How many atoms are effectively contained in a Body-Centered Cubic unit cell?

• A. 3
• B. 1
• C. 4
• D. 2

Answer: (D)

In a body-centered cubic lattice, atoms at the corners are shared with neighboring unit cells, while the center atom lies entirely within the cell. There are eight corners, and each corner contributes one eighth of an atom to this unit cell, so the corner contribution is 8 × (1/8) = 1. The atom at the center adds a full atom. Together, 1 + 1 = 2 atoms are effectively contained in a BCC unit cell. This is why the BCC structure corresponds to two atoms per unit cell.