CONTEMPORARY ADVANCES IN PHYSICS 289 



atom as a whole is neutral ; to the atom-nuclei of the Zth element of 

 the periodic table it therefore assigns the positive charge Ze. 



In so far as the circling or "orbital" electrons are concerned, the 

 details of this atom-model have suffered change after change in the 

 lapse of thirty years. Classical mechanics has given way to one form 

 after another of "quantum" mechanics; the electron-orbits at times 

 have been defined with the utmost exactitude, at other times they 

 have been merged into wide and hazily-bounded zones; the electrons 

 themselves have appeared sometimes as simple corpuscles, sometimes 

 as corpuscles with a magnet superadded, sometimes as particles 

 implicated with a wave-motion and sometimes as a continuous haze 

 of fluid charge. All the while, however, some of the features of the 

 model have remained undisturbed. Among these are the total number 

 of the electrons chosen equal to Z, and the conception of the nucleus 

 seated at the heart of the electronic system with the positive charge 

 Ze and most of the mass of the atom concentrated upon itself. To the 

 problems of this nucleus we now address ourselves. 



First a few words about its size, which incidentally will recall the 

 best of the evidence for its existence. The nuclear atom-model was 

 transformed from a pretty speculation into almost a reality, w^hen in 

 1913 Rutherford, Geiger and Marsden observed the deviations of a 

 shower of alpha-particles projected against a sheet of gold foil.^ 

 Alpha-particles are atom-nuclei of the second element of the periodic 

 table, helium (Z = 2) ; gold is the seventy-ninth element (Z = 79). 

 The observed law of the deviations — that is to say, the distribution- 

 in-angle of the deflected alpha-particles— is superbly well accounted 

 for by assuming that wdthin every atom of gold there is a center of 

 force, the origin of just such an inverse-square central field as would 

 surround a charge + 19e; and that the alpha-particles are themselves 

 point-charges of amount + 2e, which are deflected by the forces which 

 they suffer in passing through these fields. The concordance between 

 the observed distribution-in-angle, and that which was deduced from 

 these assumptions, extends to angles of deflection as great as 150°. 

 Now under these assumptions, a particle which has had its path bent 

 by as much as 150° has passed within 3.1 -lO-^^ cm. of the center of 

 the central field. Inward as far as this, then, comes the inverse-square 

 field; and whatever meaning we may later attach to such a vague 

 expression as "size of the nucleus" — for size is an indefinite concept, 

 in regard to anything which is neither tangible nor visible — the radms 

 of the nucleus of the gold atom must assuredly be put at a value 



iSee my "Introduction to Contemporary Physics," pp. 72-92; or the second 

 article of this series {Bell Sys. Tech. Jour., January, 1924). 



