Increasing order of ionic size for the ions, F–, O₂–, Na⁺, Al³⁺ is (AMU 2009)

Isoelectronic species are the species that have the same number of electrons. In isoelectronic species, the greater the nuclear charge or atomic number the smaller will be the ionic radius. Therefore, the correct order of the ionic radius is + 3 + 2 - Al < Na < F < O .