Voids: tetrahedral and octahedral
Solid State · Chemistry
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Study notes
In NaCl, Cl⁻ forms FCC. Where do Na⁺ ions sit and how many per cell? Step 1: FCC has 4 Cl⁻ per cell → 4 octahedral voids per cell. Step 2: Na⁺ (small, r⁺/r⁻ = 0.52) fits octahedral voids (needs 0.414–0.732). Step 3: Na⁺ occupies ALL octahedral voids: 4 Na⁺ per cell. Step 4: Formula: 4 Na⁺ : 4 Cl⁻ = NaCl ✓. Voids decide stoichiometry — the holes write the formula.