Section III, 1921 [37] Trans. R.S.C. 



Ionization Potential and the Size of the Atom 



By Dr. A. S. Eve, F.R.S.C. 



(Read May Meeting, 1921) 



It is known that there is for different elements a relation between 

 the ionizing potential and atomic volume, the one increasing as the 

 other diminishes. Hughes, in his book on Photoelectricity (p. 51), 

 indicates that the work in removing an electron wholly from an atom 

 might be expected to vary inversely as the radius. In other words 

 the ionizing potential might be inversely proportional to the cube- 

 root of the atomic volume. 



Now W. L. Bragg, in the Philosophical Magazine (August, 1920), 

 has given the diameters of atoms in Angstrom units (10" cm.) on 

 the assumption of close packing in crystal structure. The diameter 

 which he determines is more strictly the distance from centre to 

 centre of contiguous atoms of the same kind. The dimensions which 

 he thus found are far smaller than those deduced from calculations 

 by kinetic theory. 



It appears desirable to make a comparison of the ionization 

 potentials, (1) with the diameters as given by W. L. Bragg, and 

 (2) with the cube-roots of the atomic volume. 



In the table following the name of the element, the ionization 

 potential and Bragg's diameter (xlO^) are set forth in the first three 

 columns. The product of the diameter and ionizing potential appear 

 in the fourth column. The cube-root of the atomic volume is stated 

 in the fifth column and its product with the ionization potential in 

 the sixth column. 



Group I 



