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Three rules govern electron filling: (1) Aufbau: fill lowest energy first. Order by (n+l) rule — lower (n+l) fills first; if tied, lower n first. Sequence: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d. (2) Pauli: max 2 electrons per orbital with opposite spins. (3) Hund: in degenerate orbitals, fill singly with parallel spins before pairing. Maximises exchange energy = n(n-1)/2 exchange pairs for n parallel-spin electrons. Extra stability: half-filled (d^5, p^3) and fully-filled (d^10, p^6) subshells. Key exceptions: Cr = [Ar]3d^5 4s^1, Cu = [Ar]3d^10 4s^1, Mo = [Kr]4d^5 5s^1, Ag = [Kr]4d^10 5s^1, Pd = [Kr]4d^10. Cation formation: 4s electrons removed BEFORE 3d (opposite of filling order). Fe^2+ = [Ar]3d^6, not [Ar]3d^4 4s^2. This happens because in cations, 3d is lower in energy than 4s due to changed shielding. Magnetic properties: unpaired electrons = paramagnetic. mu = sqrt(n(n+2)) BM.