The factors responsible for the ionization enthalpy of the main group elements to decrease

down a group are listed below:

(i) Increase in the atomic size of elements: As we move down a group, the number of

shells increases. As a result, the atomic size also increases gradually on moving down a

group. As the distance of the valence electrons from the nucleus increases, the electrons 

are not held very strongly. Thus, they can be removed easily. Hence, on moving down a

group, ionization energy decreases.

(ii) Increase in the shielding effect: The number of inner shells of electrons increases on

moving down a group. Therefore, the shielding of the valence electrons from the nucleus

by the inner core electrons increases down a group. As a result, the valence electrons are

not held very tightly by the nucleus. Hence, the energy required to remove a valence

electron decreases down a group.