Aufbau principle
= Aufbau principle
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An approximate scheme for constructing an atomic <electron configuration> by filling lower-energy <atomic orbitals> before higher-energy ones, subject to the <Pauli exclusion principle>. Orbital ordering can change on ionization and has exceptions in interacting <chemical atoms>. Selecting the free-ion ground term within a specified open-shell configuration additionally requires <Hund's rules> or a more detailed interaction calculation.