In the iso-electronic species the ionic radii of ${\mathrm{N}}^{3-},{\mathrm{O}}^{2-}$ and ${\mathrm{F}}^{-}$ are respectively given be

• N³⁻,O²⁻ and F⁻ all have 10 electrons each. 
• Thus, they are iso-electronic. 
• For iso-electronic species, atomic size is inversely proportional to the effective nuclear charge. 
• Effective nuclear charge is inversely proportional to negative charge. 
• N³⁻ has maximum negative charge and hence, maximum atomic size. 
• Similarly, size of O²⁻>F⁻.

In the isoelectronic species the ionic radii (A) of N3−,Ne and AI+3 are respectively given by 171,140,136. All of these species contains same number of electrons. As the atomic number increases, the effective nuclear charge increases. The attraction of the nucleus for the valence electrons increases. Hence, the ionic radii increases.