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.
$\Rightarrow 8\times \frac{8}{18}+6\times \frac{1}{2}=4C{l}^{-}$ions
$N{a}^{+}$at octahedral voids 12 at centre edges plus one at body centre.
$\Rightarrow 12\times \frac{1}{4}+1=4N{a}^{+}\text{ions }$
i.e $4NaCl$ molecules per unit cell.