Dictionary of Chemistry [6th Ed.]

or group approaches the molecule as

the other leaves. The transition state

is an intermediate state in which

both attacking and leaving groups

are partly bound to the molecule, e.g.

B + RA →B—-R—-A →BR + A

In the theory of reaction rates, the

reactants are assumed to be in equi-

librium with this activated complex,

which decomposes to the products.

transmission electron micro-

scopeSee electron microscope.

transmutationThe transforma-

tion of one element into another by

bombardment of nuclei with parti-

cles. For example, plutonium is ob-

tained by the neutron bombardment

of uranium.

transport coefÜcientsQuantities

that characterize transport in a sys-

tem. Examples of transport coefÜ-

cients include electrical and thermal

conductivity. One of the main pur-

poses of *nonequilibrium statistical

mechanics is to calculate such coefÜ-

cients fromÜrst principles. It is

difÜcult to calculate transport coefÜ-

cients exactly for noninteracting sys-

tems and it is therefore necessary to

use approximation techniques. A

transport coefÜcient gives a measure

forÛow in a system. An inverse

transport coefÜcientgives a measure

of resistance toÛow in a system.

transport numberSymbol t. The

fraction of the total charge carried by

a particular type of ion in the con-

duction of electricity through elec-

trolytes.

transuranic elementsElements

with an atomic number greater than

92, i.e. elements above uranium in

the *periodic table. Most of these el-

ements are unstable and have short

half-lives. See also transactinide el-

ements.

triacetone triperoxide (TATP) An

organic peroxide, C 9 H 18 O 6 , derived

from acetone; r.d. 1.18; m.p. 91°C. It

is a highly explosive white crystalline

substance, very sensitive to heat and

shock. It is favoured by some terror-

ist groups because it can easily be

made from commonly available com-

pounds. For example, it is probable

that it was used in the London bomb-

ings of 7 July 2005.

triacylglycerol See triglyceride.

triatomic moleculeA molecule

formed from three atoms (e.g. H 2 O or

CO 2 ).

triazineSee azine.

transmission electron microscope 534

t

O

O

O

O

O

O

3 CH CH 3

3 CH

CH 3

CH 3

CH 3

Triacetone triperoxide

Triazine

tribologyThe study of friction, lu-

brication, and lubricants.

triboluminescence*Lumines-

cence caused by friction; for exam-

ple, some crystalline substances emit

light when they are crushed as a re-

sult of static electric charges gener-

ated by the friction.

tribromomethane (bromoform) A

colourless liquid *haloform, CHBr 3 ;

r.d. 2.9; m.p. 8°C; b.p. 150°C.

tricarbon dioxide (carbon subox-

ide)A colourless gas, C 3 O 2 , with an