The largest possible dense packing in a cubic system is achieved in the FCC crystal system. That is why it is sometimes referred to as close-packed cubic (CPC) crystal structure. Each cube face (i. e., six of them in a cube) has one atom at the center of the cube face in addition to eight corner atoms (Figure 2.6a). The effective number of atoms in an FCC unit cell is then given by (6 x 1/2) + (8 x 1/8) = 4, because each face-centered atom is shared by two unit cells and each corner atom is shared by eight unit cells. This structure has a coordination number of 12. The packing factor can be calculated in much the same way as the BCC crystal structure, and is found to be about 0.74. Following a similar method, the relationship between the atom radius (r) and the lattice constant (a) can be found for an FCC unit cell. Figure 2.6b shows the cube face of an FCC unit cell. The face diagonal is (r + 2r + r) = 4r, and now applying the Pythagorean rule, we also know that the face diagonal is ^/2a when the cube edge is a so that 4r = 2a.

Metals such as aluminum, y-Fe, gold, silver, platinum, lead, nickel, and many others have FCC crystal structures. The theoretical density of FCC metals can also be derived from the first principles if the lattice constant and the atomic weight of the metal are known as shown earlier for BCC in Example 2.1.