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

The correct option is D 2 and 4

The bcc cell consistes of 8 atoms at the corners and one atom at centre.
$\therefore n=(8\times \frac{1}{8})+1=2$.
The fcc cell consists of 8 atoms at the eight corners and one atom at each of the six faces.This atom at the faces is shared by two unit cells.
$\therefore n=8\times \frac{1}{8}+(6\times \frac{1}{2})=4$