ionization of a solvent the ionic product is also called the autoprotolysis
constant. See also solubility product;
ph scale.
ionic radius A value assigned to
the radius of an ion in a crystalline
solid, based on the assumption that
the ions are spherical with a deÜnite
size. X-ray diffraction can be used to
measure the internuclear distance in
crystalline solids. For example, in
NaF the Na – F distance is 0.231 nm,
and this is assumed to be the sum of
the Na
+ and F
– radii. By making certain assumptions about the shielding
effect that the inner electrons have
on the outer electrons, it is possible
to assign individual values to the
ionic radii – Na
+ 0.096 nm; F
– 0.135
nm. In general, negative ions have
larger ionic radii than positive ions.
The larger the negative charge, the
larger the ion; the larger the positive
charge, the smaller the ion. See also
hydrodynamic radius.
ionic strength Symbol I. A function expressing the effect of the
charge of the ions in a solution,
equal to the sum of the molality of
each type of ion present multiplied
by the square of its charge. I = Σm i z i
2 .
ionization The process of producing *ions. Certain molecules (see
electrolyte) ionize in solution; for
example, *acids ionize when dissolved in water (see also solvation):
HCl → H
+ + Cl
–
Electron transfer also causes ionization in certain reactions; for example, sodium and chlorine react by the
transfer of a valence electron from
the sodium atom to the chlorine
atom to form the ions that constitute
a sodium chloride crystal:
Na + Cl → Na
+ Cl
–
Ions may also be formed when an
atom or molecule loses one or more
electrons as a result of energy gained
in a collision with another particle or
a quantum of radiation (see photoionization). This may occur as a
result of the impact of *ionizing radiation or of thermal ionization and
the reaction takes the form
A → A
+ + e
Alternatively, ions can be formed by
electron capture, i.e.
A + e → A
–
ionization energy (IE) See ionization potential.
ionization gauge A vacuum gauge
consisting of a three-electrode system
inserted into the container in which
the pressure is to be measured. Electrons from the cathode are attracted
to the grid, which is positively biased. Some pass through the grid but
do not reach the anode, as it is maintained at a negative potential. Some
of these electrons do, however, collide with gas molecules, ionizing
them and converting them to positive ions. These ions are attracted to
the anode; the resulting anode current can be used as a measure of the
number of gas molecules present.
Pressure as low as 10
–6 pascal can be
measured in this way.
ionization potential (IP) Symbol I.
The minimum energy required to remove an electron from a speciÜed
atom or molecule to such a distance
that there is no electrostatic interaction between ion and electron. Originally deÜned as the minimum
potential through which an electron
would have to fall to ionize an atom,
the ionization potential was measured in volts. It is now, however,
deÜned as the energy to effect an ionization and is conveniently measured
in electronvolts (although this is not
an SI unit) or joules per mole. The
synonymous term ionization energy
(IE) is often used.
The energy to remove the least
ionic radius
290
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constant. See also solubility product;
ph scale.
ionic radius A value assigned to
the radius of an ion in a crystalline
solid, based on the assumption that
the ions are spherical with a deÜnite
size. X-ray diffraction can be used to
measure the internuclear distance in
crystalline solids. For example, in
NaF the Na – F distance is 0.231 nm,
and this is assumed to be the sum of
the Na
+ and F
– radii. By making certain assumptions about the shielding
effect that the inner electrons have
on the outer electrons, it is possible
to assign individual values to the
ionic radii – Na
+ 0.096 nm; F
– 0.135
nm. In general, negative ions have
larger ionic radii than positive ions.
The larger the negative charge, the
larger the ion; the larger the positive
charge, the smaller the ion. See also
hydrodynamic radius.
ionic strength Symbol I. A function expressing the effect of the
charge of the ions in a solution,
equal to the sum of the molality of
each type of ion present multiplied
by the square of its charge. I = Σm i z i
2 .
ionization The process of producing *ions. Certain molecules (see
electrolyte) ionize in solution; for
example, *acids ionize when dissolved in water (see also solvation):
HCl → H
+ + Cl
–
Electron transfer also causes ionization in certain reactions; for example, sodium and chlorine react by the
transfer of a valence electron from
the sodium atom to the chlorine
atom to form the ions that constitute
a sodium chloride crystal:
Na + Cl → Na
+ Cl
–
Ions may also be formed when an
atom or molecule loses one or more
electrons as a result of energy gained
in a collision with another particle or
a quantum of radiation (see photoionization). This may occur as a
result of the impact of *ionizing radiation or of thermal ionization and
the reaction takes the form
A → A
+ + e
Alternatively, ions can be formed by
electron capture, i.e.
A + e → A
–
ionization energy (IE) See ionization potential.
ionization gauge A vacuum gauge
consisting of a three-electrode system
inserted into the container in which
the pressure is to be measured. Electrons from the cathode are attracted
to the grid, which is positively biased. Some pass through the grid but
do not reach the anode, as it is maintained at a negative potential. Some
of these electrons do, however, collide with gas molecules, ionizing
them and converting them to positive ions. These ions are attracted to
the anode; the resulting anode current can be used as a measure of the
number of gas molecules present.
Pressure as low as 10
–6 pascal can be
measured in this way.
ionization potential (IP) Symbol I.
The minimum energy required to remove an electron from a speciÜed
atom or molecule to such a distance
that there is no electrostatic interaction between ion and electron. Originally deÜned as the minimum
potential through which an electron
would have to fall to ionize an atom,
the ionization potential was measured in volts. It is now, however,
deÜned as the energy to effect an ionization and is conveniently measured
in electronvolts (although this is not
an SI unit) or joules per mole. The
synonymous term ionization energy
(IE) is often used.
The energy to remove the least
ionic radius
290
i
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