, and
require no comment. The symmetry number
of the neutral atom is in effect the number of spectroscopic valency electrons given in Bohr’s table,[377] for the atoms for which it is known. In all the applications made by Fowler and Milne the quantity was equated to 1 or 2, and it is very probable that this number is not in any case exceeded. For carbon, where the chemical valency is equal to 4, the value of
is still 2, as has been shown by Fowler’s analysis of the spectrum of ionized carbon.[378] The value of
is not known for atoms in the long periods, but in the present work it is assumed to be 1 and 2 for atoms with arc spectra which show even and odd multiplicities, respectively. The uncertainty in the value of