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 compounds would be

(A) Occupies the corner positions. There are 8 corner positions and each position contributes one eighth to the unit cell. Hence, total number of ions 1 = ×8=1 8 Unit cell consists of (B) ion at the center of the six faces. The number of ions 1 = ×6=3 2 The formulae is 3 AB Hence the correct answer is option (b)