Hund's Rule of Maximum Multiplicity
A rule stating that when filling degenerate atomic orbitals, electrons occupy separate orbitals singly with parallel spins before pairing up.
Governing formula
S_total = ∑ s_i (Maximized spin in degenerate orbitals)
SI unit
Dimensionless Rule
In depth
Maximizing total spin minimizes electron-electron electrostatic repulsion because electrons with parallel spins are kept further apart spatially due to the Pauli exchange interaction, lowering total atomic energy.
Examples in the real world
Nitrogen (Z=7) filling its three 2p orbitals with one electron each (2p_x¹ 2p_y¹ 2p_z¹) with parallel spins rather than pairing them.