Which of the following ions is the largest in size?

Solution

1. The ions ${\text{Na}}^{+}$, ${\text{Mg}}^{2+}$, ${\text{F}}^{-}$, and ${\text{O}}^{2-}$ are isoelectronic with $10$ electrons.
2. For isoelectronic species, the ionic radius increases as the nuclear charge decreases.
3. The atomic numbers are $\text{O}=8$, $\text{F}=9$, $\text{Na}=11$, and $\text{Mg}=12$.
4. The oxide ion has the lowest atomic number, resulting in the largest radius.
5. Hence the answer is (D).