Question

Select the correct statement (s) for the rock-salt structure $\left(NaCl\right)$ :

• A. The tetrahedral voids are smaller than octahedral voids
• B. In lattice of $NaCl$, FCC arrangement of $C{l}^{-}$ in which all octahedral voids are occupied by $N{a}^{+}$
• C. The radius ratio $\left(r_c/r_a\right)$ is $0.225$
• D. The radius ratio $\left(r_c/r_a\right)$ $>0.732$

Answer 1

Answer: (A), (B)

In lattice of $NaCl$, FCC arrangement of $C{l}^{-}$ in which all octahedral voids are occupied by $N{a}^{+}$

(a) Tetrahedral void : (coordination number =$4$ octahedral void : co-ordination no. = $6$) Due to more coordination number, the size of octahedral void is large.
The radius ratio of tetrahedral sites is smaller than octahedral sites.
(b) In lattice of $NaCl$, FCC arrangement of $C{l}^{-}$ in which all octahedral voids are occupied by $N{a}^{+}$ or FCC arrangement of $N{a}^{+}$ in which all octahedral voids are occupied by $C{l}^{-}$.
(c) Radius ratio $=\frac{r_c}{r_a}$
$NaCl$ has octahedral structural arrangement (geometry of voids), which have a radius ratio ranging between $0.414-0.732$.