Hund's Rule

Hund’s rule is an empirical rule that determines the ground state electron configuration of atoms and molecules, specifying how electrons fill degenerate orbitals.

Three Rules

1. Maximum multiplicity: Electrons fill orbitals to maximize total spin
2. Maximum orbital angular momentum: For a given multiplicity, choose the state with maximum L
3. Minimum total angular momentum: For atoms with less than half-filled shells, J = |L - S|; for more than half-filled, J = L + S

First Rule (Most Common)

1. When filling degenerate orbitals, electrons occupy separate orbitals with parallel spins
2. Electrons pair up only after all degenerate orbitals contain one electron
3. This minimizes electron-electron repulsion by maximizing spatial separation
4. Formulated by Friedrich Hund in 1925

Physical Basis

1. Electrons with parallel spins avoid each other due to exchange interaction
2. Reduces Coulomb repulsion between electrons in degenerate orbitals
3. Results in lower energy configuration compared to paired electrons

Examples

1. Carbon: 1s² 2s² 2p² - two unpaired electrons in different p orbitals
2. Nitrogen: 1s² 2s² 2p³ - three unpaired electrons, one in each p orbital
3. Oxygen: 1s² 2s² 2p⁴ - two unpaired electrons (one orbital has paired electrons)