Worked example: Diamond has lattice constant a = 0.357 nm. Find the nearest-neighbour C–C distance and the theoretical density (carbon 12 u). Step 1: In diamond, nearest neighbours are the FCC atom and its (¼,¼,¼) partner: distance = √(3)×a/4. Step 2: d = 1.732 × 0.357/4 = 0.1546 nm — the standard C–C single bond length. Step 3: 8 atoms per cell; mass = 8 × 12 × 1.661×10⁻²⁷ = 1.595×10⁻²⁵ kg. Step 4: Cell volume = (3.57×10⁻¹⁰)³ = 4.55×10⁻²⁹ m³; density = 1.595×10⁻²⁵/4.55×10⁻²⁹ ≈ 3505 kg/m³. Step 5: Measured diamond density ≈ 3510 kg/m³ — near-perfect agreement. Conclusion: the tetrahedral (¼,¼,¼) offset reproduces both bond length and density — structure determines everything.