ion channel Enables ions to flow rapidly through
membranes in a thermodynamically downhill direction
after an electrical or chemical impulse. Their structures
usually consist of four to six membrane-spanning
DOMAINs. This number determines the size of the pore
and thus the size of the ion to be transported.
See alsoION PUMP.
ion exchange A reversible process where ions are
released from an insoluble permanent material in
exchange for other ions in a surrounding solution. The
direction of the exchange depends on (a) the affinities
of the ion exchanger for the ions present and (b) the
concentrations of the ions in the solution. In water
treatment, this process adds and removes ions from
water. In nuclear chemistry, it is a common method for
concentrating uranium from a solution. The uranium
solution is passed through a resin bed, where the ura-
nium-carbonate complex ions are transferred to the
resin by exchange with a negative ion like chloride.
After buildup of the uranium complex on the resin, the
uranium is eluted with a salt solution, and the uranium
is precipitated in another process.
ionic bond A chemical bond or link between two
atoms due to an attraction between oppositely charged
(positive-negative) ions.
ionic bonding Chemical bonding that results when
one or more electrons from one atom or a group of
atoms is transferred to another. Ionic bonding occurs
between charged particles.
ionic compounds Compounds where two or more
ions are held next to each other by electrical attraction.
ionic radius The radius of an ion.
ionic strength I (SI unit: mol dm–3) In a solution of
fully dissociated electrolytes, the ionic strength is
defined as I = 0.5 ΣiciZi^2 , in which ciis the concentra-
tion and Zithe charge number of ionic species i. μis
also defined as Im = 0.5 ΣimiZi^2 , where mi is the
molality.ionization The generation of one or more ions. It can
occur by loss of an electron from a neutral MOLECULAR
ENTITY, by the UNIMOLECULARheterolysis of such an
entity into two or more ions, or by a heterolytic SUBSTI-
TUTION REACTIONinvolving neutral molecules, such as
CH 3 CO 2 H + H 2 O →H 3 O++ CH 3 CO 2 –
Ph 3 CCl + AlCl 3 →Ph 3 C++ AlCl 4 – (electrophile
assisted)
Ph 3 CCl →Ph 3 C+Cl– (ion pair, in benzene)
The loss of an electron from a singly, doubly, etc.,
charged cation is called second, third, etc., ionization.
This terminology is used especially in mass spectroscopy.
See alsoDISSOCIATION; HETEROLYSIS; IONIZATION
ENERGY.ionization constant The equilibrium constant for
the ionization of a weak electrolyte. An example is
1.75 × 10 –5for acetic acid (CH 3 COOH).ionization energyEi (SI unit: kJ mol–1 or J per
molecule) The minimum energy required to remove
an electron from an isolated MOLECULAR ENTITY(in its
vibrational GROUND STATE) in the gaseous phase. If the
resulting molecular entity is considered to be in its
vibrational ground state, one refers to the energy as the
“adiabatic ionization energy.” If the molecular entity
produced possesses the vibrational energy determined
by the Franck-Condon principle (according to which
the electron ejection takes place without an accompa-
nying change in molecular geometry), the energy is
called the “vertical ionization energy.” The name “ion-
ization energy” is preferred to the somewhat mislead-
ing earlier name “ionization potential.”
See alsoIONIZATION.ionizing power A term to denote the tendency of a
particular solvent to promote IONIZATION of an
uncharged or, less often, charged solute. The term has
been used both in a kinetic and in a thermodynamic
context.ionizing power 143