temporary hardness See hard-
ness of water.
tera-Symbol T. A preÜx used in the
metric system to denote one million
million times. For example, 10^12 volts
= 1 teravolt (TV).
terbium Symbol Tb. A silvery
metallic element belonging to the
*lanthanoids; a.n. 65; r.a.m. 158.92;
r.d. 8.23 (20°C); m.p. 1356°C; b.p.
3123 °C. It occurs in apatite and xeno-
time, from which it is obtained by an
ion-exchange process. There is only
one natural isotope, terbium–159,
which is stable. Seventeen artiÜcial
isotopes have been identiÜed. It is
used as a dopant in semiconducting
devices. It was discovered by Carl
Mosander (1797–1858) in 1843.
A
- Information from the WebElements site
terephthalic acid (1,4-benzenedi-
carboxylic acid)A colourless crys-
talline solid, C 6 H 4 (COOH) 2 , m.p.
300 °C. It is made by oxidizing p-
xylene (1,4-dimethylebenzene) and
used for making polyesters, such as
Terylene.
term An electronic energy level in
an atom. A term is characterized by a
term symbol, which is given by a cap-
ital letter indicating the total orbital
angular momentum L of the atom,
with a left superscript that gives the
value of 2S+ 1, where S is the total
spin angular momentum of the
atom. Analogously to the letters used
for single electron angular momen-
tum the letters S, P, D, F correspond
to L = 0, 1, 2, 3 respectively. For ex-
ample, a term for which L = 1 and S =
1 has the term symbol^3 P.
In the absence of *spin–orbit cou-
pling the degeneracy of a term is
(2L+ 1)(2S+ 1). In the presence of
spin–orbit coupling a term splits up
into several closely spaced energy
levels, with this set of closely spaced
energy levels being called a multiplet.
Themultiplicityof a multiplet is
given by (2S+ 1) since the number of
atomic energy levels a term splits
into because of spin–orbit coupling is
(2S+ 1). This is the case because the
total electronic angular momentum J
of an atom can have the (2S+ 1) pos-
sible values: L+S, L+S–1, etc. Each
level in a multiplet has the value of J
written as a right subscript to the
term symbol. The degeneracy of a
level in a multiplet is 2J+ 1. The
names given to multiplets with 2S+1
= 1, 2, 3, 4 are singlet,doublet,
triplet,quartet, respectively, to corre-
spond to the number of atomic levels
in the multiplet. For example,^3 P 1 is
the J = 1 level in a P triplet.
ternary compound A chemical
compound containing three different
elements.terpenesA group of unsaturated
hydrocarbons present in plants (see
essential oil). Terpenes consist of
isoprene units, CH 2 :C(CH 3 )CH:CH 2.
Monoterpenes have two units,
C 10 H 16 , sesquiterpenes three units,
C 15 H 24 , diterpenes four units, C 20 H 32 ,
etc. Terpenoids, which are derivatives
of terpenes, include abscisic acid and
gibberellin (plant growth substances)
and the *carotenoid and *chloro-
phyll pigments.
A- Information about IUPAC nomenclature
terpineneA cyclic *terpene,
C 10 H 16 , used in perfumery and as a
Ûavouring. It exists in three isomeric
forms. α-Terpinene is an oily liquid
that smells of lemons, b.p. 182°C. It
occurs in the herbs cardamom, co-
riander and marjoram although it is
usually extracted from the essential
oils of other plants. β-Terpinene, b.p.
174 °C, is generally synthesized from
oil of savin. γ-Terpinene, b.p. 182°C,
occurs in various plant oils, including
521 terpinene
t