Question

How many molecules are there in the unit cell of sodium chloride?

• A. $2$
• B. $4$
• C. $6$
• D. $8$

Answer 1

The correct option is B $4$

In $NaCl$ (rock salt):

Number of $N{a}^{+}$ ions $=12$ (at edge centers) $\times \frac{1}{4}+1$ (at body centre) $\times 1=4$.

Number of $C{l}^{-}ions=8$ (at corners) $\times \frac{1}{8}+6$ (at face centre) $\times \frac{1}{2}=4$.

Thus $4$ formula units per unit cell.