The term ionization energy (EI) of an atom or molecule is the minimum energy required to remove (to infinity) an electron from the atom or molecule isolated in free space and in its ground electronic state. 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).
In chemistry, the value is usually given in kJ/mol (or formerly kcal/mol). This value is strictly the "molar ionization energy" and corresponds to the energy required to remove (to infinity) one mole of electrons from one mole of gaseous atoms or molecules. However it is often just called "ionization energy".
More generally, the nth ionization energy is the energy required to strip off the nth electron after the first n − 1 electrons have been removed. It is considered a measure of the "reluctance" of an atom or ion to surrender an electron, or the "strength" by which the electron is bound; the greater the ionization energy, the more difficult it is to remove an electron. The ionization energy is, thus, 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.
The term "ionization energy" is sometimes used as a name for the work needed to remove (to infinity) the topmost electron from an atom or molecule adsorbed onto a surface. However, due to interactions with the surface, this value differs from the ionization energy of the atom or molecule in question when it is in free space. So, in the case of surface-adsorbed atoms and molecules, it may be better to use the more general term "electron binding energy", in order to avoid confusion. Both these names are also sometimes used to describe the work needed to remove an electron from a "lower" orbital (i.e., not the topmost orbital) to infinity, both for free and for adsorbed atoms and molecules; in such cases it is necessary to specify the orbital from which the electron has been removed.
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).
Values and trends
Generally the (n+1)th ionization energy is larger than the nth ionization energy. Always, the next ionization energy involves removing an electron from an orbital closer to the nucleus. Electrons in the closer orbitals experience greater forces of electrostatic attraction; thus, their removal requires increasingly more energy.
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