## Chemistry: A Molecular Approach (3rd Edition)

$S^{2-}$
$S^{2-}$ has the larger radius because $S^{2-}$ and $Ca^{2+}$ are isoelectronic, meaning that they have the same number of electrons. However, $S^{2-}$ has four fewer protons than $Ca^{2+}$, resulting in a lesser pull on the electrons and therefore, a larger radius.