V
vacancy See crystal defect.
vacuum A space in which there is a
low pressure of gas, i.e. relatively few
atoms or molecules. A perfect vacuum would contain no atoms or molecules, but this is unobtainable as all
the materials that surround such a
space have a Ünite *vapour pressure.
In a soft (or low) vacuum the pressure is reduced to about 10
–2 pascal,
whereas a hard (or high) vacuum has
a pressure of 10
–2 –10
–7 pascal. Below
10
–7 pascal is known as an ultrahigh
vacuum. See also vacuum pump.
vacuum distillation Distillation
under reduced pressure. The depression in the boiling point of the substance distilled means that the
temperature is lower, which may
prevent the substance from decomposing.
vacuum pump A pump used to reduce the gas pressure in a container.
The normal laboratory rotary oil-seal
pump can maintain a pressure of
10
–1 Pa. For pressures down to 10
–7 Pa
a *diffusion pump is required. *Ion
pumps can achieve a pressure of
10
–9 Pa and a *cryogenic pump combined with a diffusion pump can
reach 10
–13 Pa.
valence See valency.
valence band See energy bands.
valence-bond theory A method
of computational chemistry in which
the electrons are considered as belonging to deÜnite bonds consisting
of pairs of electrons associated with
pairs of atoms in the molecule. The
actual state of the molecule can be
regarded as the result of a set of
canonical forms (see resonance). See
also density-function theory; molecular-orbital theory.
valence electron An electron in
one of the outer shells of an atom
that takes part in forming chemical
bonds.
valency (valence) The combining
power of an atom or radical, equal to
the number of hydrogen atoms that
the atom could combine with or displace in a chemical compound (hydrogen has a valency of 1). It is equal
to the ionic charge in ionic compounds; for example, in Na 2 S,
sodium has a valency of 1 (Na
+ ) and
sulphur a valency of 2 (S
2–
). In covalent compounds it is equal to the
number of bonds formed; in CO 2 oxygen has a valency of 2 and carbon
has a valency of 4.
valine See amino acid.
Valium See diazepam.
vanadium Symbol V. A silverywhite metallic *transition element;
a.n. 23; r.a.m. 50.94; r.d. 5.96; m.p.
1890°C; b.p. 3380°C. It occurs in a
number of complex ores, including
vanadinite (Pb 5 Cl(VO 4 ) 3 ) and carnotite
(K 2 (ClO 2 ) 2 (VO 4 ) 2 ). The pure metal can
be obtained by reducing the oxide
with calcium. The element is used in
a large number of alloy steels. Chemically, it reacts with nonmetals at
high temperatures but is not affected
by hydrochloric acid or alkalis. It
forms a range of complexes with oxidation states from +2 to +5. Vanadium was discovered in 1801 by
Andrés del Rio (1764–1849), who allowed himself to be persuaded that
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vacancy See crystal defect.
vacuum A space in which there is a
low pressure of gas, i.e. relatively few
atoms or molecules. A perfect vacuum would contain no atoms or molecules, but this is unobtainable as all
the materials that surround such a
space have a Ünite *vapour pressure.
In a soft (or low) vacuum the pressure is reduced to about 10
–2 pascal,
whereas a hard (or high) vacuum has
a pressure of 10
–2 –10
–7 pascal. Below
10
–7 pascal is known as an ultrahigh
vacuum. See also vacuum pump.
vacuum distillation Distillation
under reduced pressure. The depression in the boiling point of the substance distilled means that the
temperature is lower, which may
prevent the substance from decomposing.
vacuum pump A pump used to reduce the gas pressure in a container.
The normal laboratory rotary oil-seal
pump can maintain a pressure of
10
–1 Pa. For pressures down to 10
–7 Pa
a *diffusion pump is required. *Ion
pumps can achieve a pressure of
10
–9 Pa and a *cryogenic pump combined with a diffusion pump can
reach 10
–13 Pa.
valence See valency.
valence band See energy bands.
valence-bond theory A method
of computational chemistry in which
the electrons are considered as belonging to deÜnite bonds consisting
of pairs of electrons associated with
pairs of atoms in the molecule. The
actual state of the molecule can be
regarded as the result of a set of
canonical forms (see resonance). See
also density-function theory; molecular-orbital theory.
valence electron An electron in
one of the outer shells of an atom
that takes part in forming chemical
bonds.
valency (valence) The combining
power of an atom or radical, equal to
the number of hydrogen atoms that
the atom could combine with or displace in a chemical compound (hydrogen has a valency of 1). It is equal
to the ionic charge in ionic compounds; for example, in Na 2 S,
sodium has a valency of 1 (Na
+ ) and
sulphur a valency of 2 (S
2–
). In covalent compounds it is equal to the
number of bonds formed; in CO 2 oxygen has a valency of 2 and carbon
has a valency of 4.
valine See amino acid.
Valium See diazepam.
vanadium Symbol V. A silverywhite metallic *transition element;
a.n. 23; r.a.m. 50.94; r.d. 5.96; m.p.
1890°C; b.p. 3380°C. It occurs in a
number of complex ores, including
vanadinite (Pb 5 Cl(VO 4 ) 3 ) and carnotite
(K 2 (ClO 2 ) 2 (VO 4 ) 2 ). The pure metal can
be obtained by reducing the oxide
with calcium. The element is used in
a large number of alloy steels. Chemically, it reacts with nonmetals at
high temperatures but is not affected
by hydrochloric acid or alkalis. It
forms a range of complexes with oxidation states from +2 to +5. Vanadium was discovered in 1801 by
Andrés del Rio (1764–1849), who allowed himself to be persuaded that
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