170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



of the radius of the complete atom surrounded bj^ its electrons, and 

 certainly less than 4 X 10"^^ cms. All the atoms were found to show 

 this nuclear structure, and an approximate estimate was made of the 

 nuclear charge of different atoms. This type of nuclear atom, based 

 on direct experimental evidence, possesses some xevy simple proper- 

 ties. It is obvious that the number of units of resultant positive 

 charge in the nucleus fixes the number of the outer planetary elec- 

 trons in the neutral atom. In addition, since these outer electrons 

 are in some way held in equilibrium by the attractive forces from 

 the nucleus, and, since we are confident from general physical and 

 chemical evidence that all atoms of any one element are identical 

 in their external structure, it is clear that their arrangement and 

 motion must be governed entirely by the magnitude of the nuclear 

 charge. Since the ordinary chemical and physical properties are 

 to be ascribed mainly to the configuration and motion of the outer 

 electrons, it follows that the properties of an atom are defined by a 

 whole number representing its nuclear charge. It thus becomes of 

 great importance to determine the value of this nuclear charge for 

 the atoms of all the elements. 



Data obtained from the scattering of a particles, and also from 

 the scattering of X rays by light elements, indicated that the nuclear 

 charge of an element was numerically equal to about half the atomic 

 weight in terms of hj^drogen. It was fairly clear from general evi- 

 dence that the hydrogen nucleus had a charge one, and the helium 

 nucleus (the a particle) a charge two. At this stage another dis- 

 covery of great importance provided a powerful method of attack 

 on this problem. The investigation by Laue on the diffraction of 

 X rays by crystals had shown definitely that X rays were electro- 

 magnetic waves of much shorter wave-length than light, and the 

 experiments of Sir William Bragg and W. L. Bragg had provided 

 simple methods for studying the spectra of a beam of X rays. It 

 was found that the spectrum in general shows a continuous back- 

 ground on Avhich is superimposed a spectrum of bright lines. At 

 this stage H. G. J. Moseley began a research with the intention of 

 deciding whether the properties of an element depended on its 

 nuclear charge rather than on its atomic weight as ordinarily sup- 

 posed. For this purpose the X-ray spectra emitted by a number 

 of elements were examined and found to be all similar in type. 

 The frequencj'^ of a given line was found to vary very nearly as 

 the square of a whole number which varied by unity in passing 

 from one element to the next. Moseley identified this whole num- 

 ber with the atomic or ordinal number of the elements when ar- 

 ranged in increasing order of atomic weight, allowance being made 

 for the known anomalies in the periodic table and for certain gaps 

 corresponding to possible but missing elements. He concluded that 



