Frederick Soddy Nobel Lecture

    1 9 2 1   F . S O D D Y

Fig. 2. Radio-elements and Periodic Law. All elements in the same vertical column

are isotopes.

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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

391

the A- and C-members must be non-separable from polonium and radium

C

2

, thorium D and actinium D from thallium. As regards radium A, thod-

rium D and actinium D these predictions were at once confirmed by

Fleck

11

.

The rules demanded that uranium X must consist of two successive prod-

ucts uranium X, and uranium X

2

, the latter a new member in the vacant

place in the Periodic Table, between uranium and thorium, and therefore

analogous to tantalum. Fajans and Göhring proved this at once

12

. Uranium

X

2 

is very short-lived product responsible for the more penetrating of the

two types of 

β

-rays given by "uranium".

Since actinium is in the IIIrd family, its parent must be chemically iden-

tical with radium in the IInd family, if it is produced in a 

β−

ray change, and

in the Vth family, in the place occupied by uranium X

2

, if it is produced in

an 

α

-ray change.

Lastly, and most important of all, the generalization showed that the ulti-

mate products of all three series in all branches must be in the place occupied

by the element lead. The atomic weight of the lead from both branches of

the thorium series is 208, and that of the lead from uranium, in the main

branch, 206. Common lead with the atomic weights 207.2 might well be a

mixture of both.

In this same year, 1913, Sir Joseph Thomson and F. W. Aston announced

the discovery by the positive-ray method of a constituent of atmospheric

neon of atomic mass 22 in addition to the main constituent of atomic mass

20.

"The discovery is a most dramatic extenions of what had been found for

elements at one extreme of the Periodic Table to the case of an element at

the other extreme, and strengthens the view that the complexity of matter

in general is greater than the periodic law alone reveals." (1913, p. 266).

The theoretical interpretation of isotopes

In 1911 Rutherford put forward, in a somewhat tentative form, his now

well-known "nuclear theory" of atomic structure, to account for the scat-

tering of 

α− 

and 

β−

particles in their passage through matter. To account es-

pecially for the large angles through which occasional 

α−

particles are deflec-

ted it was necessary to suppose that these were the result of single encounters

between the 

α

-particle and an atom struck. For this to be possible, there

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392

    1 9 2 1   F . S O D D Y

must exist within the atom a much more powerful electric field than, for

example, in the Thomson atom. Rutherford’s supposed that, at the centre

of the atom, there existed a nucleus of very minute dimensions relatively to

the atomic volume, upon which was concentrated a large charge of one

sign, the rest of the atom being occupied by a number of single charges of

the opposite sign which neutralized the central or nuclear charge. At first

nothing was postulated in the theory as to the sign of this central charge,

whether positive or negative, or as to the constitution of the nucleus. But it

was natural to regard the central charge as positive, and the rest of the atom

to be occupied by a system of negative electrons. In the next two years,

much further experimental evidence on scattering accumulated, and the

theory began to assume a more definite form.

"Single scattering" on this theory should be proportional to the square of

the nuclear charge. The experimental results indicated that it was approx-

imately proportional to the square of the atomic weight of the scattering

atom, and that there must be one unit of nuclear charge for two units of

atomic weight, approximately. In 1911 Barkla

14 

arrived at the same conclu-

sion, as to the number of electrons in the atom from the scattering of X-rays.

In 1911, Van den Broek conceived the idea that "to each possible* intra-

atomic charge there corresponds a possible element". This necessitates that

successive elements in the Periodic Table should differ by one unit of nuclear

charge and by one electron in the outer shell, as was subsequently practically

directly established by the periodic law generalization already considered.

To account for the supposed experimental result that there was one unit

of charge for two units of atomic weight, Van den Broek at first attempted to

revive the old and discarded "cubic" Periodic System of Mendeleeff, because

it accommodated 120 elements. Since the last, uranium, has the atomic

weight 240 it satisfies this requirement of giving on the average two units of

mass per "place", i.e. per unit of charge. His attempts to fit the radioactive

elements into this scheme appeared purely fanciful. But he then found that

the actual experimental results for scattering were in entire accord with his

own idea, on the accepted Periodic Table, which accomodates some 90

elements. Thus uranium in the last place with an intra-atomic charge of

about 90, must have between 2 and 3 units of mass per unit of charge. So

that, if its nucleus be imagined to be composed of 60 

α

-particles with charge

120, there must be present is also 30 electrons to give the nuclear charge 90.

* "Possible" here meant "integral"

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