'Carbon' (
IPA: ) is a
chemical element that has the
symbol 'C' and
atomic number 6. An abundant
nonmetallic,
tetravalent element, carbon has several
allotropic forms.
The abundance of carbon in the universe, along with the unusual polymer-forming ability of carbon-based compounds at the common temperatures encountered on
Earth, make this element the basis of the chemistry of all known life.
The name "carbon" comes from
Latin language ''carbo'',
coal. In some
Romance languages, the word can refer both to the element and to coal.
Overview of carbon's importance on Earth
As the free element it forms
allotropes from differing kinds of carbon-carbon bonds, such as in
graphite and
diamond.
Coal is the main source of carbon in mineral form, containing up to 95% of carbon in
anthracite. Recently discovered nanostructured forms called
fullerenes include
buckyballs such as
C60,
nanotubes, and
nanofibers. Because of their high strength-to-weight ratio, it is hoped that many of these carbon compounds will soon be practical for use in advanced structural composite materials.
Not only can carbon also
bond with itself, but it can also form chains with a wide variety of other elements, forming nearly ten million known
compounds.
Carbon-containing polymers, often with oxygen and nitrogen atoms included at regular intervals in the main polymer chain, form the basis of nearly all industrial commercial
plastics.
Carbon occurs in all
organic life and is the basis of
organic chemistry. When united with
oxygen, carbon forms
carbon dioxide, which is the main carbon source for
plant growth. When united with
hydrogen, it forms various flammable compounds called
hydrocarbons which are essential to industry in the form of
fossil fuels, and also other important living plant components like
carotenoids and
terpenes. When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including
sugars,
celluloses,
lignans,
chitins,
alcohols,
fats, and aromatic
esters. With nitrogen it forms
alkaloids, and with the addition of sulfur also it forms
antibiotics,
amino acids and
proteins. With the addition of phosphorus to these other elements, it forms
DNA and
RNA, the chemical codes of life.
Notable characteristics of carbon
Carbon exhibits remarkable properties, some paradoxical. Different forms include the hardest naturally occurring substance (
diamond) and one of the softest substances (
graphite) known. Moreover, it has a great affinity for
bonding with other small
atoms, including other carbon atoms, and is capable of forming multiple stable
covalent bonds with such atoms. Because of these properties, carbon is known to form nearly ten million different compounds, the large majority of all chemical compounds. Carbon compounds form the basis of all life on
Earth and the
carbon-nitrogen cycle provides some of the energy produced by the
Sun and other
stars. Moreover, carbon has the highest
melting/
sublimation point of all elements. At
atmospheric pressure it has no actual melting point as its
triple point is at 10 MPa (100
bar) so it sublimates above 4000 K. Thus it remains solid at higher temperatures than the highest melting point metals like
tungsten or
rhenium, irrespective of its allotropic form. Although thermodynamically prone to oxidation, it resists oxidation more effectively than some elements (like iron and even copper)that are weaker reducing agents at room temperature.
Although it forms an incredible variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperature and pressure, it resists all but the strongest oxidizers (such as
fluorine and
nitric acid). It does not react with
sulfuric acid,
hydrochloric acid,
chlorine or any alkalis. At elevated temperatures it of course reacts with oxygen in
flames and with
sulfur vapors; it also combines with some metals at high temperatures to form metallic carbides and reduces such metal oxides as iron oxide.
Formation of the carbon atomic nucleus requires a nearly simultaneous triple collision of
alpha particles (
helium nuclei). This happens in temperature and helium concentration conditions that the rapid expansion and cooling of the early universe prohibited, and therefore no significant carbon was created during the
Big Bang. Instead, the interiors of stars in the
horizontal branch transform three helium nuclei into carbon by means of this
triple-alpha process. In order to be available for formation of life as we know it, this carbon must then later be scattered into space as dust, in
supernovae explosions, as part of the material which later forms second-generation star systems which have planets accreted from such dust. The solar system is one such second-generation star, made from carbon in the dust of dozens of supernovae in its local area of the galaxy.
Applications
Carbon is essential to all known living systems, and without it life as we know it could not exist (see
alternative biochemistry). The major economic use of carbon not in living or formerly-living material (such as food and wood) is in the form of hydrocarbons, most notably the
fossil fuel methane gas and
crude oil (petroleum). Crude oil is used by the
petrochemical industry to produce, amongst others,
gasoline and
kerosene, through a
distillation process, in
refineries. Crude oil forms the raw material for many synthetic substances, many of which are collectively called
plastics.
Other uses
★ The isotope
carbon-14 was discovered on
February 27 1940 and is used in
radiocarbon dating.
★
Industrial diamonds are used in cutting, drilling, and polishing technologies.
★
Graphite is combined with
clays to form the 'lead' used in
pencils. It is also used as a
lubricant and a pigment.
★
Diamond is used for decorative purposes, and also as drill bits and other applications making use of its hardness.
★ Carbon (usually as
coke) is used to reduce iron ore into iron.
★ Carbon is added to
iron to make
steel.
★ Carbon is used as a
neutron moderator in
nuclear reactors.
★
Carbon fiber, which is mainly used for composite materials, as well as high-temperature gas filtration.
★
Carbon black is used as a
filler in
rubber and
plastic compounds.
★ Graphite carbon in a powdered, caked form is used as
charcoal for
grilling,
artwork and other uses.
★ Activated charcoal is used in medicine (as powder or compounded in tablets or capsules) to
absorb toxins, poisons, or gases from the digestive system.
★ Carbon, due to its non-reactivity with many substances that corrode most materials, is often used as an electrode.
★ Carbon is the most commonly used element in nanotubes.
★ Rotational transitions of various isotopic forms of carbon monoxide (e.g.
12CO,
13CO, and
18CO) are detectable in the
submillimeter regime, and are used in the study of
newly forming stars in
molecular clouds.
The chemical and structural properties of fullerenes, in the form of
carbon nanotubes, has promising potential uses in the nascent field of nanotechnology.
History and etymology
It was discovered in prehistory and was known to the ancients, who manufactured it by burning organic material in insufficient oxygen (making
charcoal). It is also found in abundance in the Sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites.
Natural diamonds are found in
kimberlite of ancient volcanic "pipes," found in South Africa, Arkansas, Northern Canada and elsewhere. Diamonds are now also being recovered from the ocean floor off the Cape of Good Hope. About 30% of all industrial diamonds used in the U.S. are now made synthetically.
The energy of the Sun and stars can be attributed at least in part to the
carbon-nitrogen cycle.
The name of Carbon comes from
Latin ''carbo'', hence comes
French ''charbon'', meaning charcoal. In
German and
Dutch, the names for carbon are ''Kohlenstoff'' and ''koolstof'' respectively, both literally meaning
coal-stuff.
In
Swedish Carbon is called kol, which is also the name for kål(
Cabbage.
Allotropes
Main articles: allotropes of carbon
The
allotropes of carbon are the different molecular configurations that pure carbon can take.
The three relatively well-known allotropes of carbon are
amorphous carbon,
graphite, and
diamond. Several exotic allotropes have also been synthesized or discovered, including
fullerenes,
carbon nanotubes,
lonsdaleite and
aggregated diamond nanorods.
In its amorphous form, carbon is essentially
graphite but not held in a crystalline macrostructure. It is, rather, present as a powder which is the main constituent of substances such as
charcoal,
lampblack (
soot) and
activated carbon.
Basic
phase diagram of carbon, which shows the state of matter for varying temperatures and pressures. The hashed regions indicate conditions under which one phase is
metastable, so that two phases can coexist.
At normal pressures carbon takes the form of
graphite, in which each atom is bonded to three others in a plane composed of fused
hexagonal rings, just like those in
aromatic hydrocarbons. The two known forms of graphite, alpha (hexagonal) and beta (
rhombohedral), both have identical physical properties, except for their crystal structure. Graphites that naturally occur have been found to contain up to 30% of the beta form, when synthetically-produced graphite only contains the alpha form. The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1000 °
C.
Because of the delocalization of the
π-cloud, graphite conducts
electricity. This accounts for the energetic stability of graphite over diamond at room temperature. Graphite is soft and the sheets, frequently separated by other atoms, are held together only by
Van der Waals forces, so easily slip past one another.
At very high pressures carbon forms an allotrope called
diamond, in which each atom is bonded to four others. Diamond has the same cubic structure as
silicon and
germanium and, thanks to the strength of the carbon-carbon
bonds, is together with the
isoelectronic boron nitride (BN) the hardest substance in terms of resistance to scratching. The transition to
graphite at room temperature,although more stable, is so slow as to be unnoticeable, due to a high activation energy barrier. Under some conditions, carbon crystallizes as
Lonsdaleite, a form similar to diamond but hexagonal.
Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, also contain pentagons (or possibly heptagons) of carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The properties of fullerenes (also called "
buckyballs" and "
buckytubes") have not yet been fully analyzed. The name ''"fullerene"'' is given after
Buckminster Fuller, developer of some
geodesic domes, which resemble the structure of fullerenes.
A
ferromagnetic nanofoam allotrope has also been discovered.
Eight allotropes of carbon: Diamond, graphite, lonsdaleite, C60, C540, C70, amorphous carbon and a carbon nanotube.
Carbon allotropes include:
★
Diamond:
Hardest known natural
mineral. Structure: each atom is bonded
tetrahedrally to four others, making a 3-dimensional network of puckered six-membered rings of atoms.
★
Graphite: One of the softest substances. Structure: each atom is bonded trigonally to three other atoms, making a 2-dimensional network of flat six-membered rings; the flat sheets are loosely bonded.
★
Fullerenes: Structure: comparatively large molecules formed completely of carbon bonded trigonally, forming
spheroids (of which the best-known and simplest is the
buckminsterfullerene or buckyball, because of its soccerball-shaped structure).
★
Chaoite: A mineral believed to be formed in
meteorite impacts.
★
Lonsdaleite: A corruption of
diamond. Structure: similar to diamond, but forming a
hexagonal crystal lattice.
★
Amorphous carbon: A
glassy substance. Structure: an assortment of carbon atoms in a non-crystalline, irregular, glassy state.
★
Carbon nanofoam (discovered in
1997): An extremely light
magnetic web. Structure: a low-density web of graphite-like clusters, in which the atoms are bonded trigonally in six- and seven-membered rings.
★
Carbon nanotubes: Tiny tubes. Structure: each atom is bonded trigonally in a curved sheet that forms a hollow
cylinder.
★
Nanobud: A hybrid
carbon nanotube/
fullerene material discovered in 2007 which combines the properties of both in a single structure in which fullerenes are covalently bonded to the outer wall of a nanotube.
★
Aggregated diamond nanorods (synthesised in
2005): The most recently discovered allotrope and the hardest substance known.
★
Lampblack: Consists of small graphitic areas. These areas are randomly distributed, so the whole structure is isotropic.
★ '
Glassy carbon': An
isotropic substance that contains a high proportion of closed
porosity. Unlike normal graphite, the graphitic layers are not stacked like pages in a book, but have a more random arrangement.
Carbon fibers are similar to glassy carbon. Under special treatment (stretching of organic fibers and carbonization) it is possible to arrange the carbon planes in direction of the fiber. Perpendicular to the fiber axis there is no orientation of the carbon planes. The result are fibers with a higher
specific strength than steel.
The system of carbon allotropes spans a range of extremes:
★ Diamond is the 'hardest' mineral known to man (although aggregated diamond nanorods are now believed to be even harder), while graphite is one of the 'softest'.
★ Diamond is the ultimate 'abrasive', while graphite is a very good '
lubricant'.
★ Diamond is an excellent 'electrical insulator', while graphite is a '
conductor' of electricity.
★ Diamond is an excellent 'thermal conductor', while some forms of graphite are used for thermal '
insulation' (i.e. firebreaks and heatshields)
★ Diamond is usually 'transparent', while graphite is 'opaque'.
★ Diamond crystallizes in the '
cubic system' while graphite crystallizes in the '
hexagonal system'.
★ Amorphous carbon is completely '
isotropic', while carbon nanotubes are among the most '
anisotropic' materials ever produced.
Occurrence
Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium, and oxygen (see
Abundance of the chemical elements). Carbon is abundant in the
Sun,
stars,
comets, and in the
atmospheres of most
planets. Some
meteorites contain microscopic diamonds that were formed when the
solar system was still a
protoplanetary disk. In combination with other elements, carbon is found in the Earth's atmosphere (around 810
gigatonnes) and dissolved in all water bodies (around 36000 gigatonnes). Around 1900 gigatonnes are present in the
biosphere.
Hydrocarbons (such as
coal,
petroleum, and
natural gas) contain carbon as well —
coal "resources" amount to around 1000 gigatonnes, and
oil reserves around 150 gigatonnes. With smaller amounts of
calcium,
magnesium, and
iron, carbon is a major component of very large masses
carbonate rock (
limestone,
dolomite,
marble etc.).
Graphite is found in large quantities in
New York and
Texas, the
United States;
Russia;
Mexico;
Greenland and
India.
Natural diamonds occur in the mineral
kimberlite found in ancient
volcanic "necks," or "pipes". Most diamond deposits are in
Africa, notably in
South Africa,
Namibia,
Botswana, the
Republic of the Congo and
Sierra Leone. There are also deposits in
Arkansas,
Canada, the Russian
Arctic,
Brazil and in Northern and Western
Australia.
According to studies from the Massachusetts Institute of Technology, an estimate of the global carbon budget is:
| Biosphere, oceans, atmosphere |
|---|
| 0.44 x 1018 kilograms (3.7 x 1018 moles) | |
| Crust | |
|---|---|
| Organic carbon | 13.2 x 1018 kg |
|---|---|
| Carbonates | 62.4 x 1018 kg |
|---|---|
| Mantle | |
|---|---|
| 1200 x 1018 kg | |
Organic compounds
Main articles: organic chemistry
The most prominent oxide of carbon is
carbon dioxide, CO
2. This is a minor component of the
Earth's atmosphere, produced and used by living things, and a common volatile elsewhere. In
water it forms trace amounts of
carbonic acid, H
2CO
3, but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable. Through this intermediate, though, resonance-stabilized
carbonate ions are produced. Some important minerals are carbonates, notably
calcite.
Carbon disulfide, CS
2, is similar.
The other oxides are
carbon monoxide, CO, the uncommon
carbon suboxide, C
3O
2, the uncommon
dicarbon monoxide, C
2O and even
carbon trioxide, CO
3. Carbon monoxide is formed by incomplete combustion, and is a colorless, odorless gas. The molecules each contain a triple bond and are fairly
polar, resulting in a tendency to bind permanently to
hemoglobin molecules, displacing oxygen, which has a lower binding affinity.
Cyanide, CN
-, has a similar structure and behaves much like a
halide ion; the nitride
cyanogen, (CN)
2, is related.
With reactive
metals, such as
tungsten, carbon forms either carbides, C
-, or acetylides, C
22- to form alloys with high melting points. These anions are also associated with
methane and
acetylene, both very weak
acids. All in all, with an electronegativity of 2.5, carbon prefers to form
covalent bonds. A few carbides are covalent lattices, like
carborundum, SiC, which resembles
diamond.
Carbon has the ability to form long, indefinite chains with interconnecting C-C bonds. This property is called
catenation. Carbon-carbon bonds are strong, and stable. This property allows carbon to form an infinite number of compounds; in fact, there are more known carbon-containing compounds than all the compounds of the other chemical elements combined except those of hydrogen (because almost all carbon compounds contain hydrogen).
The simplest form of an organic molecule is the
hydrocarbon - a large family of
organic molecules that, by definition, are composed of
hydrogen atoms bonded to a chain of carbon atoms. Chain length, side chains and
functional groups all affect the properties of organic molecules.
Nearly ten million carbon compounds are known, and thousands of these are vital to life processes. They are also many organic-based reactions of economic importance.
Carbon cycle
Main articles: carbon cycle
Under terrestrial conditions, conversion of one element to another is very rare. Therefore, for practical purposes, the amount of carbon on Earth is constant. Thus processes that use carbon must obtain it somewhere and dispose of it somewhere. The paths that carbon follows in the environment make up the carbon cycle. For example, plants draw carbon dioxide out of the environments and use it to build biomass, as in
carbon respiration. Some of this biomass is eaten by animals, where some of it is exhaled as carbon dioxide. The carbon cycle is considerably more complicated than this short loop; for example, some carbon dioxide is dissolved in the oceans; dead plant or animal matter may become
petroleum or
coal which can burn with the release of carbon dioxide should bacteria not consume it.
Isotopes
Main articles: Isotopes of carbon
Carbon has two stable, naturally-occurring
isotopes:
carbon-12, or
12C, (98.89%) and
carbon-13, or
13C, (1.11%), and one unstable, naturally-occurring,
radioisotope;
carbon-14 or
14C. There are 15 known isotopes of carbon and the shortest-lived of these is
8C which decays through
proton emission and
alpha decay. It has a half-life of 1.98739x10
-21 s.
In 1961 the
International Union of Pure and Applied Chemistry adopted the isotope
carbon-12 as the basis for
atomic weights.
Carbon-14 has a
half-life of 5730
y and has been used extensively for
radiocarbon dating carbonaceous materials.
The exotic
19C exhibits a
nuclear halo.
Precautions
Although carbon is relatively safe due to low toxicity and resistance to most chemical attacks (including fire and the acidic contents of the digestive tract) at normal temperatures, inhalation of fine soot in large quantities can be dangerous. Diamond dust can do harm as an abrasive if ingested or inhaled. Carbon may also spawn flames at very high temperatures and burn vigorously and brightly (as in the
Windscale fire).
The great variety of carbon compounds include such lethal poisons as
cyanide (CN
-), and
carbon monoxide; and such essentials to life as
glucose and
protein.
See also
★
Organic chemistry
★
Inorganic chemistry of carbon
★
Allotropes of carbon
★
Diamond
★
Material properties of diamond
★
Carbon nanotube
★
Low-carbon economy
★
Atomic carbon
★
Carbon chauvinism
References
★ ''
On Graphite Transformations at High Temperature and Pressure Induced by Absorption of the LHC Beam'', J.M. Zazula, 1997
★ http://WebElements.com
★ http://EnvironmentalChemistry.com
External links
★
Los Alamos National Laboratory – Carbon
★
WebElements.com – Carbon
★
Chemicool.com – Carbon
★
It's Elemental – Carbon
★
– Carbon Fullerene and other Allotropes models by Vincent Herr
★
Extensive Carbon page at asu.edu
★
Electrochemical uses of carbon
★
Computational Chemistry Wiki
★
Carbon - Super Stuff. Animation with sound and interactive 3D-models.
★
BBC Radio 4 series "In Our Time", on ''Carbon, the basis of life'', 15 June 2006
★
Introduction to Carbon Properties geared for High School students.
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