Arrange $K^{+}$,$Cl^{-}$,$S^{2-}$,and $Ca^{2+}$ in decreasing order of their ionic radii with an explanation.

(A) The ions $K^{+}$,$Cl^{-}$,$S^{2-}$,and $Ca^{2+}$ are isoelectronic,meaning they all contain $18$ electrons.
For isoelectronic species,the ionic radius decreases as the nuclear charge (number of protons) increases.
$S^{2-}$ has $16$ protons.
$Cl^{-}$ has $17$ protons.
$K^{+}$ has $19$ protons.
$Ca^{2+}$ has $20$ protons.
As the number of protons increases,the effective nuclear charge increases,which pulls the electron cloud closer to the nucleus,resulting in a smaller ionic radius.
Therefore,the decreasing order of ionic radii is: $S^{2-} > Cl^{-} > K^{+} > Ca^{2+}$.