In a face centred cubic lattice, atom A occupies the corner positions and atom B occupies the face centre positions. If one atom of B is missing from one of the face centred points, the formula of the compound is

Number of effective A atoms= 8 corners x 1/ 8 per corner atom share= 1 atoms/unit cellNumber of atoms on faces of a cube = 6 atomsIf one B atom is missing from one face, number of B atoms left = 5∴Number of effective ,B atoms = 5 faces x 1/2per face atomshare =52per unit cellThe formula of the compound is A2B5.