ion

/uy"euhn, uy"on/, n. Physics, Chem.

1. an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons, as a cation (positive ion), which is created by electron loss and is attracted to the cathode in electrolysis, or as an anion (negative ion), which is created by an electron gain and is attracted to the anode. The valence of an ion is equal to the number of electrons lost or gained and is indicated by a plus sign for cations and a minus sign for anions, thus: Na⁺, Cl-, Ca++, S⁼.

2. one of the electrically charged particles formed in a gas by electric discharge or the like.

[ < Gk ión going, neut. prp. of iénai to go; term introduced by Michael Faraday in 1834]

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Atom or group of atoms with one or more positive or negative electric charges.

Positively charged ions are cations, negatively charged ones anions. Ions are formed when electrons are added to or removed from neutral molecules or other ions, as sodium (Na) and chlorine (Cl) atoms react to form Na⁺ and Cl^-; when ions combine with other particles, as hydrogen cations (H⁺) and ammonia (NH₃) combine to form ammonium cations (NH₄⁺); or when a covalent bond between two atoms is ruptured in such a way that the resulting particles are charged, as water (H₂O) dissociates (see dissociation) into hydrogen and hydroxide ions (H⁺ and OH^-). Many crystalline substances (see crystal) are composed of ions held in regular geometric patterns by the attraction of oppositely charged particles for each other. Ions migrate to the electrode of opposite charge in an electric field and are the conductors of current in electrolytic cells (see electrolysis). Compounds that form ions are called electrolytes. Ions are also formed in gases when heated to very high temperatures or when an electrical discharge passes through them (see plasma).

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▪ physics

      any atom or group of atoms that bears one or more positive or negative electrical charges. Positively charged ions are called cations; (cation) negatively charged ions, anions (anion). Ions are formed by the addition of electrons to, or the removal of electrons from, neutral atoms or molecules or other ions; by combination of ions with other particles; or by rupture of a covalent bond between two atoms in such a way that both of the electrons of the bond are left in association with one of the formerly bonded atoms. Examples of these processes include the reaction of a sodium atom with a chlorine atom to form a sodium cation and a chloride anion; the addition of a hydrogen cation to an ammonia molecule to form an ammonium cation; and the dissociation of a water molecule to form a hydrogen cation and a hydroxide anion.

      Many crystalline substances are composed of ions held in regular geometric patterns by the attraction of the oppositely charged particles for each other. Ions migrate under the influence of an electrical field and are the conductors of electric current in electrolytic cells.

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