Question

A substance A_x B_y crystallizes in a face centred cubic (fcc) lattice in which atoms ‘A’ occupy each corner of the cube and atom ‘B’ occupy the centres of each face of the cube. Identify the correct composition of the substance A_x B_y . 

(A) AB₃ 

(B) A₄ B₃ 

(C) A₃B 

(D) Composition cannot be specified

Answer 1

Correct option (A) AB₃

Explanation:

 In cubic system, a corner contribute 1/8th part of atom to one unit cell and a face centre contribute 1/2 part of atom to one unit cell. Therefore,

Number of A per unit cell = 1/8 x 8 = 1

Number of B per unit cell = 1/2 x 6 = 3

⇒ Formula = AB₃