1. As we move down a group, generally there is a steady decrease in ionisation enthalpy of elements due to increase in their atomic radius.
2. In p-block elements there are some minor deviations to this general trend. In group 13, from B to Al the ionisation enthalpy decreases as expected. But from Al to Tl there is only a marginal difference. This is due to the presence of inner d- and f-elements which has poor shielding effect compared to s and p electrons. As a result, the effective nuclear charge on the valance electrons increase
3. A similar trend is also observed in group 14. The remaining groups (15-18) follows the general trend, in these groups the ionisation enthalpy decreases as we move down the group. Here poor shielding effect of d- and f-electrons are overcome by the increased shielding effect of the additional p-electrons.
4. The ionisation enthalpy of elements in successive groups is higher than the corresponding elements of the previous group as expected.
