The structure of a crystalline solid, whether metal or not, is best described by considering its simplest repeating unit, which is referred to as its unit cell. The unit cell consists of lattice points that represent the locations of atoms or ions.
- An FCC unit cell contains four atoms: one-eighth of an atom at each of the eight corners (8X18 = 18 X 18 = 1 atom from the corners) and one-half of an atom on each of the six faces (6 x 12 = 36 x 12 = 3 atoms from the faces).
- The face-centred cubic (FCC) has a coordination number of 12 and contains 4 atoms per unit cell. The atoms in the corner are shared by 8 unit cells and contribute to 1/8 of themselves to the crystal lattice.