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.