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.

Radon

Radon Rn is the 86^th 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.

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 X₂O­₇                                  Re₂O₇

Melting point of the higher

Oxide 400-500^oC                                                 220^oC

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 20^th 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.

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

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.

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.

Inert Gases 

The discovery of inert gases ranks among the four great scientific events of the end of the 19^th 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. 

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 in 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 analyzing 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. 

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 7^th 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 19^th 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 3^rd 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.