Hund's Rule of Maximum Multiplicity
The rule that electrons occupy degenerate orbitals singly with parallel spins before any orbital is doubly occupied.
Formula / equation
maximise total spin S
Unit
Dimensionless
In depth
Singly occupying separate orbitals minimises electron–electron repulsion, and parallel spins gain additional exchange stabilisation energy. The rule explains the paramagnetism of nitrogen and oxygen atoms and the extra stability of half-filled d⁵ and f⁷ configurations.
Examples in the real world
Nitrogen's three 2p electrons occupy 2pₓ, 2p_y and 2p_z singly with parallel spins, giving three unpaired electrons.