34 SECTIONAL ADDRESSES. 



details from that first given by Bohr. In the new scheme the inner 

 sub-levels are completed at an earlier stage, and there is a greater concen- 

 tration of electrons in the outer sub-levels of each group. For the present 

 the nature of the modification will be sufficiently indicated by comparing 

 the electron distribution among the sub-levels in helium, neon, argon and 

 krypton, according to the old and new arrangements. 



With the_^' values assigned above, the total number of electrons required 

 to fill each n^. group is double the sum of the inner quantum numbers for 

 the group. From the relation of the j's and k's, however, the number may 

 also be considered as being equal to 2{2k — 1). 



In this connection it should be observed that Sommerfeld and others 

 now assign half-integral values to j in term systems of even multiplicity, 

 so that, although integral values as above are most frequently adopted 

 for convenience of writing and printing, it is to be understood that they 

 are to be reduced by ^ for certain purposes. With this modification 

 the number of electrons in each sub-level is equal to 2j-\-l, which Sommer- 

 feld calls the ' quantum weight,' representing the number of possible 

 orientations of the angular momentum, j, in a magnetic field. 



The new scheme of electron distribution was shown by Stoner to be 

 supported by a consideration of the intensities of X-ray lines, the absorption 

 of X-rays, chemical and magnetic properties, and optical spectra. It 

 retains all the essential features of Bohr's picture of the building up of 

 atoms, and is equally in accord with chemical considerations, as is especially 

 shown by the work of Main Smith. 



The electronic arrangements of all the elements from 1 to 92, in their 

 normal states, may now be specified with considerable confidence. They 

 are indicated in Table IX, which, with slight modifications, has been 

 taken from a paper by Foote.^' The spectroscopic ground term associated 

 with the normal state of each neutral atom is shown in the fourth column, 

 the values directly determined from spectra being marked with an asterisk, 

 while the remainder are the ground terms predicted by the new theory of 

 complex spectra. 



" Amer. Inst, Mining and Metallurgy, Sc. Paper No. 1547D (1926). 



