Here Mendeleev was groping in the dark, darling only to estimate atomic masses and suggest formulas of oxides. Mendeleev thought that it was difficult to predict the properties of the unknown elements (including those of REEs) whose places were at the boundaries of the system because there were few known elements around them. This was the “grey” area of uncertain prediction. Of course, they included the rare–earth elements. Finally, in some parts of the periodic table prediction was completely unreliable. They included those mysterious stretches extending in the directions of hypothetical elements lighter than hydrogen and heavier than uranium. Mendeleev never thought that the periodic system had to begin with hydrogen. He even wrote a paper in which he described two elements preceding hydrogen. Only when physicists explained the meaning of the periodic law, his mistake became clear: the nucleus of the hydrogen atom had the smallest charge equal to 1. As regards elements which are heavier than uranium, Mendeleev conceded the existence of a very restricted number of them and never took the liberty of predicting, even approximately, their possible properties. Predictions of this kind did not come until much later when they signaled important events in the history of science.

 

Discovery of Elements : Germanium

Germanium

Among the three elements predicted by Mendeleev eks-silicon was the last to be discovered and its discovery was to a greater extent than in the case of the two others, due to a chance. Indeed, the discovery of gallium by P Lecoq de Boisbaudran was directly related to his spectroscopic investigations, and the separation of scandium by L. Nilson and P. Cleve was associated with thorough investigation of REEs, which was going on at the time.

Predicting the existence of eka-silicon, Mendeleev assumed that it would be found in minerals containing Ti, Zr, Nb, and Ta; he himself was going to analyse some rare minerals in search for the predicted element. Mendeleev, however, was not fated to do it and 15 years had to pass before eka-silicon was discovered.

In summer 1885, a new mineral was found in the Himmels–furst mine near Freiberg. It was named “argyrodite” since chemical analysis showed the presence of silver the Latin for which is argentum. The Freiberg Academy of Mining asked the chemist C. Winkler to determine the exact composition of the mineral. Analysis was comparatively easy and soon Winkler found the mineral to contain 74.72% silver, 17.43% Sulphur, 0.66% iron (II) oxide, 0.22% zinc oxide, and 0.31% mercury. But what surprised him was that the percentage of all the elements found in argyrodite added up to only 93.04 per cent instead of 100 per cent. No matter how many times Winkler repeated the analysis 6.96 per cent was missing.

Then Winkler made an assumption that the elusive amount had to be an unknown element. Inspired by the idea he began to study the mineral carefully and in February 1886 the principal events in the discovery of eka-silicon took place.

On February 6, Winkler reported to the German Chemical Society that he had succeeded in preparing some compounds of the new element and isolating it in a free state. The scientist’s report was published and sent to many scientific institutions all over the world. Here is the text received by the Russian Physico-Chemical Society: “The signatory has the honour to inform the Russian Physico-Chemical Society that he found in argyrodite a new non-metal element close in its properties to arsenic and antimony which he named “germanium”. Argyrodite is a new mineral found by Weisbach in Freiberg and consisting of silver, Sulphur, and germanium.”

Three points in this letter deserve attention: firstly, Winkler considered the new element to be a non-metal; secondly, he assumed its analogy with arsenic and antimony, and, thirdly, the element had already been named. Originally, Winkler wanted to name it “neptunium” but the name had already been given to another element–a false discovery–and the scientist proposed the name “germanium” after “Germany”. The name became widely accepted although not immediately.

Later it become clear that germanium is to a great extent amphoteric in nature and, hence, Winkler’s description of germanium as a non-metal cannot be considered completely erroneous. Much sharper debates revolved around the question the analogue of which element in the system germanium was. In his first report Winkler suggested arsenic and antimony but the German chemist Richter disagreed with Winkler saying that germanium, most likely, was identical to eka-silicon. Richter’s opinion seemed to affect the opinion of the discoverer of germanium and in his letter of February 26 to Mendeleev Winkler wrote: “At first I thought this element would fill the gap between antimony and bismuth in your remarkable and thoughtfully composed periodic system and that the element would coincide with your eka-antimony, but the facts indicate that here we are dealing with eka-silicon.”

Such as Winkler’s reply to Mendeleev’s letter of congratulation. It is interesting that the antimony-germanium analogy was considered erroneous by Mendeleev but he did not think of germanium as eka-silicon either. Probably, Mendeleev was surprised that the natural source of the new element proved to have nothing in common with that predicted by him earlier (titanium and zirconium ores). The discoverer of the periodic law proposed another hypothesis: germanium is an analogue of cadmium, namely eka–cadmium. It the nature of gallium and scandium was established beyond any doubt, as regads germanium, Mendeleev was less certain. This uncertainty, however, soon gave way to certainty and already on March 2 Mendeleev wired to Winkler conceding the identity of germanium and eka-silicon.

Soon an exhaustive article by Winkler entitled “Germanium–a new element” was published in the “Journal of Russian Physico-Chemical Society”. It was a new illustration of the brilliant similarity between the predicted properties of eka-silicon and real properties of germanium.

Scandium

Scandium

We have already briefly mentioned the discovery of scandium in the chapter devoted to REEs (see p. 130). Although many of scandium’s properties are similar to those of rare earths, D. I. Mendeleev predicted that the element would be a boron analogue in the third group of the periodic system. His prediction proved to be accurate enough. Scandium was discovered by the Swedish chemist L. Nilson; on March 12, 1879, his article “On Scandium, a New Rare Metal” was published and on March 24 it was discussed at a session of the Paris Academy of Sciences.

Nilson’s results, however, were in many respects erroneous. He considered scandium to be tetravalent and gave, therefore, the formula of its oxide as ScO2. He did not measure the atomic mass and gave only its probable range (160-180). And, finally, Nilson suggested that scandium should be placed in the periodic table between tin and thorium, which ran counter to Mendeleev’s prediction.

The discovery of scandium excited the scientific community and Nilson’s compatriot P. Cleve set out to study the newly discovered element. He studied it thoroughly for almost five months and came to the conclusion that many results obtained by Nilson were erroneous. Cleve reported to the Paris Academy of Science on August 18, and the academicians learnt much new about scandium. It turned out to be trivalent; its oxide’s formula was Sc2O; its properties differed somewhat from those determined by Nilson. According to Cleve (and this was especially important) scandium was the eka-boron predicted by Mendeleev; Cleve showed a table in the left-hand column of which eka-boron properties were given and in the right-hand one those of scandium the following day Cleve sent a letter to Mendeleev in which he wrote: “I have the honour to inform you that your element eka–boron, has been obtained”. It is scandium discovered by L. Nilson this spring.

And, finally, on September 10 Cleve published a long article about scandium from which it is clear that he had a much better understanding of the new element than Nilson. Therefore, those historians are who consider Cleve and Nilson as co–discoverer of scandium right.

For a long time Nilson was working under an illusion about some of scandium’s properties and refused to recognize its identity with eka-boron. Cleve’s investigations, however, impressed Nilson very much; in the long run he was forced to admit that he was wrong, thus doing justice to the prediction power of the periodic system.

All of Mendeleev’s predictions were confirmed in the long run. The last to be confirmed was the prediction of the density of metallic scandium; only in 1937 did the German chemistry W. Fischer succeed in preparing 98 per cent scandium. Its density was 3.0 g/cm3, that is exactly the figure predicted by Mendeleev.

Discovery of Elements : Gallium

Gallium

The time of discovery of gallium is known to an hour. “One Friday of August 27, 1875, between 3 p.m. and 4 p.m. I discovered some signs that there can be a new simple body in the by-product of chemical analysis of zinc blende from the Pierfitt mine in the Argele valley (Pyrenees).” With these words P. E. Lecoq de Boisbaudran began his report to the Paris Academy of Sciences. He described some of the new element’s properties and noted that its presence in the ores was ascertained by spectra; analysis just as predicted by Mendeleev five years before. Boisbaudran extracted an extremely small amount of the substance and, therefore, could not study its properties properly.

On August 29, Boisbaudran suggested to name the element “gallium” after Gaul, the ancient name of France. The scientist continued the investigation of the new element and obtained additional information which he included into his report to the Paris Academy and then sent it to be academic journal. In the middle of November the journal with the article reached Petersburg where Mendeleev was impatiently waiting for it. There is every reason to believe that Mendeleev had already learnt about gallium though at second hand. Two weeks earlier the Russian Chemical Society had received a report from Paris signed by P. de Clermont. It recounted the discovery of gallium and contained a brief description of its properties. However, it was much more important for Mendeleev to read what the discoverer himself had written. Mendeleev’s reaction was prompt; on November 16, he delivered a report to the Russian Physical Society. According to the minutes of the session, Mendeleev declared that the discovered metal was, most probably, eka-aluminium. Next day he wrote an article in French entitled “Note on the discovery of Gallium”. And finally, on November 18, Mendeleev spoke about gallium at a session of the Russian Chemical Society. Such a spurt of activity is understandable: the great chemist saw an element predicted by him becoming a reality. Mendeleev believed that if further investigation confirmed the similarity of eka-aluminium properties of those of gallium, this would be an instructive demonstration of the periodic law’s usefulness.

Six days later (a surprisingly short time!) the “Note on the Discovery of Gallium” appeared in the journal of the Paris Academy of Sciences. Boisbaudran’s reaction to it is of particular interest. He continued his experiments and prepared the new results for publication. The next article by the French scientist was published on December 6. As before, he complained of the difficulties caused by the extreme scarcity of gallium, described the preparation of the metal by the electrochemical method and discussed some of its properties, and suggested that the formula of gallium oxide had to be Ga2O3.

Only at the end of the article were there a few words about Mendeleev’s note. Boisbaudran admitted that he had read it with great interest since classification of simple substances interested him for a long time. He had never known about Mendeleev’s prediction of eka-aluminium properties but it did not matter; Boisbaudran believed that his discovery of gallium was facilitated by his own laws of spectral lines of elements with similar chemical properties. In his opinion, spectral analysis played a decisive role. And not a word that Mendeleev in his prediction of eka-aluminium also underlined the prominent role of spectral analysis in the discovery of the new element. According to Boisbaudran, Mendeleev’s predictions had nothing to do with the discovery of gallium.

However, as Boisbaudran went on studying the properties of metallic gallium and its compounds, his results continued to coincide with Mendeleev’s predictions. For instance, in May 1876, the Franch scientist established that gallium was readily fusible (its melting point is 29.5oC), its appearance remained the same after storage in air, and it was slightly oxidized when heated to redness. The same properties of eka-aluminium were predicted by Mendeleev in 1870, who calculated the density of eka-aluminium to be 5.9-6.0 on the basis of the periodic system and the densities of eka-aluminium’s neighbours. Lecoq de Boisbaudranm, however, making use of his spectral laws, found that the density of eka-aluminium was 4.7 and confirmed the value experimentally. Such a difference (less than two units) might seem small to a layman but it was essential for the future of the periodic law. Up to that time only qualitative characteristics of the predicted properties had been confirmed and density was the first quantitative parameter. And it turned out to be erroneous.

There is a widely known story that Mendeleev, having received Boisbaudran’s article citing a low (4.7) density of gallium, wrote him that the gallium obtained by the French chemist was contaminated most likely by sodium used in the process of gallium preparation. Sodium has a very low density (0.98), which could substantially decrease the density of gallium. Hence, it was required to purify gallium thoroughly.

This letter has not been found either in French or in the Mendeleev’s archives. There is only indirect evidence from Mendeleev’s daughter and the eminent historian of chemistry B. Menshutkin that the letter did exist. However, that may be Mendeleev’s views became known to Boisbaudran who decided to repeat the measurements of gallium’s density. This time he took into account that Mendeleev’s calculations for the hypothetical element’s density this time he took into account that Mendeleev’s calculations for the hypothetical element’s density gave 5.9. And be obtained this value at the beginning of September, 1876. His report about this fact needs no comments. The French scientist became firmly convinced of the extreme importance of the confirmation of Mendeleev’s predictions about the density of the new element. Sometime later Lecoq de Boisbaudran send his photo to the great Russian chemist with the inscription: “With profound respect and an ardent wish to count Mendeleev among my friends. L. de B.” Mendeleev wrote under it: “Lecoq de Boisbaudran. Paris. Discovered eka-aluminium in 1875 and named it “gallium”, Ga=69.7.”

In autumn 1879, F. Engels became acquainted with a new detailed chemistry textbook by H. Roscoe and C. Shorlemmer. For the first time it contained the story about the prediction of eka-aluminium by Mendeleev and its discovery as gallium. In an article to be later included in his Dialectics of Nature Engels quoted the corresponding text from the book and concluded: “by means of the unconscious application of Hegel’s law of the transformation of quantity into quality, Mendeleev achieved a scientific feat which is not too bold to put on a par with that of Leverrier in calculating the orbit of the still unknown planet Neptune”.

Discovery of Inert Gasses

Inert Gases 

The discovery of inert gases ranks among the four great scientific events of the end of the 19th century that led to revolutionary changes in natural sciences, the other three being the discovery of X-rays by Roentgen, discovery of radioactivity, and the discovery of electron.

This prominence given by scientists to inert gases has many reasons.The history of their discovery is colourful and exciting. Helium, the mysterious solar element, was discovered on the earth and this fact alone illustrates how inventive and penetrating man’s mind became in his striving for deeper and better understanding of nature.

No less mysterious argon sowed confusion among scientists. Its chemical inertness made it impossible to be classified as a chemical element in the ordinary sense of the term since it revealed no chemical properties. There was nothing left for the researchers but to grow accustomed to the idea that there can be elements unable to enter into chemical reactions. The idea proved extremely fruitful. The discovery of inert gases contributed to development of the zero valence concept. Moreover, forming an independent zero group they added harmony to the periodic system. Almost twenty five years after their discovery the inert gases helped N. Bohr to develop his theory of the electron shells of atoms. This theory, in its turn, explained the chemical inactivity of the inert gases and their atomic structure became the basis of the concepts of ionic and covalent bonds. Thus, the discovery of inert gases contributed greatly to the development of theoretical chemistry.

In the early 60’s they surprised the scientific community once more. Scientists showed that Xenon (mainly) and krypton can form chemical compounds. Now more than 150 such compounds are known. Such late “debunking” of the myth about the complete chemical inactivity of inert gases is a paradoxical and interesting feature in their history.

Inert gases are among the rarest stable elements on the earth. Here are the data given by Ramsay: there is one part by volume of helium per 245 000 parts of atmospheric air, one of neon per 81 000 000, and one of argon per 106, one of krypton per 20 000 000 and one of xenon per 170 000 000. Since then these figures have remained almost unchanged. Ramsay said that xenon content in air is less than that of gold is sea water. This alone shows how excruciatingly difficult was the discovery of inert gases.

Krypton, Neon, and Xenon

In the history of inert gases, many problems stuck in starting. Out of many problems and their several reasons ; one of them was that scientists were dealing with very small amounts of Argon and Helium. To isolate them from air, one had to chemically remove oxygen, nitrogen, hydrogen, and carbon dioxide. All inert gases constitute a negligible part of the earth’s atmosphere.

Detection and isolation of so minor traces was difficult specially when Helium and Argon were known.  

Another reason was chemical inactivity of Argon and Helium. Even the most active reagents (for instance, fluorine) were powerless to combine with these gases and isolate them . Chemists had no way of studying inert gases and only physical methods could bring some results. Therefore better physical methods were required and they were being developed during this discovery period.

Scientists developed experimental techniques for analysing small amounts of gases, perfected spectroscopes and devices for determining gas densities.

Finally, an event took place that was of extreme importance for the history of inert gases. Two engineers, U. Hampson from England and G. Linde from Germany, invented and effective process for liquefaction of gases. Hampson built an apparatus that produced on liter of liquid air per hour. The success gave an impetus to the creative though of scientists.

In early 1898 M. Travers, Ramsay’s assistant, began to design a refrigerating apparatus for preparing large amounts of liquid argon. Since atmospheric gases liquefy at different temperatures, they can easily be separated from one another. The discoveries of argon and helium are remarkable also in that they set the chemists thinking not only about the nature of chemical inertness ( the phenomenon was understood only about a quarter of a century later) but about the periodic law and periodic system which were under a serious threat. Three most important characteristics of argon and helium (atomic masses, zero valence, monatomic molecule) put both gases outside the system. That is why Mendeleev was so readily attracted by the convenient thought about N3.

History has a striking power of prediction. Argon had not been properly discovered yet, when on May 24, 1894, Ramsay wrote a letter to Rayleigh in which he asked whether it had ever occurred to him that there was indeed a place in the periodic table for gaseous elements. For instance:

Li         Be        B         C          N         O          F          X          X          X

                                                                                    Cl

                                                                                    Mn       Fe        Co        Ni

                                                                                    Br

                                                                                    ?           Rd        Ru        Pd…

Ramsay assumed that the system’s small period could contain a triad to elements similar to those of iron and platinum metals in the great periods. The discoveries of argon and helium gave rise to an idea that these gases could occupy the places of two Xs in Ramsay’s graph. The atomic masses of these elements, however (4 and 40, respectively), proved to be too different for He and Ar to be placed in the same period. Gradually, the idea about new triads was relegated to the background and Ramsay proposed to place inert gases at the end of each period. In this case one could even expect the discovery of an element with the atomic mass 20, an intermediate between helium and argon. Ramsay’s report at the session of British Association in Toronto in August 1897, was devoted just to this element. The report was entitled “Undiscovered Gas”. Ramsay wanted to describe interesting properties of the gas but though it unwise not to mention its most remarkable property: the gas had not been discovered yet.

And here again we see the same certainty which permeated Ramsay’s letter to his wife on the eve of argon’s discovery. But not it was not audacity of a romantic but conviction multiplied by experience. The undiscovered gas turned out to be neon. Owing to a whim of fate (a frequent thing in science) the discovery was preceded by another event. The new gas could, obviously, be discovered by gradual evaporation of liquid air and by analysis of the resulting fractions, the ones lighter than argon being especially interesting. On May 24, 1898, Ramsay and Travers received a Dewar flask with liquid air. Unfortunately (or, rather, fortunately) the amount of air was too small to search for argon’s predecessor and the scientists decided to use the material for perfecting the procedure of liquid air fractionation. Having done so, Ramsay and Travers discovered by the end of the day that the fraction remained was the heaviest one. For a week the fraction remained neglected until on May 31 Ramsay decided to investigate it. The gas was scrubbed from possible impurities of nitrogen and oxygen and subjected to spectral analysis. Ramsay and Travers were dumbfounded when they saw a bright yellow line which could belong neither to helium nor sodium. Ramsay wrote down in this diary: “May 31. A new gas. Krypton.” Recall that this name was previously given to undiscovered helium. Now the name found its place in the history of inert gases. Krypton, however, was not the gas about which Ramsay made a report. Its density and atomic mass were higher than the predicted ones.

The discovery of neon promptly followed. Ramsay and Travers selected light fractions formed on the distillation of air and discovered a new inert gas in one of them. Ramsay later recollected that the name “neon” (from the Greek neos for “new”) had been proposed by Ramsay’s twelve-year-old son. In this case the experiment was performed by Travers alone since Ramsay was away. It was on the 7th of June. Then a whole week was required to confirm the result, obtain greater amounts of neon, and determine its density. Neon, as had been expected, turned out to be an intermediate between helium and argon although t had not yet been isolated as a pure gas. The problem of complete separation of neon and argon was solved later.

Still another inert gas was to be discovered by Ramsay and Travers. The scientists, however, did not feel as certain as in the case of neon. One day in July, 1898, the colleagues were busy with distilling liquid air and separating it into fractions. By midnight they collected more than 50 fractions discovering krypton in the last of them (No. 56). After that upon heating the apparatus one more fraction was collected (No. 57) consisting, mainly, of carbon dioxide traces. Ramsay and Travers argued about the expediency of studying it and at last decided to proceed with the of experiment. Next morning the scientists observed the spectrum of fraction No. 57, which turned out to be highly unusual. Ramsay and Travers concluded that it could be attributed to a new gas. pure xenon, however was prepared only in the middle of 1900. The name “xenon” originates from the Greek xenos, which means “stranger”

Argon

If you saw the statement “Inert gases were discovered by H. Cavendish in 1785” you would treat it as a joke. But no matter how paradoxical it seems, it is essentially true. Only the word “discovered” is misused here. One would be equally justified in declaring that hydrogen was discovered by R. Boyle in 1660 or by M.V Lomonosov in 1745. In his experiments Cavendish only observed “something” whose nature became clear on hundred years later. In one of his laboratory records Cavendish wrote that, passing an electric spark through a mixture of nitrogen with an excess of oxygen, he obtained a small amount of residue, no more than 1/125 the initial volume of the mixture. This mysterious gas bubble remained unchanged under the subsequent action of the electric discharge. It is clear now that it contained a mixture of inert gases, the fact which Cavendish could neither understand nor explain.

The famous English physicist’s experiment was described in 1849 by his biographer H. Wilson in the book Life of Herny Cavendish. In the early 80’s of the 19th century Ramsay studied the reaction of gaseous nitrogen with hydrogen and oxygen in the presence of a platinum catalyst. Nothing came out of these experiments and Ramsay did not even publish his results. As he recalled later, he had just read the book by Wilson and wrote “Pay attention” against the description of Cavendish’ experiment. He even asked his assistant C. Williams to repeat the experiment but we do not know the result of the attempt. Most likely, nothing came out of it. The episode, however, turned out to be unforgettable for Ramsay (his “hidden memory”, as he called it) and played a certain role in the prehistory of argon’s discovery. At first, the English physicist J. Rayleigh was the main character in it and the need for a further development of the atomic and molecular theory was its historic background. It was essential to specify the atomic masses of the elements for the development of the theory. Numerous experiments showed that in the majority of cases the atomic masses were not integers. Meanwhile, as early as 1815–1816 the English physician W. Prout advanced a hypothesis, a landmark in the history of natural sciences, that atoms of all chemical elements consist of hydrogen atoms; thus, atomic masses had to be integers. Therefore, either Prout was wrong, or the atomic masses were determined incorrectly.

To remove the discrepancy, new studies of the composition and nature of the gases were required. Rayleigh thought it necessary to determine, first of all, the densities of the main atmospheric gases, nitrogen and oxygen, since their atomic masses could then be calculated on the basis of the density values.

Rayleigh published a short article in the influential English journal Nature on September 29, 1892. It might seem that the article was about a mere trifle; the density of nitrogen separated from atmospheric air differed from that of nitrogen obtained by passing a mixture of air and ammonia over a red-hot copper wire. The difference was very small, only 0.001, but it could not be explained by an experimental error. Atmospheric nitrogen was heavier. Thus, a mystery appeared which was described as “an anomalously high density of atmospheric nitrogen”. Nitrogen obtained by any other chemical techniques was always lighter by the same value.

What was the cause of the discrepancy? Ramsay became interested in the problem. On April 19, 1894, he met with Rayleigh and discussed the situation. Each of them, however, remained firm in his previous conviction. Ramsay believed that atmospheric nitrogen contained an admixture of a heavier gas and Rayleigh, on the contrary, felt that an admixture of a lighter gas in “chemical” nitrogen was responsible for the discrepancy.

Rayleigh’s view seemed more attractive. The composition of atmosphere had been thoroughly studied for more than a hundred years and it was hardly possible that some components of the air could have remained undetected. It is just the time to remember Cavendish’s experiment and for Ramsay’s “hidden memory” to work. On April 29, Ramsay sent a letter to his wife in which he wrote that nitrogen, probably, contained some inert gas which had escaped their attention; Williams is combining nitrogen with magnesium and is trying to establish what remains after the reaction. “We can discover a new element.”

The latter breathes confidence: an unknown gas is a new element which, like nitrogen, is inactive, i.e., it hardly enters into chemical reactions. To separate the “stranger” from nitrogen, Ramsay tried to bond nitrogen chemically and used the reaction of nitrogen with red-hot magnesium shaving (3Mg+N2 = Mg3N2); this is the only example when chemistry played a role in the discovery of inert gases. Entering into polemics with himself Ramsay, however, assumed another possibility: the unknown gas is not a new element but an allotropic variety of nitrogen whose molecular consists of three atoms (N3) like oxygen (O2–molecular oxygen and O3–ozone). The absorption of nitrogen with magnesium must be accompanied with the decomposition of the N2 molecule into atoms; the single N atom could then be added to N2 forming N3. Such was Ramsay’s thinking and later the assumption about the existence of N3 became a trump card in the hands of argon’s opponents. Fruitless attempts to separate an ozone-like nitrogen continued for more than two months but by the 3rd of August Ramsay had 100 cm3 of a gas which was nitrogen with a density of 19.086.

The scientist wrote about his success to Crookes and Rayleigh. He send an ampoule with the gas Crookes for spectroscopic investigations; Rayleigh himself collected a small amount of the new gas. In the middle of August Ramsay and Rayleigh met at a scientific session and made a joint report. They described the spectrum of the gas and underlined its chemical inactivity. Many scientists listened to the report with interest but were surprised: how could it be that air contained a new component? The eminent physisist O. Lodge even asked: “Didn’t you, gentlemen, discover the name of the new gas as well?

The difficulty about the name was settled in early November when Ramsay suggested to Rayleigh to name it argon (from the Greek for “inactive”) taking into account its exceptional chemical inactivity and to assign the symbol A to it (which later became Ar.) On November 30, the president of the Royal Society Lord Kelvin (W. Thomson who in 1871 was the first to use the name “helium”) Publicly described the discovery of a new constituent of the atmosphere as the outstanding scientific event of the year. The nature of the constituent, however, was unclear. Was it a chemical element? Such authorities as D. I. Mendeleev and J. Dewar, the inventor of the flask for storage of liquid air, believed that argon was N3. The absolute chemical inactivity of argon was a new property previously unknown to chemists and, therefore, it was difficult to study the gas (in particular, to determine its atomic mass). In addition, it became clear that argon, unlike all known elemental gases, is monatomic, i.e. its molecule consists of one atom. At a session of the Russian Chemical Society on March 14, 1895, Mendeleev declared: argon’s atomic mass of 40 does not fit the periodic system, hence, argon is condensed nitrogen N3.

Much time had passed before the many problems presented by the discovery of argon were solved. A certain role was played here by the discovery of helium, which also turned out to be an inert and monatomic gas. The argon-helium pair allowed an assumption to be made that the existence of such gases is a regularity rather than a mere chance and one could expect the discovery of new representatives of this family. However, they were not discovered until three years passed. In the meantime scientists thoroughly studied the properties of helium and argon, made precise determination of their atomic masses, and put forward ideas about the location of both elements in the periodic table.

Discovery of Helium element

Helium

Helium’s unusual story attracted attention of many scientists and science historians, but the real sequence of events was distorted in numerous descriptions which overgrew with a lot of fictional details. Even a legend was invented beautiful and impressive–about the discovery of the sun element but it was far from the truth.

The French astronomer J. Janssen and the English astronomer N. Lockyer are considered to be the discoverers of helium. They studied the total solar eclipse of 1868 which was especially convenient to observe on the Indian ocean shores. In letters sent to the Paris Academy of Sciences and read out at one of its sessions they wrote that the spectra of the sun photographed during the eclipse contained a new yellow line D3 corresponding to an unknown element. To commemorate this remarkable event (the discovery of a new element existing on the sun but not on the earth) a special medal was minted.

Everything is wrong in this fascinating story except two dates. First of all, in August 1868, Lockyer was not on the Indian Ocean coast and did not observe the total solar eclipse. Janssen made his observations after the eclipse. They were of great importance for astronomy but not for the history of helium. The French astronomer was the first to observe solar prominences (gigantic ejections of solar matter) not during an eclipse and to describe their nature. Here is the text of the telegram sent by him to the Paris Academy of Science: “the eclipse and prominences were observed, the spectrum is remarkable and unexpected; Prominences are of a gaseous nature.”

Up to that time scientists had known nothing about the nature of prominences. Now it became clear they were clouds of gaseous matter and had a complex chemical composition. A detailed description of his observation was given by Janssen in a letter which reached Paris only 40 days later and was two weeks behind the letter of another French astronomer S. Raye. The latter also observed the prominences and made certain conclusions about them. And what was Lockyer doing at the time? Without leaving England, he observed the prominences with the help of a specially designed spectroscope and determined the positions of lines in their spectra. On October 23 he sent a letter to the Paris Academy of Sciences; by a surprising coincidence it was received on the same day as J. Janssen’s letter.

On October 26 the letters of Janssen and Lockyer were read to the session of the Academy but they did not contain a word about either the hypothetical sun element or the line which was later identified as the characteristic line of the helium spectrum. It was only pointed out in the letters that prominences had been observed when the sun was not eclipsed. And the medal was minted precisely to mark this event. Thus, no helium was discovered on August 18, 1868, either by Janssen or by Lockyer. Their observations provided an impetus for an intensive study of prominences by many astronomers. And only then was it noticed that the spectra of prominences contained a line which could be assigned to none of the elements known on the earth. Most clearly the line was observed by the Italian astronomer A. Secci who later designated it as D3. Secci’s name ought to be placed side by side with those of Janssen and Lockyer. His role in discovering helium was no less than that of his predecessors. Secci, however, assumed that the D3 line could belong to some known element, for instance, hydrogen, under high pressures and temperatures. If this assumption had not been confirmed, Secci would have agreed to consider D3 line as corresponding to some element unknown on Earth. N. Lockyer and E. Frankland tried to solve the problem posed by Secci but they did not notice any changes in the hydrogen spectrum. Therefore, in his article of April 3, 1871, Lockyer already used the expression “a new element X”. There are indications that the name “helium” (from the Greek helios for “solar”) was proposed by Frankland. The word “helium” was first uttered at a British Association Session by its president V. Thomsov (Lord Kelvin) on August 3, of the same year. Even if we regard the discovery of helium as “fait accompli”, then, it still remained unusual. It was the only element which could not be isolated in a material form. What is helium under ordinary conditions–gas, liquid, or solid? What are its properties? What is its atomic mass and where is its place in the natural series of elements?

None of these questions could be answered even approximately. Besides, Secci’s doubt was still not cleared. Thus, a period began in the history of helium when it was only a hypothetical element. There was no consensus on helium. Mendeleev firmly supported Secci’s point of view, feeling that the bright yellow line could belong to some other known element at high temperatures and pressures. W. Crookes, however, completely recognized helium’s independence and considered it to be a primary matter which gave rise to all other elements via successive transformations.

Sometimes it seemed that helium was not unique in its mysteriousness. Astronomers discovered new lines in the spectra of various cosmic objects: the sun, the stars, and nebulae. A number of hypothetical elements appeared, namely coronium, arconium, nebulium, protofluorine. Several years later they were all recognized to be nonexistent and only helium survived.

To receive recognition, helium had to show its “earth face” and its “earth” history began with a chance event.

On February 1, 1895, W. Ramsay received a short letter from K. Miers, a British museum employee. By that time Ramsay had already been acclaimed as the discoverer of argon and we may think Miers did not choose him by chance. Miers wrote about the experiments of the American researcher. W. Hildebrand, performed at the US Geological Institute as early as 1890. Upon heating of some thorium and uranium minerals (for instance, cleveite) a chemically inactive gas was liberated; its spectrum was similar to that of nitrogen and contained new lines.

Later Hilderbrand himself confessed to Ramsay that he had a temptation to attribute these lines to a new element. However, his colleagues were sceptical about the results and Hildebrand stopped his experiments. Miers, however, believed that in the light of numerous cases of nitrogen presence in natural uranates it was reasonable to stage another experiment.

Evidently, Ramsay believed that Hildebrand’s inactive gas could be argon; therefore, he agreed with Miers and on February 5, he acquired a small amount of cleveite. Ramsay himself, however, was busy with studying argon and attempting to prepare its compounds and, therefore, asked his pupil D. Matthews to carry out the experiment. Matthews treated the mineral with hot sulphuric acid and, like Hildebrand, observed the formation of bubbles of a gas resembling nitrogen.

When a sufficient amount of the gas was collected, Ramsay performed its spectral analysis (March 14). The picture was unexpected: the spectrum had a bright band whose lines were not found in the spectra of nitrogen and argon.

Although Ramsay had no sufficient facts to make definitive conclusions he assumed that cleveite contained, in addition to argon, another unknown gas. Ramsay spent a whole week to obtain this gas in as pure a form as possible. On March 22, he compared the spectra of argon and the unknown gas in the presence of B. Brauner. Ramsay provisionally named this gas “krypton” from the Greek for “secret”, “covered”. The name later passed to another inert gas. On March 23. The scientist wrote down in his diary that the bright yellow line of “krypton” did not belong to sodium and was not observed in the argon spectrum. (In the late sixties it was necessary to prove that the D3 line of solar helium was not the bright yellow line of sodium; history, as we see, repeated itself.)

Not quite sure of his result, Ramsay sent an ampoule with the gas to W. Crookes. A day later a telegram was received from Crookes which read: “krypton is helium, 587.49; come and see.” The figure 587.49 corresponded to the wavelength of the solar helium on a specially calibrated scale. Although these data facilitated the identification of helium on the earth, otherwise this discovery was independent. It became possible for the scientists to comprehensively study helium–a new chemical element which was no longer hypothetical. Helium’s complete chemical inactivity was not suspicious: similar inactivity of argon had already been known by that time (1894).

A brief communication about the discovery of helium on the earth was first published by Ramsay on March 29, 1895, in the “Chemical News” edited by Crookes. It is interesting that almost simultaneously terrestrial helium was discovered in cleveite by the Swedish scientist P. Cleve (in whose honour the mineral had been named) and by his assistant. A Lunglet. They, however, were a little too late with their experiments and could only express their disappointment, by no means claiming their priority. Terrestrial helium received full recognition and no attempts were made to refute Ramsay’s results. A little time passed and helium was discovered in other minerals and mineral spring waters. In 1898 helium was found in the earth atmosphere.

Discovery of Elements: Indium

Indium

In the history of chemical elements the discovery of a new element often directly affected the discovery of another one. Thus, the discovery of thallium was a catalyst for the discovery of indium–the last of the classic group of four elements identified by spectral analysis.

The stage was set in the German town of Freiberg; and the main characters were F. Reich, professor of physics in the Mining Academy and his assistant Th. Richter. The time was the year of 1863. Interested in some properties if thallium, discovered two year earlier, F Reich decided to obtain a sufficient amount of the metal for his experiments. Searching for natural sources of thallium, he analysed samples of zinc ores mined at Himmelsfürst. In addition to zinc the ores were known to contain Sulphur, arsenic, lead, silicon, manganese, tin, and cadmium, in a word, quite a number of chemical elements. Reich believed that thallium could be added to the list. Although time-consuming chemical experiments did not produce the desired element, he obtained a straw-yellow precipitate of an unknown composition. It was told that when C. Winkler (subsequently the discoverer of germanium) entered Reich’s laboratory the latter showed him a test-tube with the precipitate and said that it contained sulphide of a new element.

It would have been surprising if F. Reich had not used spectroscopy to prove his assumption. Of course, Reich did use it but, unfortunately, he was colour-blind and, therefore, asked his assistant Richter to perform spectral analysis. Th. Richter succeeded in the very first attempt: in the spectrum of the sample he saw an extremely bright blue line which could not be confused with either cesium blue line or any other line. In a word, the observation was quite definite. Reich and Richter came to the conclusion that the ores of Himmelsfurst contained a new chemical element. They named it “indium” after “indigo”, a bright blue dye. There is an interesting fact that does credit to F. Reich. The first reports about the discovery of indium were signed by the two scientists. Reich, however, believed that this was unjust and that the honour of the discovery belonged solely to Richter.

Soon after the two scientists had proved the existence of natural indium with the help of spectroscopy, they obtained a small amount of it. Indium compounds turn the flame of a Bunsen burner blue-violet and so bright that presence of the new element could be established without a spectroscope. Subsequently Reich and Richter studied some properties of indium, with Winkler giving them considerable help.

When metallic indium, although contaminated, was prepared, Richter submitted the samples to the Paris Academy of Sciences in 1867 and estimated their value at 800 pounds sterling which was quite a lot of money at the time.

Chemical properties of indium were described soon after its discovery but its atomic mass was at first determined incorrectly (75.6). Mendeleev saw that this atomic mass would not correctly place indium in the periodic table and suggested to increase it by about 50 per cent. Mendeleev proved to be right and indium occupied its place in the third group of the periodic table.