= Noninteracting Coulomb-shell filling
{title2=$E_n=-C/n^2,\quad g_n=2n^2$}
In a fixed attractive <Coulomb potential>, neglecting electron-electron interactions leaves hydrogenic one-particle levels with spin-inclusive degeneracy $2n^2$. The <Pauli exclusion principle> fills these levels in increasing <energy>. A partially filled degenerate shell gives many states at the same total <energy>; the first higher <energy> requires promotion to a different principal shell. This model does not describe the screening and configuration-dependent energies of a real many-electron atom.
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