IONIZATION POTENTIAL

also called Ionization Energy, in chemistry, the amount of energy required to remove an electron from an isolated atom or molecule. There is an ionization potential for each successive electron removed; the ionization potential associated with removal of the first (most loosely held) electron, however, is most commonly used. The ionization potential of a chemical element, expressed in electron volts, is usually measured in an electric discharge tube in which a fast-moving electron generated by an electric current collides with a gaseous atom of the element, causing it to eject one of its electrons. For a hydrogen atom, composed of an orbiting electron bound to a nucleus of one proton, an ionization energy of 13.6 electron volts is required to force the electron from its lowest energy level entirely out of the atom. The magnitude of the ionization potential of an element is dependent on the combined effects of the electric charge of the nucleus, the size of the atom, and its electronic configuration. Among the chemical elements, removal of an electron is hardest for the noble gases and easiest for the alkali metals. The ionization energy required for removal of electrons increases progressively as the atom loses electrons, because the positive charge on the nucleus of the atom does not change, and therefore, with each removal of an electron, the remainder are held more firmly. The ionization potential is a measure of the capability of an element to enter into chemical reactions requiring ion formation or donation of electrons. It is also generally related to the nature of the chemical bonding in the compounds formed by the elements. See also binding energy; electron affinity.