NaCl = rock salt structure. Cl- forms FCC lattice. Na+ occupies ALL octahedral voids. Octahedral voids in FCC: 12 edge centres $\frac{x 1}{4}$ + 1 body centre = 4. So 4 Na+ and 4 Cl- per unit cell = 4 NaCl. CN: Na+ has 6 Cl- neighbours (octahedral), Cl- has 6 Na+ neighbours. Edge length: a = 2($r_{Na+}$ + $r_{Cl-}$). Examples: NaCl, KCl, MgO, CaO, LiF. Radius ratio for NaCl: 0.95/1.81 = 0.52 (within 0.414-0.732 range).