`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 hcc cell consists of `8` atoms at the corners and one atoms at centre.
`:. 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.
`:. n = 8 xx (1)/(8) +(6 xx (1)/(2)) = 4`