Which coordination number is typical for an FCC lattice?

• A. 6
• B. 8
• C. 12
• D. 14

Answer: (C)

Coordination number is the number of nearest neighbor atoms surrounding a given atom. In an FCC lattice, atoms are packed as tightly as possible, and each atom touches twelve others. This arises because the closest contacts lie along the face diagonals of the cube, giving twelve distinct directions to neighboring centers (four in each of the three perpendicular orientations). So the typical coordination number is twelve. This high value reflects the efficient close-packed arrangement; for reference, simpler lattices have fewer nearest neighbors (six in a simple cubic, eight in a body-centered cubic).