The ions F−,Na+,O2− and Mg2+ are isoelectric species having same electronic configuration but their nuclear charges differ from each other because of their difference in the number of protons in the nucleus. With increase in the number of protons in the nucleus, the electrons are more attracted towards nucleus thereby causing the decrease in ionic radius.Therefore, the given ions are F−: no.of protons = 9 and no. of electron = 10 Na+:no.of protons = 11 and no. of electron =10 O2−::no.of protons = 8 and no. of electron =10 Mg2+:no.of protons = 12 and no. of electron =10