Dictionary of Chemistry [6th Ed.]

perfumes, to scent tobacco, and as an
anticoagulant in medicine; *warfarin
is derived from it.

coumarone See benzfuran.

countercurrent ÛowFlow of two
Ûuids in opposite directions with
transfer of heat or matter between
them. Compare cocurrent flow.

counter ionAn ion of opposite
charge to a given ion. For example,
in a crystal of sodium chloride, the
chloride ions can be regarded as
counter ions to the sodium ions. In
certain colloids, the charge on the
surface of colloidal particles is neu-
tralized by oppositely charged
counter ions in the surrounding solu-
tion.

coupling 1.An interaction be-
tween two different parts of a system
or between two or more systems.
Examples of coupling in the spectra
of atoms and nuclei are Russell–
Saunders coupling, *j-j coupling, and
spin–orbit coupling. In the spectra of
molecules there areÜve idealized
ways (called the Hund coupling cases)
in which the different types of angu-
lar momentum in a molecule (the
electron orbital angular momentum
L, the electron spin angular momen-
tum S, and the angular momentum
of nuclear rotation N) couple to form
a resultant angular momentum J. (In
practice, the coupling for many mol-
ecules is intermediate between
Hund’s cases due to interactions,
which are ignored in the idealized

cases.) 2.A type of chemical reaction

in which two molecules join to-

gether; for example, the formation of

an *azo compound by coupling of a

diazonium ion with a benzene ring.

covalent bondSee chemical

bond.

covalent carbideSee carbide.

covalent crystalA crystal in

which the atoms are held together

by covalent bonds. Covalent crystals

are sometimes called macromolecular

or giant-molecular crystals. They are

hard high-melting substances. Exam-

ples are diamond and boron nitride.

covalent radiusAn effective ra-

dius assigned to an atom in a cova-

lent compound. In the case of a

simple diatomic molecule, the cova-

lent radius is half the distance be-

tween the nuclei. Thus, in Cl 2 the

internuclear distance is 0.198 nm so

the covalent radius is taken to be

0.099 nm. Covalent radii can also be

calculated for multiple bonds; for in-

stance, in the case of carbon the val-

ues are 0.077 nm for single bonds,

0.0665 nm for double bonds, and

0.0605 nm for triple bonds. The val-

ues of different covalent radii can

sometimes be added to give internu-

clear distances. For example, the

length of the bond in interhalogens

(e.g. ClBr) is nearly equal to the sum

of the covalent radii of the halogens

involved. This, however, is not al-

ways true because of other effects

147 covalent radius

c

coumarin coumarinic acid

CHCHCH

COCOCO

OOO

H

HHH

COCOCO 222 HHH

OHOHOH

CCC

Coumarin