What are the various factors due to which the ionisation enthalpy of the main group elements tends to decrease down a group?

Explanation

Within the main group elements, the ionization enthalpy decreases regularly as we move down the group. The is due to the following factors :
(i) Atomic size: On moving down the group, there is a gradual increase in atomic size due to an additional main energy shell (n).
(ii) Shielding effect. There is increase in shielding effect on the outermost electron due to increase in the number inner electrons.
(iii) Nuclear charge. In going from top to bottom in a group, the nuclear charge increase. The effect of increase in atomic size and the shielding effect is much more than the effect of increase in nuclear charge. As a result, the electron becomes less tightly held to the nucleus as we move down the group. Hence there is a gradual decrease in the ionisation enthalpies in a group.