Question

Identify the correct order of the size of the following:

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

Answer 1

The correct option is C

$C{a}^{2+}<K^{+}<Ar<C{l}^{-}<S^{2-}$

Calcium and potassium belong to the fourth period whereas the rest all belong to the third period. Usually, anions have the largest sizes followed by neutral atoms and then cations for a respective period. So the correct order should be $C{a}^{2+}<K^{+}<Ar<C{l}^{-}<S^{2-}$

$C{a}^{2+}$ has more protons is thus able to exert a greater electrostatic force on the valence electrons as compared to $K^{+}$. This translates to a smaller ionic radius for $C{a}^{2+}$ than $K^{+}$.