$word_{count}$: 220

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$\frac{n-1}{2}$ 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]3$d^5$ 4$s^1$, Cu = [Ar]3$d^{10}$ 4$s^1$, Mo = [Kr]4$d^5$ 5$s^1$, Ag = [Kr]4$d^{10}$ 5$s^1$, Pd = [Kr]4$d^{10}$. Cation formation: 4s electrons removed BEFORE 3d (opposite of filling order). $Fe^{2+}$ = [Ar]3$d^6$, not [Ar]3$d^4$ 4$s^2$. 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.