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Ion I"on ([imac]"[o^]n), n. [Gr. 'io`n, neut, of 'iw`n, p. pr. of 'ie`nai to go.] 1. (Elec. Chem.) an atom or goup of atoms (radical) carrying an electrical charge. It is contrasted with neutral atoms or molecules, and free radicals. Certain compounds, such as sodium chloride, are composed of complementary ions in the solid (crystalline) as well as in solution. Others, notably acids such as hydrogen chloride, may occur as neutral molecules in the pure liquid or gas forms, and ionize almost completely in dilute aqueous solutions. In solutions (as in water) ions are frequently bound non-covalently with the molecules of solvent, and in that case are said to be solvated. According to the electrolytic dissociation theory, the molecules of electrolytes are divided into ions by water and other solvents. An ion consists of one or more atoms and carries one unit charges of electricity, 3.4 x 10^{-10} electrostatic units, or a multiple of this. Those which are positively electrified (hydrogen and the metals) are called {cations}; negative ions (hydroxyl and acidic atoms or groups) are called {anions}.

Note: Thus, hydrochloric acid ({HCl}) dissociates, in aqueous solution, into the hydrogen ion, {H+}, and the chlorine ion, {Cl-}; ferric nitrate, {Fe(NO3)3}, yields the ferric ion, {Fe+++}, and nitrate ions, {NO3-}, {NO3-}, {NO3-}. When a solution containing ions is made part of an electric circuit, the cations move toward the cathode, the anions toward the anode. This movement is called migration, and the velocity of it differs for different kinds of ions. If the electromotive force is sufficient, electrolysis ensues: cations give up their charge at the cathode and separate in metallic form or decompose water, forming hydrogen and alkali; similarly, at the anode the element of the anion separates, or the metal of the anode is dissolved, or decomposition occurs. Aluminum and chlorine are elements prepared predominantly by such electrolysis, and depends on dissolving compounds in a solvent where the element forms ions. Electrolysis is also used in refining other metals, such as copper and silver. Cf. {Anion}, {Cation}. [Webster 1913 Suppl.]

2. One of the small electrified particles into which the molecules of a gas are broken up under the action of the electric current, of ultraviolet and certain other rays, and of high temperatures. To the properties and behavior of ions the phenomena of the electric discharge through rarefied gases and many other important effects are ascribed. At low pressures the negative ions appear to be electrons; the positive ions, atoms minus an electron. At ordinary pressures each ion seems to include also a number of attached molecules. Ions may be formed in a gas in various ways. [Webster 1913 Suppl.]