ATOMIC STRUCTURE 
v. 4] 
79 
Pauli,* * * § Heisenberg,f and Hund.J Through their work " the 
foundations have been laid for the interpretation of spectra in 
terms of atomic states, and it appears that we can now predict, 
almost with certainty, the structure and chief characteristics of 
any optical spectrum of the atom of any element when the extra- 
nuclear electronic configuration that gives rise to it is known. 
Conversely, if the characteristics of any optical spectrum of an 
atom be known, it is possible likewise definitely to describe the 
extra-nuclear electronic states of the atom involved in the 
production of such spectrum." § 
The new theory arises out of the work of Russell and Saunders || 
on the spectra of the alkaline earths. Here we have to do with 
two outer electrons and it appears that the so-called “ anomalous 
terms,” which had been discussed independently by Wentzel,^ 
require for their theoretical explanation the resultant momentum 
of these electrons to be quantized. The corresponding quantum 
number, called the “ group quantum number,” is denoted by l 
in Sommerfeld’s notation (instead of k used by Russell and 
Saunders). 
Bohr has suggested that in elements like the alkaline earths, 
the two valency electrons may both be displaced to outer orbits. 
From a study of certain anomalous terms in the spectrum of 
calcium, Russell and Saunders concluded that it is possible for 
an atom to remain neutral while absorbing more energy than is 
required for the removal of the series electron. This may be 
explained, in accordance with a suggestion made earlier by 
Bohr, by supposing that “ the energy must be divided between 
two (or more) electrons, each of which is displaced to a higher 
energy level, without the removal of either of them.” “ The 
detailed numerical evidence led inevitably to the conclusion 
that both valency electrons might jump at the same time from 
outer to inner orbits, and that the net loss of energy would be 
then radiated as a single quantum, i.e. as monochromatic 
emission.” 
There is here an extension of the earlier conception of Bohr 
as to the emission of monochromatic radiation constituting a 
* Pauli, Zeits. f. Physik, voi. 31, p. 765, 1925. 
t Heisenberg, Zeits. f. Physik, voi. 32, p. 841, 1925. 
X Hund, Zeits. f. Physik, voi. 33, p. 345 ; voi. 34, p. 296, 1925. 
§ McLennan, McLay, and Grayson Smith, Proc. Roy. Soc., voi. 112, 
p. 76, 1926. A summary is given of the Heisenberg-Hund theory, and 
its application illustrated by examples. An outline of the scheme is also 
given in a paper by Fowler and Hartree, Proc. Roy. Soc., voi. in, p. 83, 
1926. 
II Russell and Saunders, Phys. Rev., voi. 22, p. 201, 1923 ; Astrophys. 
Jour., voi. 61, p. 38, 1925. 
H Wentzel, Phys. Zeits., voi. 24, p. 106, 1923 ; voi. 25, p. 182, 1924«