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

Within the main group elements,the ionization enthalpy decreases regularly as we move down the group. This 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 an increase in shielding effect on the outermost electron due to an increase in the number of inner electrons.
$(iii)$ Nuclear charge: In going from top to bottom in a group,the nuclear charge increases.
The effect of the increase in atomic size and the shielding effect is much more than the effect of the 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 ionization enthalpies in a group.