Were the Last to Be Discovered
The elements with the atomic numbers of 72 and 75 were the last stable elements to be discovered in nature—only in the twenties of this century. They are rare, especially rhenium which is one of the least abundant elements. How-ever, the rareness of hafnium and rhenium is hardly respon-sible for their late discovery. The reason is the peculiar geochemistrv of these elements: thev are known as trace elements which do not form ores and minerals in the earth's crust hut appear in ores and minerals of other elements as low-concentration impurities. Tsomorohism (replacement of ions of some elements in crvstal lattices of compounds by those of others when the ionic radii are close) largely ac-counts for their behaviour. The ionic radii of zirconium and hafnium are almost the same, which is responsible for their chemical similarity (their separation is a difficult problem even now). Hafnium in small amounts often accompanies zirconium and, because of their similarity, is not detected against its background.
Rhenium has no special affinity to minerals of any one of the abundant elements. Therefore, while the existence of hafnium was oroved rather easily, rhenium was not discov-ered definitely until after several years of painstaking search.
Scientists knew what thev were looking for, planning be-forehand what, where, andhowthev were going to discover:
thev were after elements No. 72 and No. 75. Hafnium was nromptly discovered; as for rhenium, brilliant theoretical predictions at, first misfired.
The fates of hafnium and rhenium had something else in common: they were discovered with the help of a new method of snectral analysis (X-rav soectroscoov) consisting in the study of X-rav spectra of elements. In 1914 the English nhvsicist H. Moselev discovered the law which related the wavelength of an element's characteristic X-ray radiation to its number in the periodic system. The law made it
possible to predict X-ray spectra. Never before was the discovery of new elements so thoroughly prepared as in the case of hafnium and rhenium.
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, 1922, although the article about the discovery appeared in a scien-tific journal only in January, 1923. The Dutch spectrosco-pist D. Coster and the Hungarian radiochemist G. Hevesy named the element after the ancient name of Copenhagen—
Hafnia. 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, scan-dium, 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
ele-ment. 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 celtium was a mixture of known rare earths. The communication remained practically unnoticed.
In a word, the discovery of celtium was for a very long time considered to be doubtful, although the symbol Ct some-times 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 for 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, Boar'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 miss-ing 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 com-plete analogue of zirconium, the whole of Bohr's periodic system theory would have been questioned. Havingr 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 more 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 defeat 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 wasnothingin common between hafnium and cel-tium. What an irony of history! Urbain had everything to be the first to discover hafnium. At the beginning of 1922 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 zirconi-um 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.
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 min-erals 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 discov-erers of rhenium: V. Noddack, I. Takke (who later married V. Noddack), and the spectroscopist 0 . Berg. Their
author-ship was never contested by anybody. This may be the only case when engineers became interested in the yet undis-covered element. They were aware of the uses of the periodic system. Since tungsten was widely used in electrical engineer-ing, there was every reason to believe that element No. 75 would possess properties even more valuable for this indus-try. It is highly probable that the first attempts of the Nod-dacks 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 X207 Re2Oj
Melting point of the higher
oxide 400-500°C 220°C
The agreement is, indeed, excellent. Only the melting point of the oxide proved to he much lower than the expect-ed one whereas on the whole Mendeleev's classical method of prediction was fully confirmed. In other words the Nod-dacks 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 elements? Predicting the geochemical behaviour 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 pi anned to investigate two groups of minerals:
platinum ores and so-called columbites (tantalites). Four years (from 1921 to 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 0 . Berg, a specialist in X-ray spectro-scopy.
Tn June 1925, V. Noddack, I. Takke, and 0 . Berg pub-lished 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 pro-vided 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 myste-rious manganese analogues. They subjected 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 exnerimentally determine their properties.
Nobody could reproduce the results obtained by the Noddacks. Their compatriot W. Prandtl even sent his assist-ant 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 scientist F. Loring and the Czechs Ya. Gei-rovskii 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 discov-erers 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 techneti-um 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 min-erals for analysis had to be considerably enlarged. The previ-ous sreochemical 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 msr of rhenium mainly from molybdenite (molyb-denum 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 Ion? time.
Hence, 1928 is the date of the reliable discovery of rheni-um, the final step in the lonsr process of search. As resrard& 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 on supposed discoveries of eka-mansraneses. Their notes were lost during the Second World War but, undoubtedly, the name of the Russian scientist S. F. Kern and the name of the element "devium" were men-tioned 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 reporls 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 results 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 Dralian ore does not contain rhenium but its presence as traces in ores of other deposits has been proven.
S. F. Kern studied a very rare sample of platinum ore from Borneo where by that time the mines had already been aban-doned. At the beginning of the 20th century the Russian chemist G. Chernik worked on the island. Analysing plati-num 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 min-erals, this would confirm Kern's discovery. Samples of plati-num ores from Borneo can be found now only in a few min-eralogical 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.