Define ionisation enthalpy.

Ionisation enthalpy is the energy required by an isolated and gaseous atom in its ground state to remove an electron. The effective nuclear charge due to the screening

effect, the valence electrons are shielded by the inner core electrons. This effective nuclear charge is less than the actual charge present on the atom. Penetrated

orbitals: It is difficult to remove an electron from the orbitals that are closer to the nucleus and are penetrated towards the nucleus. The order of penetration is given by

s>p>d>fStability of orbitals: Half-filled and filled orbitals have a high ionisation enthalpy as they don’t want to lose their stability. Across a period the ionisation enthalpy

increases along the period. Down the group, the ionisation enthalpy decreases.