Question

A binary solid $\left(AB\right)$ has a rock salt structure. If the edge length is $400\text{pm}$ and radius of cation is $80\text{pm}$, the radius of anion is:

• A. $60\text{pm}$
• B. $180\text{pm}$
• C. $120\text{pm}$
• D. $100\text{pm}$

Answer 1

The correct option is C $120\text{pm}$

In the rock salt type structure, the $B^{-}$ ions are present in FCC manner and $A^{+}$ ions are present at octahedral voids.
So, the edge length is $a=2\times (r_{+}+r_{-})\Rightarrow 400=2\times (80+r_{-})\Rightarrow r_{-}=120\text{pm}$