The ionic radii ( in Å ) of N 3 - , O 2 - and F - are respectively:

As $\frac{\text{Z}}{\text{e}}\uparrow$ ionic radius decreases for isoelectronic species.

$$\begin{gathered} \mathrm{Z}=\mathrm{number} \mathrm{of} \mathrm{protons} \\ \mathrm{e}=\mathrm{number} \mathrm{of} \mathrm{electrons} \end{gathered}$$

${\text{N}}^{3-}\left(\frac{\text{Z}}{\text{e}}\right)=\frac{7}{10}$

${\text{O}}^{2-}\left(\frac{\text{Z}}{\text{e}}\right)=\frac{8}{10}$

${\text{F}}^{-}\left(\frac{\text{Z}}{\text{e}}\right)=\frac{9}{10}$

${\text{Trend of ionic radius: N}}^{3-}>{\text{O}}^{2-}>{\text{F}}^{-}$