Consider the isoelectronic species, Na+, Mg2+, F– and O2–. The correct order of increasing length of their radii is _________.

Radii of Mg²⁺< Na⁺ due to an increase in effective nuclear charge on the outermost electrons as we go down the group. Na⁺ and Mg²⁺ are cations so they have a smaller radii than F^- and O2^-.

In the case of F^- and O2^- fluoride has an extra proton as compared to oxygen and the number of electrons gained by oxygen is more which increases the nuclear charge on fluoride.