Factors Influencing Ionization Enthalpy
The ionization enthalpy of an atom depends on the following factors.
(i) Size of the atom
As the distance between the electron and the nucleus increases, i.e., as the size of the atom increases, the outermost electrons are less tightly held by the nucleus. Thus, it becomes easier to remove an outermost electron. Thus ionization enthalpy decreases with increases in atomic size.
(ii) Charge on the nucleus
Ionization enthalpy increases with increase in nuclear charge because of the increase in the attractive force between the nucleus and the electron.
(iii) Screening effect of inner electrons
Ionization enthalpy decreases when the shielding effect of inner electrons increases. This is because when the inner electron shells increases, the attraction between the nucleus and the outermost electron decreases.
(iv) Penetration effect of electrons
The penetration power of the electrons in various orbitals decreases in a given shell (same value of n) in the order: s>p>d>f. Since the penetration power of s-electron towards the nucleus is more, it will be closer to the nucleus and will be held firmly. Thus, for the same shell, the ionization enthalpy would be more to remove the s-electrons in comparison with the p-electron which in turn would be more than that for d-electron and so on.
(v) Effect of half-filled and completely filled sub-levels
If an atom has half-filled or completely filled sub-levels, its ionization enthalpy is higher than that expected normally from its position in the periodic table. This is because such atom, have extra stability and hence it is difficult to remove electrons from these stable configurations.