Na and Mg crystallize in BCC and FCC type crystals respectively, then the

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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: Option D
:
D

The bcc cell consistes of 8 atoms at the corners and one atom at centre.
$∴ n = (8 \times \frac{1}{8}) + 1 = 2$ .
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.
$∴ n = 8 \times \frac{1}{8} + (6 \times \frac{1}{2}) = 4$

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