ionization potential

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Ionization potential

The potential difference through which a bound electron must be raised to free it from the atom or molecule to which it is attached. In particular, the ionization potential is the difference in potential between the initial state, in which the electron is bound, and the final state, in which it is at rest at infinity.

The ionization potential for the removal of an electron from a neutral atom other than hydrogen is more correctly designated as the first ionization potential. The potential associated with the removal of a second electron from a singly ionized atom or molecule is then the second ionization potential, and so on.

ionization potential

See ionization.

Ionization Potential


a physical quantity determined by the ratio of the least energy necessary for single ionization of an atom (or molecule) in the ground state to the charge of the electron. The ionization potential is a measure of the ionization energy, which is equal to the work expended in emitting the electron from the atom or molecule and characterizes the electron’s bond strength in the atom or molecule. The ionization potential commonly is expressed in volts and is numerically equal to the ionization energy in electron volts.

The values of the ionization potential can be determined experimentally by studying the ionization induced by an electron impact or by measuring the energy of photons during photoionization. Highly accurate values of the ionization potential for atoms and the simplest molecules can be obtained from spectroscopic data on energy levels and their convergence toward the ionization boundary.

For atoms the values of the first ionization potential, which corresponds to the removal of the most weakly bound electron from a neutral atom in its ground state, range from 3.894 V for cesium to 24.587 V for helium. They change periodically as a function of the atomic number Z (see Figure 1). The first ionization potentials of molecules are of the same order of magnitude as for atoms (usually 5–15 V). The ionization potential increases with increasing degree of ionization of the atom. For example, the ionization potential for a neutral lithium (Li) atom is 5.392 V (the first ionization potential); for Li+, 75.638 V (the second ionization potential); and for Li++, 122.451 V (the third ionization potential).

Figure 1. Curve of the change in ionization potentials as a function of the atomic number Z. As Z increases, the values of the ionization potential increase within a period but decrease within a group. The points on the curve correspond to chemical elements.


Shpol’skii, E. V. Atomnaia fizika, vol. 1, 5th ed. Moscow, 1963.
Moore, C. E. Ionization Potentials and Ionization Limits Derived From the Analysis of Optical Spectra. NSRDS-NBS 34. Washington, D. c, 1970.


ionization potential

[‚ī·ə·nə′zā·shən pə′ten·chəl]
(atomic physics)
The energy per unit charge needed to remove an electron from a given kind of atom or molecule to an infinite distance; usually expressed in volts. Also known as ion potential.