If the ions derived from different atoms are isoelectronic species, then they all have the same number of electrons in their electronic shells and have the same electronic configuration but their nuclear charge will differ because of their difference in number of protons in the nucleus. The increasing number of protons in the nucleus, the electrons gets highly attracted towards nucleus thereby causing the decrease in ionic radius. The given ions are: Mg2+: number of protons =12 and number of electrons =10 Na+: number of protons =11 and number of electrons =10 O2−: number of protons =8 and number of electrons =10 N3−: number of protons =7 and number of electrons =10 Thus, the highest and least ionic radius respectively is Mg2+,N3−