In a face-centered cubic (FCC) metal, how many atoms touch a given atom at the nearest-neighbor distance (the coordination number)?

Answer: B. 12

In FCC, each atom touches 6 atoms within its own {111} plane and 3 atoms in each of the {111} planes above and below, so the coordination number is 12. This is the maximum possible for packing spheres of equal size, and HCP also has 12. Eight is the coordination number of BCC (the body-center atom touches the 8 corner atoms of the cube), and 6 is that of simple cubic (SC). Structures with larger coordination numbers also have higher packing factors: about 0.52 for SC, about 0.68 for BCC, and about 0.74 for FCC and HCP. Cu, Al, Ni, Au and γ-iron are typical FCC examples.