Among the isoelectronic ions$\left({\mathrm{O}}^{2-}, {\mathrm{N}}^{3-}, {\mathrm{Mg}}^{2+}, {\mathrm{Na}}^{+}\right),$ the ions with the least and the highest ionic radius are, respectively

For isoelectronic species,

With increase in number of protons in the nucleus the electrons are more attracted towards nucleus, thereby causing the decrease in ionic radius.

Order of ionic radius: ${\mathrm{Mg}}^{2+}<{\mathrm{Na}}^{+}< {\mathrm{O}}^{2-}< {\mathrm{N}}^{3-}$.