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 19^th 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 19^th 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 20^th 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

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

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

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.

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: “61^st 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 20^th 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.

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

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.