`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

Options

A. 4 and 2
B. 9 and 14
C. 14 and 9
D. 2 and 4

Answer

D

Explanation

in bcc - 8 atoms are at corner and one in the centre of the unit cell .
number of atoms per unit cell `n = 8 xx 1/8 + 1 = 2`
In fcc - 8 atoms are at the corners and also centre of the six faces of each cell .
Number of atoms per unit cell `n = 8 xx 1/8 + 6 xx 1/2 = 4`