Question

How many units of $NaCl$ is present per unit cell of $NaCl$?

• A. $4$
• B. $6$
• C. $2$
• D. $8$

Answer 1

Answer: (A)

$C{l}^{-}$ ions form ccp lattice i.e 8 at corners + 6 at face centres.
$\Rightarrow 8\times \frac{1}{8}+6\times \frac{1}{2}=4C{l}^{-}ions$
$N{a}^{+}$ at octahedral voids 12 at centre edges plus one at body centre.
$\Rightarrow 12\times \frac{1}{4}+1=4N{a}^{+}ions$
i.e $\text{4NaCl}$ molecules per unit cell.