Ionization Energy

The energy required to remove an electron from an atom is called the ionization energy (IE). The zero of Energy for H is ionization so that IE = -E1 = 13.60 eV. (One electron volt is the energy acquired by an electron on acceleration through an electric potential difference of 1 V, 1 eV = 1.6022 × 10-19 J.) First ionization energies (IE1) for other atoms are given in Table 1.1. In general, IE1 decreases as we descend a column in the periodic table and increases as we move from left to right across the table. Table 1.1 also lists second ionization energies (IE2), the energy required to remove a second electron. IE2 is always greater than IE1, and the difference is greatest when the second electron comes from a different shell than the first, e.g., the alkali metals of Group I.

Although the transition metals (Sc-Cu, for example) appear to fill the 4s orbital before the 3d orbitals, the 4s electrons are the first to be ionized; thus the electronic configuration of Cr²⁺ is [Ar] 3d4, and that of Zn²⁺ is [Ar] 3d¹⁰.

 Table 1.1. Ionization Energies (eV), Electron Affinities (eV), and Electronegativities