Frederick Soddy Nobel Lecture

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elements into the Periodic classification. In 1911 in the first edition of my

Chemistry of the Radio-elements (pp. 29 and 30) I discussed the effect on the

chemical character of the loss of the 

α

-particle, which is a helium atom carry-

ing two atomic charges of positive electricity.

"The loss of a helium atom or 

α

-particle appears to cause the change of the

radio-element not into the next family but into the next but one... Thus we

have in each series a well-marked sequence from the tetravalent family

(radiothorium, ionium) into the divalent family (radium, etc.) and into the

non-valent family ( emanations). Again the product derived from polonium

is probably not bismuth but lead, in each case the step being from the family

of even valency into the next, the family of odd valency being missed. But

this is not all. The progress is certainly not so straightforward as this. In

several cases the matter appears to alternate in its passage, passing through the

same family not once but twice. The product from thorium (group IV) is

mesothorium (group II). The product of the latter is radiothorium (group

IV), which in turn produces thorium X (group II). Again radium D is chem-

ically non-separable from lead (group IV), its product is polonium (group

VI), while the product of the latter is almost certainly lead (group IV)."

This 

α

-ray generalization enabled me to see that uranium X must be inter-

mediate between the two uraniums (1912, p. 321) though his conclusion was

published by several others as well as by myself before the Report was issued.

But I was at fault in one point, in that I thought the "active deposit" prod-

ucts, such as radium A, B and C, were unstable transition links between the

beginning of the period and the end of the last, analogous to the VIIIth

group between gold and tungsten, and not represented in the table.

In 1912, the chemical character of the post-emanation, or active deposit,

products began to attract more attention. The ease with which these prod-

ucts are separated from all inactive matter from the volatile emanations,

and, after the discovery of recoil methods, partly from one another, without

chemical methods accounted probably for the fact that none of them were

known to resemble known elements. However, von Lerch from as far back

as 1905 had studied their electrochemical deposition. His rule that the suc-

cessive products are successively more electrochemically "noble", i.e. more

easily deposited on metals, and on the cathode during electrolysis, in acid

solution, though true for radium A, and for the B and C members, was

found to fail for thorium D, which is less easily deposited than thorium B.

Rutherford in his original discovery of the thorium active deposit, had

shown that it volatilized at a bright red heat. Makower in 1909 had found for

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Y

three constituents of the radium active deposit, the following volatilization

points, radium A 900

o

, radium B 600

o 

and radium C 1200

o 

C. In 1912

Schrader and, after him, A. S. Russell, observed that the volatility of the ac-

tive deposit is much affected by exposure to various gaseous reagents such

as chlorine, and by volatilizing in different atmospheres. Thus in hydrogen

radium C commences to volatilize at 360

o 

and, for the three products to-

gether, volatilization is complete at 650

o

. This clearly pointed to definite

chemical and physico-chemical properties even of these excessively ephem-

eral elements, which before had been thought to exist in too infinitesimal

quantities to show such definite reactions. In this year, von Hevesy studied

the electrochemistry of these products from the standpoint of modern

theory, and showed that they might be used to test the consequences of this

theory at degrees of concentration far below anything that can be attempted

by any other method. He also obtained the important result that in their

electrochemical behaviour the three B-members are identical among them-

selves, and also the three C-members.

In 1912, A.Fleck, commenced to publish the results of a long series of

systematic studies into the chemistry of such of the radio-elements, with

period over one minute, as remained uncharacterized, from the definite

point of view of finding which known element each most resembled in

chemical character, and then whether it was separable or not from that ele-

ment when mixed with it

4

. He confirmed the characterization of uranium X

as chemically non-separable from thorium, showed that radioactinium,

which Strömholm and Svedberg had found to be isomorphous with thorium,

was in fact non-separable from it, and, most important of all, proved that

thorium B is non-separable from lead. Hitherto from von Lerch’s electro-

chemical researches it had been supposed that these products were chemically

allied to the noble metals. In his next communication, Fleck showed that

mesothorium 2 was non-separable from actinium, that radium B and ac-

tinium B were extremely similar to lead and probably non-separable from

it, that the three C-members are closely allied and probably chemically

similar to bismuth, whilst radium E has properties in all respects identical with

those of bismuth

5

. These results, as concerns the B- and C-members and

radium E, were at variance with those found at the same time by von Heve-

sy

6 

 from the speed of diffusion, a method that previously had given a fase

result for thorium X. Hevesy classed the B-members as univalent and the C-

members and radium E as divalent, but later showed these conclusions were

in error.

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