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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-.