G

GABASee gamma-aminobutyric
acid.

Gabriel reaction A method of
making a primary *amine (free from
any secondary or tertiary amine im-
purities) from a haloalkane (alkyl
halide) using potassium phthalimide.
It is named after Siegmund Gabriel
(1851–1924).

gadoliniumSymbol Gd. A soft sil-
very metallic element belonging to
the *lanthanoids; a.n. 64; r.a.m.
157.25; r.d. 7.901 (20°C); m.p. 1313°C;
b.p. 3266°C. It occurs in gadolinite,
xenotime, monazite, and residues
from uranium ores. There are seven
stable natural isotopes and eleven
artiÜcial isotopes are known. Two of
the natural isotopes, gadolinium–155
and gadolinium–157, are the best
neutron absorbers of all the el-
ements. The metal has found limited
applications in nuclear technology
and in ferromagnetic alloys (with
cobalt, copper, iron, and cerium).
Gadolinium compounds are used in
electronic components. The element
was discovered by Jean de Marignac
(1817–94) in 1880.

A

• Information from the WebElements site

galactoseA simple sugar, C 6 H 12 O 6 ,
stereoisomeric with glucose, that oc-
curs naturally as one of the products
of the enzymic digestion of milk
sugar (lactose) and as a constituent of
gum arabic.

galenaA mineral form of lead(II)
sulphide, PbS, crystallizing in the
cubic system; the chief ore of lead. It
usually occurs as grey metallic cubes,

frequently in association with silver,

arsenic, copper, zinc, and antimony.

Important deposits occur in Australia

(at Broken Hill), Germany, the USA

(especially in Missouri, Kansas, and

Oklahoma), and the UK.

gallic acid (3,4,5-trihydroxybenzoic

acid)A colourless crystalline aro-

matic compound, C 6 H 2 (OH) 3 COOH;

m.p. 253°C. It occurs in wood, oak

galls, and tea, and is a component of

tannins. It can be made from tannin

by acid hydrolysis or fermentation.

Gallic acid is used to make ink and

various dyes. On heating it yields

*pyrogallol.

OH

OH

OH

OH O

Gallic acid

galliumSymbol Ga. A soft silvery

metallic element belonging to group

13 (formerly IIIB) of the periodic

table; a.n. 31; r.a.m. 69.72; r.d. 5.90

(20°C); m.p. 29.78°C; b.p. 2403°C. It

occurs in zinc blende, bauxite, and

kaolin, from which it can be ex-

tracted by fractional electrolysis. It

also occurs in gallite, CuGaS 2 , to an

extent of 1%; although bauxite only

contains 0.01% this is the only com-

mercial source. The two stable iso-

topes are gallium–69 and gallium–71;

there are eight radioactive isotopes,

all with short half-lives. The metal