Question

Na and Mg crystallize in bcc and fcc type crystals respectively, then the number of atoms of Na and Mg present in the unit cell of their respective crystal is

(a) 4 and 2 

(b) 9 and 14

(c) 14 and 9 

(d) 2 and 4.

Answer 1

(d) : bcc - Points are at corners and one in the centre of the unit cell.
Number of atoms per unit cell = 8 x 18 + 1 = 2.

fcc - Points are at the corners and also centre of the six faces of each cell.
Number of atoms per unit cell = 8 x 18 + 6 x 12 = 4.