An ionic compound has a unit cell consisting of A ions at the corners of a cube and B ions on the centres of the faces of the cube. The empirical formula for this compound would be

'A' atoms are at eight corners of the cube. Therefore, the no. of atoms in the unit cell =8×18 =1. B atoms are at the face centre of six faces. Therefore, its share in the unit cell =6×12=3. The formula is AB3