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

Answer

Within the main group elements, the ionization enthalpy decreases regularly as we move 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 increase. consequenctly, the force of attraction of the nucleus for the valece electron, decreases and hence the ionization enthalpy decreased.
(ii) Screening effect. with the addition of new shells, the number of inner electron shells, which shield the valence electrons, increase. In other words, the shielding effect or the screening effect increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence, the ionisation enthalpy decreases.