C

cacodylAn oily liquid,
(CH 3 ) 2 AsAs(CH 3 ) 2. It has a characteris-
tic odour of garlick and is poisonous.
Cacodyl was one of theÜrst organo-
metallic compounds to be synthe-
sized (by heating arsenic with
potassium acetate). The group
(CH 3 ) 2 As– is the cacodyl group.

cadmiumSymbol Cd. A soft bluish
metal belonging to *group 12 (for-
merly IIB) of the periodic table; a.n.
48; r.a.m. 112.41; r.d. 8.65; m.p.
320.9°C; b.p. 765°C. The element’s
name is derived from the ancient
name for calamine, zinc carbonate
ZnCO 3 , and it is usually found associ-
ated with zinc ores, such as spha-
lerite (ZnS), but does occur as the
mineral greenockite (CdS). Cadmium
is usually produced as an associate
product when zinc, copper, and lead
ores are reduced. Cadmium is used in
low-melting-point alloys to make sol-
ders, in Ni–Cd batteries, in bearing
alloys, and in electroplating (over
50%). Cadmium compounds are used
as phosphorescent coatings in TV
tubes. Cadmium and its compounds
are extremely toxic at low concentra-
tions; great care is essential where
solders are used or where fumes are
emitted. It has similar chemical prop-
erties to zinc but shows a greater ten-
dency towards complex formation.
The element was discovered in 1817
by F. Stromeyer.

A

• Information from the WebElements site

cadmium cell See weston cell.

cadmium sulphideA water-
insoluble compound, CdS; r.d. 4.82.
It occurs naturally as the mineral

greenockite and is used as a pigment

and in semiconductors andÛuores-

cent materials.

caesiumSymbol Cs. A soft silvery-

white metallic element belonging to

*group 1 (formerly IA) of the periodic

table; a.n. 55; r.a.m. 132.905; r.d.

1.88; m.p. 28.4°C; b.p. 678°C. It oc-

curs in small amounts in a number

of minerals, the main source being

carnallite (KCl.MgCl 2 .6H 2 O). It is ob-

tained by electrolysis of molten cae-

sium cyanide. The natural isotope is

caesium–133. There are 15 other

radioactive isotopes. Caesium–137

(half-life 33 years) is used as a gamma

source. As the heaviest alkali metal,

caesium has the lowest ionization po-

tential of all elements, hence its use

in photoelectric cells, etc.

A

• Information from the WebElements site
  caesium chloride structure A
  type of ionic crystal structure in
  which the anions are at the eight cor-
  ners of a cubic unit cell with one
  cation at the centre of the cell. It can
  equivalently by described as cations
  at the corners of the cell with an
  anion at the centre. Each type of ion
  has a coordination number of 8. Ex-
  amples of compounds with this
  structure are CsCl, CsBr, CsI, CsCN,
  CuZn, and NH 4 Cl.
  caesium clockAn *atomic clock
  that depends on the energy differ-
  ence between two states of the cae-
  sium–133 nucleus when it is in a
  magneticÜeld. In one type, atoms of
  caesium–133 are irradiated with
  radio-frequency radiation, whose fre-
  quency is chosen to correspond to