On moving down the group, atomic size increases, hence ionisation energy decreases.
(ii) With the addition of new shells the number
of inner shell electrons which shield the valence electrons from the nucleus increases. In other words, shielding or screening effect increases. As a result, the force of attraction of the nucleus for the valence electron further decreases and hence the ionisation energy decreases.
(iii) Nuclear charge increases down the group with increase in atomic number. As a result the force of attraction of the nucleus for the valence electrons should increase thus I.E., should increase. The combined effect of the increase in the atomic size and the screening effect more them compensates the effect of increased nuclear charge