Dictionary of Chemistry [6th Ed.]

1802 by Joseph *Gay-Lussac. Thus the
law is also known as Gay-Lussac’s
law. An equation similar to that
given above applies to pressures for
ideal gases: p = p 0 (1 + t/273), a rela-
tionship known as Charles’ law of
pressures. See also gas laws.

cheddite Any of a group of high ex-
plosives made from nitro compounds
mixed with sodium or potassium
chlorate.

chelateAn inorganic complex in
which a *ligand is coordinated to a
metal ion at two (or more) points, so
that there is a ring of atoms includ-
ing the metal. The process is known
as chelation. Chelating agents are
classiÜed according to the number of
points at which they can coordinate.
A ligand such as diaminoethane
(H 2 N(CH 2 ) 2 NH 2 ), which can coordi-
nate at two points, is said to be
bidentate. The angle made between
two bonds coordinating to a metal
atom is the bite angle of the ligand.
See also sequestration.

115 chemical bond

c

H 2 C

H 2 C

NH 2

Cu^2 +

NH 2

NH 2

CH 2

CH 2

NH 2

Chelate

chelate effect The effect in which
a chelate complex is generally more
stable than the analogous complex
formed with monodentate ligands.
For example, the complex ion [Cu(en)
(OH 2 ) 4 ]2+is more stable than the com-
plex ion [Cu(NH 3 ) 2 (OH 2 ) 4 ]2+. Here, en
denotes the bidentate ethylene di-
amine (1,2-diaminoethane) ligand.
The main cause of the chelate effect
is the effect of reaction entropy
when the complex is formed. Thus,
the reaction

[Cu(OH 2 ) 6 ]2++ en →[Cu(en)(OH 2 ) 4 +

2H 2 O

results in a net increase in the num-

ber of molecules (from 2 to 3). The

reaction

[Cu(OH 2 ) 6 ]2++ 2NH 3 →

Cu(NH 3 )2(OH 2 ) 4 + 2H 2 O

involves no net increase in the num-

ber of molecules. As a result, the

chelate reaction has a larger reaction

entropy and is more favourable.

cheletropic reaction A type of ad-

dition reaction in which a conjugated

molecule forms two single bonds

from terminal atoms of the conju-

gated system to a single atom on an-

other molecule to give a cyclic

adduct. Cheletropic reactions are

types of *pericyclic reaction.

ChemDrawA widely used chemi-

cal drawing and modelling program

produced by CambridgeSoft.

A

• The CambridgeSoft website, giving
  details about ChemDraw and associated
  software

chemical bondA strong force of

attraction holding atoms together in

a molecule or crystal. Typically chem-

ical bonds have energies of about

1000 kJ mol–1and are distinguished

from the much weaker forces be-

tween molecules (see van der waals’

force). There are various types.

Ionic(or electrovalent) bonds can be

formed by transfer of electrons. For

instance, the calcium atom has an

electron conÜguration of [Ar]4s^2 , i.e.

it has two electrons in its outer shell.

The chlorine atom is [Ne]3s^23 p^5 , with

seven outer electrons. If the calcium

atom transfers two electrons, one to

each chlorine atom, it becomes a

Ca2+ion with the stable conÜgura-

tion of an inert gas [Ar]. At the same

time each chlorine, having gained

one electron, becomes a Cl–ion, also