How many atoms are contained in a body-centered cubic unit cell?

• A. 2
• B. 4
• C. 1
• D. 8

Answer: (A)

In a body-centered cubic lattice, atoms at the eight corners each belong to eight neighboring unit cells, so their contribution to a single unit cell is 8 × (1/8) = 1. There is also one atom at the center that lies entirely within the cell. Adding these together gives 1 + 1 = 2 atoms per unit cell.

The other numbers don’t fit because they imply counting corner atoms as full atoms or missing the center. Four would come from counting corners as full atoms plus face-centered contributions (as in a face-centered cubic lattice), which isn’t the case here. One would ignore the corner atoms entirely, which isn’t correct since they do contribute. Eight would count all corner atoms as full, without accounting for sharing. The correct total for a body-centered cubic unit cell is two atoms.