Ionisation Enthalpy

On descending any group of the periodic table, atomic radius generally increases, and ionisation enthalpy -- the energy needed to remove the outermost electron -- generally falls in step with it. The p-block follows this rule overall, but with a couple of instructive local deviations.

In group 13, ionisation enthalpy falls as expected from boron to aluminium, but from aluminium to thallium the fall becomes only marginal. This is because the heavier group 13 elements have intervening filled d and f subshells, whose electrons shield the nucleus far less effectively than s and p electrons do; the resulting rise in effective nuclear charge on the valence electrons largely cancels out the expected fall from increasing size. A closely similar deviation is seen in group 14. …