The ionic radii of O-2,F-1,Na+and Mg2+ are 1.35, 1,.34, 0.95 and 0.66 A respectively. The radius of the Ne-atom is

Ionic radii of O2−>F−>Na+>Mg2+We take van der Waals' radius of Ne which should have maximum size.Alternatively we can say that Ne must be just smaller that F- and larger than Na+