The energy required by an isolated, gaseous atom in its ground state to remove an electron is known as ionization enthalpy. The valence electrons are shielded by the inner electrons, which reduces the effective nuclear charge. This effective nuclear charge is lower than the original atomic charge. It is difficult to remove an electron from orbitals that are closer to the nucleus and that have been penetrated. s>p>d>f denotes the order of penetration.

Orbital stability is defined as the ability of an orbital to remain stable throughout time. Because they don’t want to lose their stability, half-filled and filled orbitals have a high ionization enthalpy. The enthalpy of ionization increases across the period. The ionization enthalpy reduces as you move tdown the group.