engines to lower its freezing point so
that it does not solidify at sub-zero
temperatures. The commonest an-
tifreeze is *ethane-1,2-diol (ethylene
glycol).
antigorite See serpentine.
anti-isomerSee isomerism.
antiknock agentA petrol additive
that inhibits preignition (‘knocking’)
in internal-combustion engines. Anti-
knock agents work by retarding
combustion chain reactions. The
commonest, lead(IV) tetraethyl,
causes environmental pollution, and
its use is being discouraged.
antimonic compoundsCom-
pounds of antimony in its +5 oxida-
tion state; e.g. antimonic chloride is
antimony(V) chloride (SbCl 5 ).
antimonous compoundsCom-
pounds of antimony in its +3 oxida-
tion state; e.g. antimonous chloride is
antimony(III) chloride (SbCl 3 ).
antimonySymbol Sb. An element
belonging to *group 15 (formerly VB)
of the periodic table; a.n. 51; r.a.m.
121.75; r.d. 6.68; m.p. 630.5°C; b.p.
1750 °C. Antimony has several al-
lotropes. The stable form is a bluish-
white metal. Yellow antimony and
black antimony are unstable non-
metallic allotropes made at low tem-
peratures. The main source is
stibnite (Sb 2 S 3 ), from which anti-
mony is extracted by reduction with
iron metal or by roasting (to give the
oxide) followed by reduction with
carbon and sodium carbonate. The
main use of the metal is as an alloy-
ing agent in lead-accumulator plates,
type metals, bearing alloys, solders,
Britannia metal, and pewter. It is also
an agent for producing pearlitic cast
iron. Its compounds are used in
Ûame-prooÜng, paints, ceramics,
enamels, glass dyestuffs, and rubber
technology. The element will burn in
air but is unaffected by water or di-
lute acids. It is attacked by oxidizing
acids and by halogens. It wasÜrst re-
ported by Tholden in 1450.
A- Information from the WebElements site
antioxidantsSubstances that slow
the rate of oxidation reactions. Vari-
ous antioxidants are used to preserve
foodstuffs and to prevent the deterio-
ration of rubber, synthetic plastics,
and many other materials. Some an-
tioxidants act as chelating agents to
sequester the metal ions that catalyse
oxidation reactions. Others inhibit
the oxidation reaction by removing
oxygen free radicals. Naturally occur-
ring antioxidants include *vitamin E
and β-carotene; they limit the cell
and tissue damage caused by foreign
substances, such as toxins and pollu-
tants, in the body.antiparallel spinsNeighbouring
spinning electrons in which the
*spins, and hence the magnetic mo-
ments, of the electrons are aligned in
the opposite direction. Under some
circumstances the interactions be-
tween magnetic moments in atoms
favour *parallel spins, while under
other conditions they favour antipar-
allel spins. The case of antiferromag-
netism (see magnetism) is an example
of a system with antiparallel spins.antiperiplanarSee torsion angle.
anti-Stokes radiationElectro-
magnetic radiation occurring in the
*Raman effect, which is of much
lower intensity than the Rayleigh
scattering in which the frequency of
the radiation is higher than that of
the incident light, i.e. displaced to-
wards shorter wavelengths. If the fre-
quency of the original light is ν, the
frequency of the anti-Stokes radia-
tion is ν+ νk, where νkis the fre-
quency of the rotation or vibration of
the molecule. The spectral lines asso-41 anti-Stokes radiation
a