electrically charged particles formed by the loss or addition of electrons or other charged particles by atoms or groups of atoms. Such groups of atoms may be molecules, radicals, or other ions. The concept and the term “ion” were introduced in 1834 by M. Faraday, who, while studying the effect of an electric current on aqueous solutions of acids, bases, and salts, assumed that the electrical conductivity of such solutions was due to the movement of ions. Positively charged ions migrating in a solution toward the negative electrode (cathode) were called by Faraday cations, whereas negatively charged ions migrating toward the positive electrode (anode) were called anions.

The charge sign of the ions is designated by plus or minus signs. The magnitude of the ionic charge is a multiple of the charge on the electron. The loss or addition of 1, 2, 3, … electrons leads to the formation by the atom of singly, doubly, or triply charged ions, respectively—for example, Na⁺, Ca²⁺, and A1³⁺ ; C1^-, SO₄^2-. Ions may be part of molecules of a substance. Ions are found as discrete particles in all states of aggregation of matter—gases (particularly in the atmosphere), liquids (in melts and solutions), and crystals.

In gases, ions are formed mainly as the result of impact of high-energy particles or upon photoionization by ultraviolet radiation, X rays, and gamma rays. Ions thus formed are shortlived under ordinary conditions because of their capability of recombination, leading to mutual neutralization. Ionization of atoms and ions at high temperatures (thermal ionization, or thermal dissociation with the loss of an electron) may also occur as an equilibrium process, in which the degree of ionization increases with increasing temperature and decreasing pressure. In this case, the gas passes into the plasma state.

Ions in gases play a large role in many phenomena. Under natural conditions, ions are formed in air by cosmic rays, by solar radiation, or by electric discharge (lightning). The presence of ions, as well as their nature and concentration, affects many physical properties of air and also its physiological activity. Many methods for the experimental study of matter (mass spectroscopy and the use of the cloud chamber) are based on the use of ions. Ionization in gases leads to ions of high chemical activity, which readily enter into interaction with other particles and give rise to various chemical reactions. Low-temperature plasma, which consists of ionized particles, is used in magnetohydrodynamic generators. High-temperature plasma is used in the development of methods for the use of controlled thermonuclear reactions.

Ions in the organism are universal participants in the metabolism. In particular, ions participate in the mechanisms responsible for the permeability of biological membranes, in the regulation of muscular contraction, and in the propagation of the excitation impulses within the nerve tissue. The continuously occurring dissociation of molecules into ions and the opposite process of recombination of the ions into molecules are balanced in the organism in such a manner that the content of ions in the cells and tissue fluids is maintained within certain specific limits. However, this level may be shifted under certain influences.