The enthalpy of ionization, also known as ionization energy, refers to the energy required to remove an electron from an atom or ion. In the case of sodium, there are two ionization energies given: the first ionization energy removing one electron from a neutral sodium atom and the second ionization energy removing a second electron from the sodium 1+ ion .The first ionization energy for sodium is given as 495 kJ/mol. This is the energy required to remove one electron from a neutral sodium atom, resulting in a sodium 1+ ion Na+ .The second ionization energy for sodium is given as 4562 kJ/mol. This is the energy required to remove a second electron from the sodium 1+ ion, resulting in a sodium 2+ ion Na2+ .In general, the enthalpy of ionization for an element refers to the energy required to remove one electron from its neutral atom. Therefore, the enthalpy of ionization for sodium is 495 kJ/mol.