In an fcc arrangement of A and B atoms A are present at the corners of the unit cell and B are present at the face centres. If one atom of A is missing from its position at the corner, what is the formula of the compound?
We know that there are eight corners in the unit cell and each contributes 1/8th. One atom (A) from the corner is missing, which means there are 7 A atoms whose contribution will be = 78 There are six face centers and ‘B’ atoms are present there. Each has a contribution = 12 So, the total contribution will be = 6 × 12=36 × 12=3 Ratio will be = A : B = 78 : 3 = 7 : 24 The formula will be = A7B24
