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.

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