Ionic solids: radius ratio rule
Solid State · Chemistry
Study notes
r(Na⁺) = 102 pm, r(Cl⁻) = 181 pm. Predict NaCl's coordination. Step 1: Ratio = 102/181 = 0.564. Step 2: 0.564 falls in 0.414–0.732 → octahedral, CN 6. Step 3: Real NaCl: each Na⁺ touched by 6 Cl⁻ ✓. For CsCl: r(Cs⁺)/r(Cl⁻) = 167/181 = 0.92 → cubic CN 8 ✓. Size ratio predicts structure — geometry from arithmetic.