What is the correct order of the ionic radii of the following ions: ${\text{Na}}^{+},{\text{Mg}}^{2+},{\text{F}}^{-},{\text{O}}^{2-}$?

Solution

1. All the given ions ${\text{Na}}^{+},{\text{Mg}}^{2+},{\text{F}}^{-},{\text{O}}^{2-}$ have 10 electrons, making them isoelectronic.
2. The ionic radius of isoelectronic species is inversely proportional to the nuclear charge ($Z$).
3. The atomic numbers are $\text{O}(8),\text{F}(9),\text{Na}(11),\text{Mg}(12)$.
4. The order of radii is ${\text{O}}^{2-}(Z=8)>{\text{F}}^{-}(Z=9)>{\text{Na}}^{+}(Z=11)>{\text{Mg}}^{2+}(Z=12)$.
5. Hence the answer is (A).