Question

If the anions A form hexagonal closest packing and cations C occupy only $\frac{2}{3}$rd of the octahedral voids in it, then the general formula of the compound is :

• A. $CA$
• B. $C{A}_2$
• C. $C_2{A}_3$
• D. $C_3{A}_2$

Answer 1

Answer: (C)

$C_2{A}_3$

For hexagonal close packed (hcp) crystal, there are six atoms per unit cell.

Hence, $A=6$ (number of octahedral voids $=$ number of atoms per unit cell).

Now $C=\frac{2}{3}$ of $6$ i.e. $C=4$.

Hence, $C:A=4:6=2:3$. Thus, the general formula is $C_2{A}_3$.