Question

From the following ions, which has the highest value of ionic radius?

• A. $O^{2-}$
• B. $B^{3+}$
• C. $L{i}^{+}$
• D. $F^{-}$

Answer 1

The correct option is A $O^{2-}$

As we know,
size of the ions $\propto$ radius of the ions
(i) As the no of shells increases, size increases. Both the species contain one shell.
$L{i}^{+}:1s^2$, $B^{3+}:1s^2$
With the increase in +ve charge, size decreases therefore $B^{+3}<L{i}^{+}$.
$O^{2-}\text{and}{F}^{-}$ both the species contain two shells.
$\therefore$ The size of $O^{2-}\text{and}{F}^{-}$ must be greater than $L{i}^{+}\text{and}{B}^{3+}$.
Among $O^{2-}\text{and}{F}^{-},O^{2-}$ is of larger size.
$\therefore$ In an isoelectronic species as charge increases, size increases.