Aufbau Principle

A principle that governs the order in which the atomic orbitals are filled in elements of successive proton number; i.e. a statement of the order of increasing energy. The order is as follows:

1s², 2s², 2p⁶, 3s², 3p6, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁶, 6s², 4f¹⁴, 5d¹⁰, 6p⁶, 7s², 5f¹⁴, 6d¹⁰
(the superscript indicates the maximum number of electrons for each level).

1. Hund’s rule of maximum multiplicity applies; i.e. degenerate orbitals are occupied singly before spin pairing occurs.
2. the unexpected position of the d-levels, which give rise to the first, second, and third transition series.
3. the unusual position of the f-levels, giving rise to the lanthanoids and actinoids. ‘Aufbau’ is a German word meaning ‘building up’.