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The Collaborative International Dictionary of English v.0.48 (gcide)

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.]