`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

The bcc cell consists of 8 atoms at the corners and one atom at centre.
`therefore n = (8 xx 1/8) + 1= 2`
The fcc cell consists of 8 atoms at the eight corners and one atom at each of the six faces. This atom at the face is shared by two unit cells.
`therefore n = 8 xx 1/8 + (6 xx 1/2) = 4`