Question

A solid AB has the NaCl structure. If the radius of the cation $A^{+}$ is 120 pm, the maximum possible value of the radius of the anion $B^{-}$ is:

• A. 240 pm
• B. 60 pm
• C. 49.6 pm
• D. 290 pm

Answer 1

The correct option is D

290 pm

For NaCl crystal, the radius ratio lies between 0.732 to 0.414. For the largest anion size, the ratio must be the smallest value.
$\frac{\text{Radius of cation}}{\text{Racius of anion}}=0.414$
$\frac{r_{A^{+}}}{r_{B^{-}}}=0.414\Rightarrow r_{B^{-}}=\frac{r_{A^{+}}}{0.414}$
= $\frac{120}{0.414}=290$ pm