Ion

History of discovery{{anchor|History of discovery}}

The term ion derives from the Greek ἰέναι (ienai), meaning "to go," introduced by Michael Faraday in 1834 following a suggestion by William Whewell. Whewell also coined the words anode, cathode, anion, and cation, reflecting that ions move toward electrodes of opposite charge: cations go down (κάτω, kato) and anions go up (ἄνω, ano). Faraday used the term for unknown species that traveled between electrodes through an aqueous medium, as metals dissolved at one electrode and emerged at the other. Svante Arrhenius, in his 1884 dissertation, explained that solid crystalline salts dissociate into paired charged particles when dissolved, proposing that ions exist even without an electric current; this work earned him the 1903 Nobel Prize in Chemistry.

Characteristics

In their gas-like state, ions are highly reactive and quickly combine with oppositely charged ions to form neutral molecules or ionic salts. In liquids or solids, salts interact with solvents such as water to produce solvated ions surrounded by a solvation shell; these are more stable due to energy and entropy changes as ions separate from each other. Such stabilized species are common in environments like seawater, derived from dissolved salts. As charged objects, ions attract opposite charges and repel like charges, and their trajectories can be deflected by magnetic fields.

Anions are larger than their parent atom or molecule because excess electrons repel each other and expand the electron cloud; cations are smaller due to a reduced electron cloud. A hydrogen cation consists of a single proton, much smaller than a hydrogen atom. The net charge of an ion equals the number of protons minus the number of electrons. An anion has more electrons than protons, giving a net negative charge; a cation has fewer electrons than protons, giving a net positive charge.

Ions with a −2 charge are called dianions, those with +2 are dications, and +3 are trications. A zwitterion is a neutral molecule with both positive and negative charges at different locations.

Related technology

Ions are created without chemical reactions using ion sources that rely on high voltage or heat, and they appear in devices like mass spectrometers, particle accelerators, ion implanters, and ion engines. These charged particles also help clean air by breaking down microbes and are found in household smoke detectors. Living cells depend on precise ionic gradients across membranes for signaling and metabolism; disrupting this balance kills cells, a trick used by natural and artificial biocides such as gramicidin and the fungicide amphotericin.

Dissolved inorganic ions make up total dissolved solids, a common measure of water quality. Radiation detection often relies on how radiation ionizes gas, creating an ion pair—a positive ion and a free electron. The simplest detector, an ionization chamber, collects all these charges with an electric field. More sensitive devices like Geiger–Müller tubes and proportional counters use a Townsend avalanche, where freed electrons gain enough energy to knock loose more electrons, multiplying the original signal.

Chemistry

In chemical formulas, an ion's net charge is written as a superscript after the molecule or atom, with the magnitude before the sign (e.g., 2+ for a doubly charged cation), but the magnitude is omitted for singly charged ions. Alternative representations include drawing multiple signs or circling the sign. Monatomic ions may be denoted with Roman numerals in spectroscopy, such as Fe III for a doubly charged iron ion, where the Roman numeral indicates formal oxidation state and the superscript numeral indicates net charge; these notations are exchangeable for monatomic ions but Roman numerals do not apply to polyatomic ions.

Radical ions contain unpaired electrons and are very reactive. Polyatomic ions containing oxygen are called oxyanions, and those with at least one carbon–hydrogen bond are organic ions.

Organic ions with charge centered on carbon are termed carbocations (positive) or carbanions (negative). Monatomic ions form by gain or loss of electrons in the valence shell; inner shell electrons are tightly bound and do not participate. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.