Determine the number of formula units of NaCl in the unit cell. NaCl is a face centred cubic crystal. See the diagram of the NaCl unit cell in the text.

Explanation

Either of `Na^+` or `CI^(-)` ions may be chosen as the lattice points. Choosing `Cl^(-)` ions (as shown in the text), the cube contains a `Cl^(-)` ion at the centre of each face as well as at the corners of the unit cell. There are `12 Na^+` ions at the centres of 12 edges and 1 `Na^(+)` ion at the centre of the unit cell.