Pauli exclusion principle: no two electrons have the same set of all four quantum numbers. Consequence: maximum 2 electrons per orbital (with opposite spins). Hund's rule of maximum multiplicity: in degenerate orbitals (same subshell), electrons occupy each orbital singly (all parallel spins) before pairing begins. This minimises electron-electron repulsion and maximises exchange energy. Exchange energy: a stabilisation that occurs when electrons with parallel spins are in different degenerate orbitals. Number of exchange pairs = nC2 = n(n-1)/2, where n = number of parallel spin electrons. Half-filled (d^5, f^7) and fully-filled (d^10, f^14) subshells have maximum exchange energy, contributing to extra stability.