Worked example: How many atoms per unit cell do SC, BCC and FCC cubic lattices contain? Step 1: Corner atoms are shared by 8 neighbouring cells, so each contributes 1/8. Step 2: SC: 8 corners × 1/8 = 1 atom per cell. Step 3: BCC: 8×1/8 corners + 1 body centre = 2 atoms per cell. Step 4: FCC: 8×1/8 corners + 6 faces × 1/2 (each face shared by 2 cells) = 1 + 3 = 4 atoms per cell. Step 5: Consequence: for the same atomic radius r, FCC packs 74% of space (close packing) versus 68% for BCC and 52% for SC — which is why most metals choose FCC or HCP. Conclusion: counting shared atoms reveals why some lattices pack better — geometry decides density.