gem Designating molecules in
which two functional groups are
attached to the same atom in a
molecule. For example, 1,1-
dichloroethane (CH 3 CHCl 2 ) is a gem
dihalide and can be named gem-
dichloroethane. Compare vicinal.
geminate pairA pair of mol-
ecules, ions, etc., in close proximity
surrounded by a solvent cage (see
cage effect).
geochemistryThe scientiÜc study
of the chemical composition of the
earth. It includes the study of the
abundance of the earth’s elements
and their isotopes and the distribu-
tion of the elements in environments
of the earth (lithosphere, atmos-
phere, biosphere, and hydrosphere).
geometrical isomerism See iso-
merism.
geradeSymbol g. Describing a mo-
lecular orbital of a homonuclear di-
atomic molecule with evenparity
(gerade is the German word for even).
This means that during the process
of inversion, the sign of the orbital is
unchanged upon going from any
point in the molecule through the
centre of inversion to the corre-
sponding point on the other side.
The symbol g is written as a sub-
script. The opposite of gerade is
ungerade, symbol u. The symbols g
and u are used to determine selection
rules for diatomic molecules. These
symbols are only applicable to
homonuclear diatomic molecules, as
heteronuclear diatomic molecules,
such as CO, do not have a centre of
inversion.
geraniolAn alcohol, C 9 H 15 CH 2 OH,
present in a number of essential oils.
germaniumSymbol Ge. A lustrous
hard metalloid element belonging to
group 14 (formerly IVB) of the peri-
odic table; a.n. 32; r.a.m. 72.59; r.d.
5.36; m.p. 937°C; b.p. 2830°C. It is
found in zinc sulphide and in certain
other sulphide ores, and is mainly
obtained as a by-product of zinc
smelting. It is also present in some
coal (up to 1.6%). Small amounts are
used in specialized alloys but the
main use depends on its semiconduc-
tor properties. Chemically, it forms
compounds in the +2 and +4 oxida-
tion states, the germanium(IV) com-
pounds being the more stable. The
element also forms a large number
of organometallic compounds. Pre-
dicted in 1871 by *Mendeleev (eka-
silicon), it was discovered by Winkler
in 1886.
A- Information from the WebElements site
German silver (nickel silver)An
alloy of copper, zinc, and nickel,
often in the proportions 5:2:2. It re-
sembles silver in appearance and is
used in cheap jewellery and cutlery
and as a base for silver-plated wire.
See also electrum.getterA substance used to remove
small amounts of other substances
from a system by chemical combina-
tion. For example, a metal such as
magnesium may be used to remove
the last traces of air when achieving
a high vacuum. Various getters are
also employed to remove impurities
from semiconductors.GHBGammahydroxybutyric acid.
See 4-hydroxybutanoic acid.gibberellic acid (GA 3 )A plant
growth substance, abundant in
young actively growing areas of the
plant, that is involved in stem elon-
gation. A *terpene, it was discovered
in 1954. Gibberellic acid and related
growth substances are called gib-
berellins.
Gibbs, Josiah Willard (1839–1903)
US mathematician and physicist,245 Gibbs, Josiah Willard
g