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 `IE` decreases regularly as we move down the group due to the following two factors:
a. Atomic size.On moving down the group, the atomic size increases gradually due to the addition of one new pricipal energy shell at each succeeding element. As a result, the distance of the valence electrons from teh nucleus increses. Consequently, the force of attraction of the nucleus for the valence electrons decreases and hence the ionisation enthalpy decreases.
Screening effect. With the addition of new shells, the number of inner electron shells which shield the valence electrons increases. In other words, the shielding effect or the screening effect increases. As the talence electrons further decreases and hence the `IE` decreases.