$word_{count}$: 220

Radius ratio (r+/r-) determines structure type. NaCl (rock salt): Cl- in FCC, Na+ in ALL octahedral voids. Z = 4, CN = 6:6. a = 2(r+ + r-). Examples: NaCl, KCl, MgO, CaO. CsCl: NOT BCC! Cl- at corners (SC lattice), Cs+ at body centre. Z = 1, CN = 8:8. a*sqrt(3) = 2(r+ + r-). CsCl doesn't have a true BCC lattice because corner and body-centre atoms are different species. ZnS (zinc blende): $S^{2-}$ in FCC, $Zn^{2+}$ in alternate tetrahedral voids (4 of 8). Z = 4, CN = 4:4. Examples: ZnS, CuCl, SiC, diamond. CaF2 (fluorite): $Ca^{2+}$ in FCC, F- in ALL tetrahedral voids (8). Z = 4, CN = 8(Ca):4(F). Antifluorite (Na2O): cations and anions swap — $O^{2-}$ FCC, Na+ in all tetrahedral voids, CN = 4(Na):8(O). The number of formula units per cell equals Z for AB compounds and Z for A2B/AB2 compounds. Understanding which voids are occupied and what fraction is filled is key to solving JEE problems on ionic solids.