Ionisation enthalpy is the energy required by an isolated and gaseous atom in its ground state to remove an electron.

A(g) A⁺(g) →+e^-

Factors affecting ionisation enthalpy

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>f

stability of orbitals

Half filled and fully filled orbitals have a high ionisation enthalpy as they don’t want to lose their stability.

Across a period: ionisation enthalpy increases along the period.

Down the group: Ionisation enthalpy decreases down the group.