Dictionary of Chemistry [6th Ed.]

conductor of heat and electricity. It

has a variety of uses including electri-

cal contacts, high-temperature equip-

ment, and as a solid lubricant.

Graphite mixed with clay is the ‘lead’

in pencils (hence its alternative

name). The third crystalline allotrope

is fullerite (see buckminster-

fullerene). There are also several

amorphous forms of carbon, such as

*carbon black and *charcoal.

There are two stable isotopes of

carbon (proton numbers 12 and 13)

and four radioactive ones (10, 11, 14,

15). Carbon–14 is used in *carbon

dating.

Carbon forms a large number of

compounds because of its unique

ability to form stable covalent bonds

with other carbon atoms and also

with hydrogen, oxygen, nitrogen,

and sulphur atoms, resulting in the

formation of a variety of compounds

containing chains and rings of car-

bon atoms.

A

• Information from the WebElements site

carbon assimilationThe incorpo-

ration of carbon from atmospheric

carbon dioxide into organic mol-

ecules. This process occurs during

*photosynthesis. See carbon cycle.

carbonateA salt of carbonic acid

containing the carbonate ion, CO 3 2–.

The free ion has a plane triangular

structure. Metal carbonates may be

ionic or may contain covalent

metal–carbonate bonds (complex car-

bonates) via one or two oxygen

atoms. The carbonates of the alkali

metals are all soluble but other car-

bonates are insoluble; they all react

with mineral acids to release carbon

dioxide.

carbonate mineralsA group of

common rock-forming minerals con-

taining the anion CO 3 2–as the funda-

mental unit in their structure. The

most important carbonate minerals

are *calcite, *dolomite, and *magne-

site. See also aragonite.

carbonationThe solution of car-

bon dioxide in a liquid under pres-

sure.

carbon bisulphideSee carbon

disulphide.

carbon blackAÜne carbon powder

made by burning hydrocarbons in in-

sufÜcient air. It is used as a pigment

and aÜller (e.g. for rubber).

carbon cycle1.One of the major

cycles of chemical elements in the

environment. Carbon (as carbon

dioxide) is taken up from the atmos-

phere and incorporated into the tis-

sues of plants in *photosynthesis. It

may then pass into the bodies of ani-

mals as the plants are eaten. During

the respiration of plants, animals,

and organisms that bring about de-

composition, carbon dioxide is re-

turned to the atmosphere. The

combustion of fossil fuels (e.g. coal

and peat) also releases carbon diox-

ide into the atmosphere. See illustra-

tion. 2.(in physics) A series of

nuclear reactions in which four hy-

drogen nuclei combine to form a he-

lium nucleus with the liberation of

energy, two positrons, and two neu-

trinos. The process is believed to be

the source of energy in many stars

and to take place in six stages. In this

series carbon–12 acts as if it were a

catalyst, being reformed at the end of

the series:

12

6 C +

1

1 H →

13

7 N + γ

(^137) N → (^136) C + e++ νe
(^136) C + (^11) H → (^147) N + γ
(^147) N + (^11) H → (^158) O + γ
15
8 O →
15
7 N + e
++ ν
e
(^157) N + (^11) H → (^126) C + (^42) He.
carbon dating (radiocarbon dating)
carbon assimilation 100
c