Electronic configuration of copper

Electrons in each shell

Copper has 29 electrons arranged as 2, 8, 18, 1 across 4 shells, which is why it sits in period 4.

How to write it

Electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… Each s orbital holds 2 electrons, p holds 6 and d holds 10. Placing Copper's 29 electrons in that order gives 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹, but copper is an exception.

Exception to the Aufbau rule. Filling orbitals in order would give 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹, but the real configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ because a completely filled 3d¹⁰ is extra stable, so one 4s electron moves into 3d.

Copper at a glance

Other elements in group 11