Frederick Soddy Nobel Lecture
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O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 387
From now on, the history of this development is extraordinarily involved, and brief reference to the actual dates of the various publications is essential before passing on to the nature of the advance itself. G. von Hevesy and A. S. Russell were both working in Rutherford’s laboratory in Manchester, and published within a fortnight papers containing a form, only in part cor- rect, of the great generalization, which was to clear up, as in a sudden flash, the whole subject. A fortnight after Russell, K. Fajans working in Karlsruhe, published this generalization complete and accurate in every detail, A fort- night after Fajans, in a paper amplifying and amending Russell’s scheme, I published exactly the same scheme as that of Fajans, whose paper only reached England after mine was written. His generalization was remarkable, in that at the dates on his two papers Dec. 31, 1912 and Jan. 15, 1913 only one of Fleck’s important results, that of thorium B, had been published. But to me in full knowledge of all Fleck’s experimental results, the complete generalization was obvious so soon as Russell’s paper directed attention to it. Till then I had scrupulously avoided even thinking about these results until they were completed, for fear of giving an unconscious bias to the work in progress in the laboratory. Hevesy 7
ments, concluded that the effect of the expulsion of the p-particle is to alter the valency by one in the opposite direction to the effect of the expulsion of the α
tion members, the direction in which the valency was altered was regarded as the opposite of that for the post-emanation members. He thus supposed that the valency, having been reduced to zero at the emanations by suc- cessive a-ray expulsions, then rises to two, making radium A divalent. This got the tetra-valent elements right, but others (RaCa 2 AcD, and ThD) were wrong. Others again could be wrong or right according as the valency is taken as the Group Number or 8 minus the Group Number. Russell* dealt with the position of the element in the Periodic Table, and not with its valency, and published the first scheme tracing the whole dis- integration series through the table. In the pre-emanation part he predicted the existence of uranium X, in the vacant place between thorium and ura- nium, and was in general correct, showing the effect of the expulsion of the β−
ray as causing a shift of one place in the direction opposite to the double shift caused by the expulsion of the a-ray. But he failed to envisage the Periodic Table as a continuous succession of places, and passed from the emanation or zero family to the preceding members in the table in the reverse order, 388 1921 F.SODDY placing radium A with mercury. Though he got the C-members correct to Fleck’s characterization, it was by arbitrarily making the effect of the β− ray in the same direction as that of the α -ray change*. In the correct generalization of Fajans 9 and myself 10 , each a-ray expelled causes a shift of two places in the direction of diminishing mass and diminish- ing valency, as I had pointed out two years before, and each β -ray expelled causes a shift of one place in the opposite direction, consistently throughout the whole series, including the very complicated branchings that occur at the C-members**. In the elucidation of these, Fajans had himself been the pio- neer the year previously. The scheme is shown in Fig. I
(from the British Association Reports, Birmingham Meeting, 1913) in substantially the form in which I first pub- lished it, except that the actinium series has been separated to make it clear- er. In its present form (1922) it is shown in Fig. 2 (from a recent paper on Isotopes, contributed to the Brussels Conference of the Institut Internatio- nale de Chimie Solvay, April 1922), which shows the actinium series linked up with the uranium series***. In the first (Fig. I ), the places in the Periodic Table are represented as differing by one in the number of electrons in the atom, a loss of one electron being regarded, purely conventionally, as equiv- alent to the gain of a single atomic charge of positive electricity. This was shown only in the diagram in the Jahrbuch der Radioaktivität, not in that * Used as we now are to the idea of Atomic Number, this error may now seem obvi- ous. Rather it is the great clarification of the nature of the Periodic System, first effected by this very generalization, which makes the error appear obvious. *** The atomic weights shown for the actinium series are purely provisional(see p. 395).
O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 389
Fig. I.
Radio-elements and Periodic Law. All elements in the same place in the Periodic Table are chemically non-separable and (probably) spectroscopically indistinguishable. in the Chemical News published simultaneously. At the time, in the ab- sence of any special theory of atomic structure, or of the nature of positive electricity, this was natural, but now it is more in accord with our theoretical knowledge to represent the abscissae as units of positive electricity (Fig. 2). Either represents the fundamental fact that in the expulsion of an α -particle twopositive charges are lost by the atom, and in the expulsion of a β -particle one negative. "Whenever, by the expulsion of α - or
β -rays, two or more elements come to occupy the same place in the Periodic Table, then, independently of all other consideration, such as the atomic weight, the disintegration series to which the element belongs, its radioactive character and the nature of the radioactive changes, in which it is produced, or by which it is transformed, these elements occupying the same place, are non-separable from one anoth- er, and are, so far as is known, identical in chemical character." (1913, p. 263). Of the radio-elements still uncharacterized, the generalization shows that
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