The Electronic Structure of Atoms with Two or More Valence Electrons

THE ELECTRONIC STRUCTURE OF ATOMS WITH TWO OR MORE VALENCE ELECTRONS 

Let us consider au atom with two s electrons, with different total quantum numbers for example, a beryllium atom with one valence electron in a 2s orbital and the other in a 3s orbital, in addition to the two electrons in the K shell. The orbital angular momenta of the two valence electrons are zero (h = 0, k = 0), and accordingly the resultant angular momentum is zero (L = 0). Each of the two electrons has spin quantum number (si = sz = ), and each spin angular mo- 

11 The BuHsell-Saunders coupling was discovered by H. N. Russell and F. A. Saunders, Astrophys. J. 61, 38 (1925). 

Experience shows that these two states differ greatly in energy. The interaction energy of the magnetic moment of the two electron spins is very small, and the observed energy difference is not due to a direct spin-spin magnetic interaction. It was shown by Heisenberg1 that 

—The interaction of the spin angular momentum vectors of two electrons to form a resultant total spin angular momentum vector, corresponding to the value of the total spin quantum number S = 0 or S = 1. 

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