STD 11 - 3. Classification of elements and periodicity in properties — NEET STD 11 Science — Question

No. of electrons in $B ^{3+}=8$

No. of electrons in $Li ^{+}=4$

No. of electrons in $F ^{-}=10$

$F ^{-}$and $O ^{2-}$ have highest no. of electrons, thus the effective nuclear charge

$\{\left.\frac{\text { nuclear charge }}{\text { No. of electrons }}\right\}$ for them is less than $B$ and $Li$.

When effective nuclear charge increases, size of an ion decreases.

Amongst $F ^{-}$and $O ^{2-}$, size of $O ^{2-}$ is more than $F ^{-}$as size decreases on moving from left to right in a period.

Therefore, $O ^{2-}$ is largest in size.