Electronic configuration of lanthanum

Electrons in each shell

Lanthanum has 57 electrons arranged as 2, 8, 18, 18, 9, 2 across 6 shells, which is why it sits in period 6.

How to write it

Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For lanthanum that gives [Xe] 5d¹ 6s².

Exception to the Aufbau rule. Filling orbitals in order would give 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹, but the real configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 5d¹ 6s² because the energies of the outer orbitals are very close, so the real ground state differs from the simple filling order.

Lanthanum at a glance

Other elements in group 3