Energy is required to overcome the attraction between nucleus and the electron while removing it from the atom. This energy is referred to as ionization energy and is expressed in kJ mol-1. It is a measure of nuclear attraction over the outermost electron in a given species (atom or ion or molecule). Previously it was denoted by ionization potential expressed in eV atom-1.
1) Atomic radius:
Ionization energy decreases with increase in atomic radius as the nuclear attraction over outer electron decreases.
2) Nuclear charge:
Ionization energy increases with increase in the effective nuclear charge.
3) Shielding or screening effect:
Reduction in nuclear attraction over the outer electrons due to repulsions by inner electrons is called screening or shielding effect.
4) Penetration power:
The ability of an electron to come closer towards nucleus is called penetration power.
5) Electronic Configuration:
Atoms with stable electronic configurations like: ns2 np6 or half filled or completely filled sub shells are extra stable. Removal of electrons from them is difficult and hence their ionization energies are comparatively high.