In an FCC unit cell, how many nearest neighbors does each atom have?

• A. Each atom has 6 nearest neighbors.
• B. Each atom has 8 nearest neighbors.
• C. Each atom has 12 nearest neighbors.
• D. Each atom has 4 nearest neighbors.

Answer: (C)

Coordination number describes how many atoms lie at the closest distance to a given atom. In a face-centered cubic lattice, this number is 12. The arrangement places atoms at corners and at the centers of each face, and the closest contacts occur along the face diagonals with distance a/√2. From any atom, there are twelve neighbors at that distance spread through the surrounding lattice—four in each of the three adjacent close-packed layers. This is why FCC has 12 nearest neighbors; simple cubic gives 6, body-centered cubic gives 8, and the fact that there are four atoms per unit cell refers to counting atoms in the cell, not to the number of nearest neighbors.