Ionization Energy

The ionization energy of a chemical species (i.e., an atom or molecule) is the energy required to remove electrons from gaseous atoms or ions. The property is alternately still often called the ionization potential, measured in volts. In chemistry it often refers to one mole of a substance (molar ionization energy or enthalpy) and reported in kJ/mol. In atomic physics the ionization energy is typically measured in the unit electron volt (eV). Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.

The ionization energy is different for electrons of different atomic or molecular orbitals. More generally, the nth ionization energy is the energy required to strip off the nth electron after the first electrons have been removed. Ionization energies for all charge states of all elements with atomic numbers at and below that of lead are tabulated in the (ADAS) database. It is considered a measure of the tendency of an atom or ion to surrender an electron, or the strength of the electron binding; the greater the ionization energy, the more difficult it is to remove an electron. The ionization energy may be an indicator of the reactivity of an element. Elements with a low ionization energy tend to be reducing agents and form cations, which in turn combine with anions to form salts.

Electron binding energy (BE), more accurately, is the energy required to release an electron from its atomic or molecular orbital when adsorbed to a surface rather than a free atom. Binding energy values are normally reported as positive values with units of eV. The binding energies of 1s electrons are roughly proportional to (Z-1)² (Moseley's law).