Question

Which one of the following series is arranged in order of increasing ionic radius ?

• A. $M{g}^{2+}<S^{2-}<C{l}^{-}<K^{+}<C{a}^{2+}$
• B. $M{g}^{2+}<C{a}^{2+}<K^{+}<C{l}^{-}<S^{2-}$
• C. $S^{2-}<C{a}^{-}<K^{+}<M{g}^{2+}<C{a}^{2+}$
• D. $S^{2-}<M{g}^{2+}<C{a}^{2+}<C{l}^{-}<K^{+}$

Answer 1

The correct option is B $M{g}^{2+}<C{a}^{2+}<K^{+}<C{l}^{-}<S^{2-}$

Consider the species in the given options. We know that for isoelectronic species, cations are smaller than neutral atoms which are in turn smaller than anions.
In this case $M{g}^{2+}$ is the smallest since it has only $10e^{-}$.
$C{a}^{2+},K^{+},C{l}^{-},S^{2-}$ are isoelectronic.
$C{a}^{2+}<K^{+}$
$C{l}^{-}<S^{2-}$
Hence the correct order is $M{g}^{2+}<C{a}^{2+}<K^{+}<C{l}^{-}<S^{2-}$.
Therefore option B is the right answer.