Ionisation Enthalpy

Ionisation enthalpy decreases down Group 16, the expected consequence of increasing atomic size — the outermost electron sits progressively farther from the nucleus and is easier to remove.

Notably, however, every element in Group 16 has a lower ionisation enthalpy than its counterpart in Group 15 of the same period. This apparent reversal of the usual left-to-right increase across a period is explained by the electronic structure of Group 15: those elements have a half-filled p-subshell ($np^3$), which is an especially stable, symmetric arrangement that resists ionisation. Group 16 elements, with one additional electron ($np^4$ …