Ionization Energy is the minimum amount of energy which is required to eject theelectron completely from the outermost shell of neutral gaseous atom so as to form a positive ion. Thus, smaller the I.E., easier is to remove the electron to form cation. Hence ionic bondis readily formed.
The atom which gains the electron must possess strong tendency to accept the additional electrons. Halogens possess high electron affinity & thus show high tendency for ionic bonding. The elements of oxygen family also exhibit remarkable electron affinity & form ionic compounds but not so readily as halogens.
(iii). High Lattice Energy