Helge Kragh Roskilde University Centre P O . Box 260 4000 Roskilde. Denmark Peter Robertson The Niels Bohr Archive Blegdarnsvej 17 2100 Copenhagen, Denmark
I I I
On the Discovery of Element 72
The fascinating story of the disroverv of hafnium. element Interesting contributions have heen made by Heilhron ( 1 ) &d hy Heimann ( 2 ) ,both of whom, however, focus on the wqrk of Moseley rather than on hafnium itself. The well-known hook of Weeks ( 3 ) contains a chapter nn hafnium, hut the chaoter is neither verv detailed nor comoletelv reliahle. Part. of the present paper &ws upon a study hy one of the authors (H. K.), on Rohr's theory of the periodic system and the discovery of hafnium (4). The authors would like to thank the Niels Rohr Institute for permission to use its archive and for the photographs. I t was toward the end of his Nohel lecture to the Swedish Academy of Sciences on Deremher 11,1922. that Niels Rohr made the surprising announrement that George de Hevesy and Dirk Coster in Cooenhaeen had iust succeeded in discovering element numher 72. The news of this discovery was soon to soread further afield. touching off one o f t h e loneest and mosi hitter rnntroversies in moiern science involving scientists in Denmark, Franre, and Endand. Celtium Rohr's announcement hecame the main suhject of conversation among the prim winners and Scandinavian scientists gathered in Storkholm for they were aware that the French chemist Georzes IJrhain had I m p a m laid claim to the discovery of this rlement. and that onlvsix months hefore he had published further evidcnre with Alrmndrr Dauvillier for the existence of the element he had named. celtium, in honor of France. Urhain's hunt fnr this element had hegun in 1907 as a consequence of his u~orkon thr disrnvcry of the elements ytterhium and lutecilm. Roth he and the Austrian chemist, Auer von Welsl)ach had indrpendently and correctly suspected that a snhstnnce identified hy Marignac as one of the rare earth elemrnts 2.5 w a r s hefore was in fact a mixture of two different elenirnts Both c h ~ m i s t gave s different names to the new elements (which itsell' I c d I r r a nrioritv d i s ~ u t e ) .
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of carrying out the lahorious and repeated process of fractionation, which saw 200 kg of minerals reduced to only half a gram, von Welshach gave up the search. ITsing the same methods, Ilrhain could annottnce, however, in 1911 that he had succeeded i n mrasuring s i z l c ~ nlines in the optical spectrum not sren in his nriginal samplcand urndurled that he had discovered the nrxt rare earth element celtium. The historv nI' the rare earth elemrnts is a tangled weh marked hy far mnre premat.ure announcements of the discovery of a new elemrnt than the artual numher existing in nature. It was not until the nmcrpt nf atomic numher put forward in 191.1. and then Mnsrlev's devalopment of the techniques nf X-ray spectroscopy, that there existed a clear and reliahle way lo identify not only the rare earths hnt also all elements. Ilrhain was quirk to realize the power of X-ray soectroscoov to deride in a few hortrs ouestions that often took G a r s to dkiide using chemical methnds. In 1914 he went to England where Moseley carried out an invest,igat.ion to determine if the celtium sample did contain the element with atomic number 72. There was however a negative result, a fact 456 1 Journal of Chemical Education
Figure 1. Dirk Coster (1480 19501 st,own in 1912 during his stay at the Institute of Ttleoreticai Physics 8n Copenhagen. H#scareer began in Leiden and Deln. and in 1920 he took a position in Lund. From 1924 until hisdeath. he was Professor of physics at the University of Groningen.
that Urhain attrihi~tedto the lark of sensitivity in the newly developed equipmrnt to detect. the small traces of this element in his sample. Urhain kept faith in celtium and with the post-war imprnvements in X-rav spectroscopy he decided to undertake his own examination in Paris with Dauvillier. In May 1922, they announced in the French journal Comptes Rendus the detection of two X-ray lines, which were in fairly good agreement with the values predicted for element 72 hy the so-called Moseley diagram, and concluded that "le dernier mot" had heen said on the existence of celtium (.5). The Discovery of Hafnium On his return fiom a series of lectures in Giittingen in June 1922, Rohr came across a summarv of these findings on celtium in a note communicated to Nature hy n o less an authority than Rutherford. At first Rohr accepted these results although they contradicted a point made in one of his Gottingen lectures that element 72 should not. he a rare earth hut .%ho~~ld hnvr chwmn.:,l propprtirq sinb~lnrto t h t w of zirnmium. k ~ h had r heen lead to thl* wncl~~sion I,? hi* I heory c,f ;~tomir strwttln,. whlrh r n l ~ ~ i r ethat d thc. ftlliny o t t h r rlectron .hell ncrllss the roll, ol'rarr carlhc in the periodic f a l h ta, cnmrtlrtld a t element 71, so that. just as in zirconium, there are four va-
lence electrons in element 72, rather than the three which characterize the rare earths, and make their chemical properties so similar. The belief that element 72 would he a homoloeue to zirconium had been indicated alreadv in the -~~ original periodic table of Mendeleev and also in a late; version nut forward hv the Danish chemist Julius Thomsen and had been furthersupported by the more recent work of the chemists Charles Bury and Harold King. I n order to clarify the claim that element 72 was a rare earth, which would according to Bohr. "place extraordinarilv meat, if not insurmountable, dilficultws in the way uirheiheors" ( h ) ,he de. cided 10 rnnsult with C + r w r tollou,ed theie e\,ent* i n two artirles, hut ~ atr~ve"the wrren little inme cautious in riving . .credit r t Scott Copenhagen scientists, who are among the foremost authorities on the X-ray method of identifying and placing the elements" ( I 1 ). In order to settle the claim for the new element Scott prnposed to name oceanium, in honor of the region where it originated, he sent a sample of his oxide to Copenhagen in February, noting that "the whole scientific world will wait your examination of it with breathless interest" (12). Following their examination of the X-rav and optical spectra of the cinnamon-colored oxide, ~ e v e sand i Goiter could only report to Scott a negative result, hut in the meantime thev offered tcg cmtillue the analy& r t , dvtrrmine its irur mnposition on theotfchnn,e that i! tnlrnt cw~tuinone uithestill unknown elements. These further tests, and also an analysis carried out a t the General Electric Research Laboratories a t Wembley, showed Scott's sample to consist of oxides of iron, aluminum, and titanium, hut no trace of a new element. In May 1923, Scott withdrew his claim.
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The Elements 71 and 72 The paper by Hevesy and Coster in Nature did not gounnnticed by Urhain and Dauvillier in Paris. In a strongly worded reply in the same journal a month later, they agreed that the finding of element 72 in zirconium ores was a "very important iesult," hut argued that Hevesy and Coster had no right to claim priority and that the original name of celtium should stand ( I . ? ) . They also reported a remeasnrement of their two X-ray lines giving values in much closer agreement with those measured fur hafnium and those predicted by the Moseley diagram. Hevesy and Coster countered by pointing out, firstly, that while these two lines were expected to be fairly weak, Urbain and Dauvillier had not detected any of the strongest lines visible in the hafnium X-ray spectrum, and secondly, Volume 56. Number 7, July 1979 1 457
that in the purification of his 1911sample, Urbain had taken special precautions to wash out all zirconium compounds, which presumably would have also taken out all of the new element 72. Their position was strengthened by a measurement the Danish physicists H. M. Hansen and S. Werner made of the optical spectrum, which had no similarity with the optical lines Urhain had measured for celtium in 1911. Further work using rare earth samples donated to Bohr's institute by Auer von Welsbach showed that Urhain's optical lines in fact belonged to element 71, the element Urbain had discovered and named lutecium in 1907. This implied however that Urbain's original lutecium had contained only a trace of element 71. and that this element was not ourified until 1911 when ~ r h a i nbelieving , it to he element 72, had labelled it celtium. In contrast. von Welshach's samole of element 71 proved to he quite pure prompting Coster td write to the aging Austrian nobleman to conaratulate him on being the true discoverer of element 71.1n-1924 the German committee on Atomic Weights, besides adopting the name hafnium for element 72, also recognized von Welsbach's priority to element 71 hychanging the name lutecium to the one he had proposed in 1907. ca&&eium. These develonments were not onlv an attack on ~ r h a i n ' sscientific credihhity, hut also a challenge to his nosition as President of the International Committee c the very organization responsible for the on ~ t d m i Weights, adontion and nomenclature of new elements. In resoonse to thih~riticiim,L'rhain wasrrndy to;ttlmit thnt his 1911 claim had been unlustified. and furthermore that celtium ;houltl indeed he classified as a homologue of zirconium and not as a rare earth element, but he and Dauvillier tenaciously maintained that, however weak and inaccurately their two X-ray lines had been measured in 1922, they still deserved credit for the discovery of this element. ~~
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The Celtium-Hafnium Controversy During the battle waged throughout 1923 in the pages of the English journals Nature, Chemical News, and Chemistry and Industry. the central issue was whether one could believe that ~ r b a i n ' sand Dauvillier's rather meagre evidence constituted enough proof to support their claim. The issue could not he decided b v strictlv scientific arguments, it was more a matter of faith whether one accepted tlheir result. In such a situation, where there was no firm obiective basis for deciding the anof the swer, the personal ambitions and national scientists involved came to play a more important role. The attack on celtium became an attack on French science. According to Hevesy, the editor of Chemical News accepted one of Hevesy's papers on hafnium with the proviso that "we adhere to the original word celtium given to it by Urhain as a rnnresentntive of the ereat French nation which was loval to ~. , 11sthrwghuur the war. We do not accept the name which w;ts given I < ,it hv the Danes whoonlv ~ c ~ k e t the e d svoil after the war" (14).while the French and the majority or the English chemists strongly supported celtium, the Copenhagen cltitm w s hncktd up by most ol'rht, irtcntists in Crrmnny. Holland, and Sc;mdinavia.'Chis polarization into twt, wmps reflectetl 10 ;i large degre~:11 d ~ e p e underlying r division in the scitmre of postwar Europe. 'l'hr hittrrnrss and prejudices inherited frmn the war werestill sostrung in 19;'s that (;t!rmany and the othrr ('entral I'oaer.; w r r excluded i n m participating in nearlv everv maior " international scientific hodv. and most of the international conferences. Some of the more extreme activists of this boycott of "Teuton" science had even wanted to exclude the neutral Scandinavian countries and Holland from those international bodies because thev had not actively supported the Allied war cause. In Copenhagen there was a general reluctance to become involved in a controversy where there was little belief in the reality of the argument on the other side. It was a dispute that, as Rutherford summed it up in his usual colorful language, was as meaningful as "flogging a dead horse" (15).In between the nutnwuus papers trying to establish the correct priurity ftgr the new elenlent, Ilewiy had made g w d pn,grt.ssdcternmining ~
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458 / Journal of Chemical Education
Figure 3. "Hafnium-element no. 72-discovered in 1922 in Copenhagen." A sample of the first preparation of the new element. Hafnium is today widely used ~~~-in the chemical and metalluraical industries. In 1923 the first oatents were taken out by the Dutch company Philips for the use of hafnium in electric light filamentsand were swn followed by further patents fa radiovalves and fireproof enamels. ~
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the properties of hafnium. With Coster, he completed measuring the X-ray spectrum and determined the hafnium content of dozens of zirconium-hearing minerals. and. in collaboration with V. Thal-Jantzen, he aiso succeeded inohtaining by a complicated process of fractional crystallization a few grams of fairly pure hafnium and the first pure sample of zirconium. enabling the atomic weights . of both to he measured accurately. Even though the efforts to have celtium recognized as the proper nmne fur ~ l ~ m I ? n72l continued thro~ighout the twcmties, outside of Fr:~ncethe acceprancr of hafniutn hy the scientiiir cmun~initywas prnrticallv unilnitnous by 11325. \Vhrn comthenting on a long paper by Hcvesy on the p r t p erties dhainium. tht. editor 11I'('hvmhlr~nnd lndrr~tr\took thi, upportunit). to reverie thclr pre\.iuusly stnmg pnr-cehiuln nosition: "liin this iuurnal we have hern citutioua in admittina the due claims of ~ e s s r sCoster . and Hevesy, we can from t h i historv of chemistrv. .oroduce some iustification for caution; Ht WS$F memoir shows us that to mainrain now our attitude of twc, years ago would nut hc caution but swnething el%e" I I ( i J .Kven though tht.controversy overrlrment 7 9 e\.entually died awav, thr ~iftermatheml~itteretlthe persunal relstions between t h e men on the opposing sides for many years. In France, the celtium claim never,lost its credibility in some quarters. In 1962 one authoritative French encyclopedia printed: "Hafnium is the chemical element with atomic number 72 and atomic weinht Hf or Ct = 178.6. I t was discovered hy G. Urbain and isolated hy Hevesy" (17). Althounh the discovervof element 72 led to a long and bitter c~mtroversy,there WHS 110 douhtin: the immediate l l t i l i r y of the new rlcment. 'l'he rrlatiw abundanre ol'hafnium in nature soon enabled Hevesy and Coster and others to pioneer a variety of technical applications. Their work on the discovery and study of the new element thus covered a wide range of science, extending from fundamental atomic theory to the field of industrial technology. Literature Cited Heilbron. J. L., "H.G. J. Moreley The Life and Letters of an English Phpieint 1887-1915," Berkeley. 1974. (2) Heimann. P. M..Ann. d 9 c r e n c a . 23.249(1967). (1)
(31 Weeks. M. A , "Dlseoveryaf the Elementa,"7th ed.,Easton, Pa., 1968. (4) Kragh. H.,"Nieis Bohr'sSeeond AtomicTheory." Hiat. Stud. Phys. Seiences,vol. 10, Editor: R. McCorrnrnech,Lokpublished. ( 5 ) Dauvillier, A., Comp. Rend., 174, 1317 (1922): Urbain, G.. Comp. Rend., 174, 1349 (1922). (61 Bohr.N.,Nnfure. 112.29(1923). (7) Cater, D., and von Hwesy, G., Nature, I l l , 79 (1923). 18) Letter of 20 Februsrv 1923:Edam of Row Motariais Review to N. Bohr
(10) (11) (12) (13) (14)
Letter of 4 February 1923: E. Rutherford to N. Bohr. The Obsarucr, January 28,1923. Letter o f 5 February 1923: A. Scott to D. Cater and G.von Hevesy.
Urbsin.G.sndDauvillier,A.,NaIure,111,218 (1923). Hevesy, G. von."Aduenturos in Rsdioisotope Rwareh." PergamanPr-,Oxford, 1962, "01. 1, p. 21. (15) Latter of26 March 1923. E. Rutherford to N. Bohr. (161 Editorial, C h e m and lnd., 14.6M (19251.
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