THE NATURAL
AND ARTIFICIAL DISINTEGRATION
OF THE ELEMENTS
AN ADDRESS BY
Professor Sir ERNEST RUTHERFORD
Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S.
ON THE OCCASION OF THE CENTENARY CELEBRATION
OF THE FOUNDING OF
THE FRANKLIN INSTITUTE
AND THE INAUGURATION EXERCISES OF THE
BARTOL RESEARCH FOUNDATION
SEPTEMBER 17, 18, 19, 1924
THE FRANKLIN INSTITUTE
PHILADELPHIA
THE NATURALAND ARTIFICIAL DISINTEGRATIONOF THE ELEMENTS
By Professor Sir ERNEST RUTHERFORD, Kt., D. Sc.,
LL. D., Ph. D., D. Phys., F. R. S.
IT is not my intention in this paper to give a detailed account of thenatural disintegration of the radio elements or of the methods employedto effect the artificial disintegration of certain light elements. Ishall assume that you all have a general knowledge of the results ofthese investigations, but I shall confine myself to a consideration ofthe bearing of these results on our knowledge of the structure of thenuclei of atoms.
There is now a general agreement that the atoms of all elements have asimilar electrical structure, consisting of a central positivelycharged nucleus surrounded at a distance by the appropriate number ofelectrons. From a study of the scattering of α particles by theatoms of matter and from the classical researches of Moseley on X-rayspectra, we know that the resultant positive charge on the nucleus ofany atom, in terms of the fundamental unit of electronic charge, isgiven numerically by the atomic or ordinal number of the element, dueallowance being made for missing elements. We know that with fewexceptions all nuclear charges, from 1 for the lightest atom, hydrogen,to 92 for the heaviest element, uranium, are represented by elementsfound in the earth. The nuclear charge of an element controls the numberand distribution of the external electrons, so that the properties of anatom are defined by a whole number, representing its nuclear charge, andare only to a minor degree influenced by the mass or atomic weight ofthe atom.
This minute but massive nucleus is, in a sense, a world of its own whichis little, if at all, influenced by the ordinary physical and chemicalforces at our command. In many respects, the problem of nuclearstructure is much more difficult than the corresponding problem of thearrangement and motions of the planetary electrons, where we have awealth of available information, both physical and chemical, to test theadequacy of our theories. The facts known about the nucleus are few innumber and the methods of attack to throw light on its structure arelimited in scope.
It is convenient to distinguish between the properties assigned to thenucleus and the planetary electrons. The movements of the outerelectrons are responsible for the X-ray and optical spectra of theelements and their configuration for the ordinary physical and chemicalproperties of the element. On the other hand, the phenomena ofradioactivity and all properties that depend on the mass of the atom areto be definitely assigned to the nucleus. From a study of theradioactive transformations, we know that the nucleus of a heavy atomnot only contains positively charged bodies but also negative electrons,so that the nuclear charge is the excess of positive charge overnegative. In recent years, the general idea has arisen that there aretwo definite fundamental units that have to do with the building up of