what is ionization enthalpy

The term ionization energy (EI) of an atom or molecule means the energy needed to remove electrons from an atom. Large atoms require low ionization energy while small atoms require high ionization energy. This quantity was formerly called ionization potential, and was at one stage measured in volts. The name "ionization energy" is now strongly preferred. In atomic physics the ionization energy is measured using the unit "electronvolt" (eV).