Question

In a rock salt structure $NaCl$ :

• A. All the tetrahedral voids are filled
• B. $C{l}^{-}$ ions form a ccp aarangment
• C. All the octahedral voids are vacant.
• D. $N{a}^{+}$ ions form a ccp aarangment

Answer 1

The correct option is B $C{l}^{-}$ ions form a ccp aarangment

In rock salt structure, $C{l}^{-}$ ions forms the ccp arrangement. Thus, it occupies the corners and face centres of the cubic unit cells.
For ccp/fcc,
$\text{No. of tetrahedral voids}=8$
There are two tetrahedral voids on the every body diagonal of fcc.
$\text{No. of octahedral voids}=4$
Octahedral voids are present at the every edge centre and at the body centre.
There is one octahedral in centre of cube which gives full contribution to the unit cell and 12 edge centred octahedral voids which contributes $\frac{1}{4}$ to the unit.
Thus, the total number of octahedral voids in ccp is 4.
In $NaCl,N{a}^{+}$ occupies all the octahedral voids and the 8 tetrahedral voids are left vacant.