Electron Configuration

Aufbau principle, orbital filling, and energy levels

Electrons in atoms occupy orbitals — regions of space described by quantum numbers (n, l, ml, ms). The Aufbau principle states that electrons fill from lowest to highest energy. Each orbital holds at most 2 electrons with opposite spins (Pauli exclusion principle). When filling degenerate orbitals (same energy), electrons spread out with parallel spins first (Hund's rule). The filling order follows the (n+l) rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... Notable exceptions: Cr is [Ar] 3d⁵4s¹ (half-filled d stability) and Cu is [Ar] 3d¹⁰4s¹ (fully-filled d stability). Electron configuration determines chemical properties and periodic trends.