Question

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

• A. $100$ pm
• B. $120$ pm
• C. $250$ pm
• D. $325$ pm

Answer 1

The correct option is B $120$ pm

$a=2(r^{+}+r^{-})$

$400=2(80+r^{-})$

$400=160+r^{-}$

$400-160=2r^{-}$

$240=2r^{-}$

$r^{-}=\frac{240}{2}$

$r^{-}=120$.

Hence option B is correct.