due to the presence of impurities. It
forms well-shaped orthorhombic
crystals. It occurs in the USA, Spain,
and SW Africa.cetaneSee hexadecane.cetane numberA number that
provides a measure of the ignition
characteristics of a Diesel fuel when
it is burnt in a standard Diesel en-
gine. It is the percentage of cetane
(hexadecane) in a mixture of cetane
and 1-methylnaphthalene that has
the same ignition characteristics as
the fuel being tested. Compare oc-
tane number.CFCSee chlorofluorocarbon.CGECapillary gel electrophoresis.
See capillary electrophoresis.c.g.s. unitsA system of *units
based on the centimetre, gram, and
second. Derived from the metric sys-
tem, it was badly adapted to use with
thermal quantities (based on the in-
consistently deÜned *calorie) and
with electrical quantities (in which
two systems, based respectively on
unit permittivity and unit permeabil-
ity of free space, were used). For sci-
entiÜc purposes c.g.s. units have now
been replaced by *SI units.chainA line of atoms of the same
type in a molecule. In a straight chain
the atoms are attached only to single
atoms, not to groups. Propane, for in-
stance, is a straight-chain alkane,
CH 3 CH 2 CH 3 , with a chain of three
carbon atoms. A branched chain is
one in which there are side groups
attached to the chain. Thus, 3-ethyl-
octane, CH 3 CH 2 CH(C 2 H 5 )C 5 H 11 , is a
branched-chain alkane in which
there is a side chain (C 2 H 5 ) attached
to the third carbon atom. A closed
chainis a *ring of atoms in a mol-
ecule; otherwise the molecule has an
open chain.Chain, Sir Ernst Boris (1906–79)German-born British biochemist,
who began his research career at
Cambridge University in 1933. Two
years later he joined *Florey at Ox-
ford, where they isolated and
puriÜed *penicillin. They also devel-
oped a method of producing the drug
in large quantities and carried out its
Ürst clinical trials. The two men
shared the 1945 Nobel Prize for phys-
iology or medicine with penicillin’s
discoverer, Alexander *Fleming.chain reaction A reaction that is
self-sustaining as a result of the prod-
ucts of one step initiating a subse-
quent step. Chemical chain reactions
usually involve free radicals as inter-
mediates. An example is the reaction
of chlorine with hydrogen initiated
by ultraviolet radiation. A chlorine
molecule isÜrst split into atoms:
Cl 2 →Cl•+ Cl•
These react with hydrogen as follows
Cl•+ H 2 →HCl + H•H•+ Cl 2 →HCl + Cl•etc.
Combustion and explosion reactions
involve similar free-radical chain re-
actions.chairSee ring conformations.chair conformationSee confor-
mation.
chalcedonyA mineral consisting
of a microcrystalline variety of
*quartz. It occurs in several forms,
including a large number of semi-
precious gemstones; for example,
sard, carnelian, jasper, onyx, chryso-
prase, agate, and tiger’s-eye.chalcogensSee group 16 elements.
chalconidesBinary compounds
formed between metals and group 16
elements; i.e. oxides, sulphides, se-
lenides, and tellurides.chalcopyrite(copper pyrites)A
brassy yellow mineral consisting of acetane 112c