The ions${\mathrm{O}}^{-2},{\mathrm{F}}^{-},{\mathrm{Na}}^{+},{\mathrm{Mg}}^{+2}, \mathrm{and} {\mathrm{Al}}^{+3}$ are isoelectronic. Their ionic radii show

Daughter anion with more negative charge will be large in size than parent atom whereas the daughter cation with large positive charge will be small in size than the parent atom therefore a significant decrease from${\mathrm{O}}^{-2} \mathrm{to} {\mathrm{Al}}^{+3}$is observed.