in physics and chemistry, a duplex of charged particles (ordinarily charged atoms or molecules), one positive, the other negative. An ion pair, for the physicist, is the positively charged particle (positive ion) and the negatively charged particle (negative ion) simultaneously produced by the addition of sufficient energy to a neutral atom or molecule to cause it to dissociate into oppositely charged fragments. Thus, an energetic electron passing near or through an oxygen molecule, O₂, may force one of the electrons out of the molecule. The result is an ion pair consisting of the positive oxygen ion, O₂⁺, and the negative detached electron, e^-.

An ion pair, in the context of chemistry, consists of a positive ion and a negative ion temporarily bonded together by the electrostatic force of attraction between them. Ion pairs occur in concentrated solutions of electrolytes (substances that conduct electricity when dissolved or molten). Thus, in concentrated solutions of sodium chloride, some positive sodium ions, Na⁺, and some negative chloride ions, Cl^-, are paired together. Upon colliding, two oppositely charged ions stay together only for a short period of time. On the average, a certain population of these pairs exists at any given time, although the formation and dissociation of ion-pairs is continuous.