Frederick Soddy Nobel Lecture

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F

REDERICK 

S

ODDY

The origins of the conceptions of isotopes

Nobel Lecture, December 12, 1922

Introduction

The work of my students and myself, for which you have so signally hon-

oured me by the award of the Nobel Prize for Chemistry for 1921, is but a

small part of much pioneering work, in many lands and stretching over a

period of nearly twenty years, into the chemistry of radio-elements and the

existence and nature of isotopes. I think I may best fulfil my duty as Nobel

laureate if I try to disentangle the real origins of the conception of isotopes,

in so far at least as I have been connected with it. What is so very simple and

clear to us today at first had a very different and puzzling character, so that

this task of tracing the birth of an idea from its earliest indications is really

both difficult and complicated. I cannot hope to achieve historical complete-

ness or finality. Rather I shall attempt to give the results of my own efforts

to trace to its beginning the idea which today is universally admitted to be of

fundamental interest and significance.

The interpretation of radioactivity which was published in 1903 by Sir Er-

nest Rutherford and myself ascribed the phenomena to the spontaneous dis-

integration of the atoms of the radio-element, whereby a part of the original

atom was violently ejected as a radiant particle, and the remainder formed a

totally new kind of atom with a distinct chemical and physical character.

These disintegrations proceed successively a large number of times, so

that there results a series of more or less unstable new elements, between the

original parent element and the ultimate unknown final product. This was a

theory sufficiently challenging to the accepted doctrines of chemistry. The

further detailed study of the chemical character of these successive unstable

elements, produced in radioactive changes, introduced an idea which was

even more subversive of the fundamental doctrines of chemistry. That idea

was that the chemical elements are not really homogeneous, but merely

chemically homogeneous. In some cases they are mixtures of different con-

stituents which. are only identical in their chemical character. Put colloqui-

ally, their atoms have identical outsides but different insides. Chemical

analysis classifies according to the external systems of electrons which sur-

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round a small massive internal nucleus, whereas radioactive changes, which

are of the character of veritable transmutations, concern the internal consti-

tution of this inner nucleus. They showed that the same exterior may con-

ceal very different interiors in the atomic structure. These elements which

are identical in their whole chemical character and are not separable by any

method of chemical analysis are now called isotopes.

I

 

may begin with a brief statement of the earlier researches into the nature

of radioactivity. The power of spontaneously emitting rays of a new kind

was discovered in 1896 by Henri Becquerel for the compounds of the ele-

ment uranium. The physicists sorted the rays emitted into three types, the

α−, β− 

and  

γ 

- 

rays, and their real nature was quickly elucidated. The 

β 

-rays

were shown to be due to the expulsion of negative electrons traveling at a far

higher speed than any that can be artificially produced in the vacuum tube

as cathode-rays. The 

γ

-rays were correctly regarded as X-rays, but of a greater

penetrating power. In due course the X-rays were shown to be waves of light

of extraordinary short wavelength and high frequency, whilst the 

γ

-rays are

also of the nature of light, but of even shorter wavelength and higher fre-

quency. The 

α 

-rays were first proved to be due to radiant atoms of matter

carrying a positive charge and were identified, after many years of contin-

uous work by Sir Ernest Rutherford, with helium atoms carrying two

atomic charges of positive electricity. Sir William Ramsay and I had shown

in 1903 that helium was being continuously generated from radium in a

spectroscopically detectable quantity, and subsequent work showed that

helium is generated, as 

α

-particles, in all the radioactive changes where 

α−

rays are expelled.

The nature of the ionization produced in gases by all these new types of

radiation was quickly elucidated, and as a consequence highly sensitive and

accurate methods of measurement were evolved, which, more than any

other single factor, contributed to the rapid development of the subject.

On the chemical side, the work of M. and Mme. Curie had shown radio-

activity to be a specific atomic property, definitely restricted to the last two

of the then known elements, uranium and thorium, though the elements po-

tassium and rubidium emit 

β

-rays, exhibiting in this respect evidence of true

radioactivity. Using the new property much as the pioneers with the spec-

troscope used the spectra in the discovery of new elements, they discovered

the powerfully radioactive new elements, polonium and radium, and, M.

Debierne, actinium, in the uranium minerals. Of the many similar new

radio-elements now known radium is still the only one the compounds of

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