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 regularly as we more down the group due to the following two factors.
(i) Atomic size. On moving down the group the atomic size increases gradually due to the addition of one new principal energy shell at each succeeding element .As a result the distance of the valence electrons from the nucleus increases . Consequently the force of attraction of the nucleus of the valence electrons decreases and hence the ionization enthalpy decreases.
(ii) 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 of the screening effect increases . As a result the force of attraction of the nucleus for the valence electrons further decreases and hence the ionization enthalpy decreases.