|
Without the element carbon, life as we
know it would not exist. Carbon provides the framework for all tissues of
plants and animals. These tissues are built of elements grouped around
chains or rings made of carbon atoms. Carbon also provides common
fuels--coal, coke, oil, gasoline, and natural gas. Sugar, starch, and paper
are compounds of carbon with hydrogen and oxygen. Proteins such as hair,
meat, and silk contain carbon and other elements such as nitrogen,
phosphorus, and sulfur.
More than six and a half million compounds of the element carbon, many
times more than those of any other element, are known, and more are
discovered and synthesized regularly. Hundreds of carbon compounds are
commercially important but the element itself in the forms of diamond,
graphite, charcoal, carbon black, and fullerene is also indispensable.
Carbon occurs in nature as the sixth most abundant element in the universe
and the 19th element in order of mass in the Earth's crust. As the
element--in the forms of graphite, diamond, and fullerene--it is a minor
part of the Earth' s crust, but compounds of carbon with other elements are
very common. The chemical symbol for an atom of carbon is C. Some common
natural substances rich in carbon are coal, petroleum, natural gas, oil
shale, limestone, coral, oyster shells, marble, dolomite, and magnesite.
Limestone, coral, and oyster shells are largely calcium carbonate. Marble,
dolomite, and magnesite also contain calcium, magnesium, and carbon.
Coal, petroleum, natural gas, and oil shale are mainly compounds of carbon
and hydrogen derived from plant and animal sources deposited in the Earth
millions of years ago and subjected to high pressure. These deposits were
once a part of what is called the carbon cycle, a dynamic system of change
still occurring. Through photosynthesis, plants use sunlight to convert
carbon dioxide from the air and convert water from the soil into plant
tissues such as cellulose and into an energy source such as sugar. Plants
release oxygen into the air as the carbohydrates sugar and cellulose are
synthesized. Animals eat the plants, breathe in oxygen from the air and
oxidize the carbohydrates, or use them as fuel, which releases energy to
the animal. Eventually the products of animal metabolism--carbon dioxide,
water, and other waste products--are returned to the atmosphere and the
Earth. The cycle repeats itself endlessly.
(See also Carbohydrates; Photosynthesis; Plant.)
|
Besides the wide occurrence of carbon in compounds, two allotropes, or
forms, of the element--diamond and graphite--are deposited in widely
scattered locations around the Earth. The third form, fullerene, is
unstable in comparison to these two forms and is thus not found widely.
A diamond, no matter what the size, may be considered to be a single
molecule of carbon atoms, each joined to four other carbons in regular
tetrahedrons, or triangular prisms. The crystal structure is called a face-
centered cubic lattice. Diamond is extremely hard but brittle and has a
high specific gravity of 3.51. Its high refractive index of 2.42 is a
measure of how far diamond can refract, or bend, light. This property gives
the diamond brilliance and fire. A diamond can be cleaved, or split, along
its crystal faces into smaller pieces with the sides of the cleavage
remaining smooth. This property is very important to the diamond cutter and
the jeweler.
Graphite, the second allotrope of carbon, was known in antiquity. Natural
deposits of graphite have been called black lead, silver lead, and
plumbago, which is another name for the lead ore galena. The largest
deposits of graphite are in Sri Lanka but the highest quality graphite
comes from Madagascar. Other sources are North Korea, Mexico, Canada,
Siberia, and New York. In contrast to that of diamond, the structure of
graphite consists of layers of carbon atoms joined in regular hexagons by
strong bonds. The layers are held together by long-range, relatively weak
attractive forces called Van der Waals forces. The layers can slide over
each other easily, which accounts in part for the lubricating property of
graphite.
|
Amorphous carbon is not generally called a third allotrope because it is a
form of graphite consisting of microscopic crystals. Amorphous carbon is
obtained by heating any of a variety of carbon-rich materials to 1,200
degrees to 1,800 degrees F (650 degrees to 980 degrees C) in a limited
amount of air so that complete combustion does not occur. Coal, for
example, is heated to give coke; natural gas or petroleum to give carbon
black (also called lampblack and channel black); wood to give charcoal;
bone to give bone char; petroleum coke or coal to give baked carbon, carbon
arcs, or carbon electrodes.
In 1785 it was discovered that activated carbon from the carbonization of
wood and charcoal removes color from solutions--for example, the brown
color from raw sugar solutions. Activated carbon is still used in the beet
sugar industry and bone char is favored for the same purpose in the cane
sugar industry. Other foodstuffs commonly decolorized by activated carbon
include vinegar, soup stock, whiskey, gelatin, and oils and fats. Activated
carbon is also used to adsorb the toxic gases used in chemical warfare, to
adsorb organic vapors, and to reclaim solvents. All these uses depend on
the adherence of impurities to the enormous surface area of the finely
divided carbon.
Most carbon black is used in the manufacture of tires; it improves the
strength of rubber and resists scraping. The rest is used in making
printing inks for newspapers and magazines, and in paints, lacquers,
enamels, and carbon paper.
Fullerene, a hollow cluster of carbon atoms that resemble the geodesic
domes made
by architect R. Buckminster Fuller, was first postulated to exist in 1985.
In 1990 its existence was confirmed, and methods of synthesizing it in mass
quantities were invented. The most studied form, known as buckminsterfullerene
or the buckyball, has 60 carbon atoms arranged into a five-sided and six-sided
geometry to resemble a soccer ball.
It is suited for use as a lubricant, superconductor, radioactive shield,
hard coating, battery, and ball bearing. Through experimentation, scientists
concluded that fullerene exists in interstellar space and in soot from the
burning of certain gases on Earth. In 1992 it was found for the first time in rock sediments formed more than 600
million years ago. Because fullerene is unstable when exposed to air it is
not found in large quantities naturally.
|
The synthesis of carbon-containing compounds starts from carbon compounds
available in nature. The sources of the starting compounds are petroleum
for aliphatic hydrocarbons (straight-chain molecules of carbon and
hydrogen) and coal or petroleum for aromatic hydrocarbons (rings of carbon
and hydrogen). Limestone, from which carbon-containing calcium carbide and
acetylene can be made, is extremely important to the chemical industries in
countries that have no native petroleum.
Carbon compounds containing boron and silicon are among the hardest
substances known. On a standardized scale of hardness called the Mohs
scale, where diamond is 10, silicon carbide (or Carborundum), is 9.15 and
boron carbide is 9.32. These carbides are used as abrasives on emery
wheels. They are chemically inert and nearly indestructible. Carbides
formed by the more metallic elements such as iron, cobalt, and nickel, in
contrast, are easily decomposed by acids to give hydrocarbons, chiefly
methane and hydrogen. (See also Carbon Dioxide; Carbon Monoxide; Limestone;
Organic Chemistry.)
An ordinary carbon atom has six protons and six neutrons in its nucleus;
so the atom is called C-12. Another isotope, or type, of carbon atom has
six protons and seven neutrons in its nucleus and is called carbon-13, or
C-13. The relative abundance of C-12 and C-13 in natural sources is 98.89
percent and 1.11 percent respectively. In the air, however, the fast-flying
neutrons from cosmic rays keep hitting nitrogen atoms (N-14, with seven
protons and seven neutrons). Each time a neutron hits, it drives a proton
from the nitrogen atom' s nucleus and takes its place. Since the atom now
has six protons, it is an atom of carbon. It has 14 particles (six protons
and eight neutrons), however, in the nucleus; so it is called C-14.
|
This form of carbon decays radioactively. The production and decay are
balanced so that C-12 and C-14 remain always at the same ratio to each
other in carbon dioxide. Since the two forms are the same chemically,
plants use them for photosynthesis in this same ratio. Because animals eat
plants the ratio is found in all living organisms.
Fossils, mummies, and wooden relics, however, no longer exchange carbon
with the air. The carbon (C-12) that was present at death remains, but the
C-14 decays radioactively and becomes less in ratio to C-12. The changing
ratio can be detected easily with a Geiger counter or a scintillation
counter; and the amount of change tells the age of the specimen. For
example, suppose that the percentage of C-14 in a specimen is only half
that in the air. Since C-14 undergoes a half-life of decay in about 5,730
years, the specimen must be this old. (See also Radioactivity.)
Radioactive carbon-13 is used as a tracer for many chemical reactions.
Chemists can introduce it into food, for example, and then trace the course
of the food through the body with a special type of Geiger counter or
scintillation counter. This method has been used to trace the steps in
photosynthesis.
Leallyn B. Clapp
Properties of Carbon
Symbol C Atomic Number 6 Atomic Weight 12.011 Group in
Periodic Table IVA Specific Gravity at 68degF (20degC) 1.88-3.53 Boiling
Point 8,721degF (4,827degC) Melting Point 6,420degF (3,550degC)
This was originally from the Compton's Encyclopedia when
it was on-line. |
