 Noninteracting Coulomb-shell filling

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.