Atomic spectroscopy configuration systematics

Both in atomic spectroscopy (describing gaseous atoms and ions sufficiently isolated from each other to be considered as free) and in M.O. theory, the approximation of configurations is introduced. In this approximation, each electron has an orbital y>, and each orbital may contain zero, one, or two electrons (with opposite spin-directions). However, in atoms and in molecules having a sufficiently high symmetry, systematically degenerate sets of orbitals may occur. In the case of such a partly filled set of orbitals, more than one energy level is often produced. The main reason for this is the interelectronic repulsion between the... 

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