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= Carbon =
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Introduction
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Carbon () is a chemical element; it has symbol C and atomic number 6.
It is nonmetallic and tetravalent--meaning that its atoms are able to
form up to four covalent bonds due to its valence shell exhibiting 4
electrons. It belongs to group 14 of the periodic table. Carbon makes
up about 0.025 percent of Earth's crust. Three isotopes occur
naturally, (12)C and (13)C being stable, while (14)C is a
radionuclide, decaying with a half-life of 5,700 years. Carbon is one
of the few elements known since antiquity.
Carbon is the 15th most abundant element in the Earth's crust, and the
fourth most abundant element in the universe by mass after hydrogen,
helium, and oxygen. Carbon's abundance, its unique diversity of
organic compounds, and its unusual ability to form polymers at the
temperatures commonly encountered on Earth, enables this element to
serve as a common element of all known life. It is the second most
abundant element in the human body by mass (about 18.5%) after oxygen.
The atoms of carbon can bond together in diverse ways, resulting in
various allotropes of carbon. Well-known allotropes include graphite,
diamond, amorphous carbon, and fullerenes. The physical properties of
carbon vary widely with the allotropic form. For example, graphite is
opaque and black, while diamond is highly transparent. Graphite is
soft enough to form a streak on paper (hence its name, from the Greek
verb "γράφειν" which means "to write"), while diamond is the hardest
naturally occurring material known. Graphite is a good electrical
conductor while diamond has a low electrical conductivity. Under
normal conditions, diamond, carbon nanotubes, and graphene have the
highest thermal conductivities of all known materials. All carbon
allotropes are solids under normal conditions, with graphite being the
most thermodynamically stable form at standard temperature and
pressure. They are chemically resistant and require high temperature
to react even with oxygen.
The most common oxidation state of carbon in inorganic compounds is
+4, while +2 is found in carbon monoxide and transition metal carbonyl
complexes. The largest sources of inorganic carbon are limestones,
dolomites and carbon dioxide, but significant quantities occur in
organic deposits of coal, peat, oil, and methane clathrates. Carbon
forms a vast number of compounds, with about two hundred million
having been described and indexed; and yet that number is but a
fraction of the number of theoretically possible compounds under
standard conditions.
Characteristics
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Carbon in its solid state exists in several allotropes, including
graphite, a soft, black, and slippery material, and diamond, the
hardest naturally occurring substance. This variation in physical
properties arises from differences in atomic arrangement: graphite
consists of layers of hexagonally arranged carbon atoms, while diamond
features a rigid three-dimensional lattice.
Chemically, carbon is notable for its ability to form stable chemical
bonds with many elements, particularly with other carbon atoms, and is
capable of forming multiple stable covalent bonds with suitable
multivalent atoms. Carbon is a component element in the large majority
of all chemical compounds, with about two hundred million examples
having been described in the published chemical literature. Carbon
also has the highest sublimation point of all elements. At atmospheric
pressure it has no melting point, as its triple point is at 10.8 ± and
4600 ±, so it sublimes at about 3900 K.
Compared to its well-known solid allotropes, the liquid and gaseous
phases of carbon are far less studied. In the vapor phase, some of the
carbon is in the form of highly reactive diatomic carbon dicarbon ().
When excited, this gas glows green. The liquid phase of carbon is a
dark, mobile, and reflective liquid that can only exist above 4000 K
and under pressures exceeding 100 atmospheres.
Carbon is the sixth element, with a ground-state electron
configuration of 1s22s22p2, of which the four outer electrons are
valence electrons. Its first four ionisation energies, 1086.5, 2352.6,
4620.5 and 6222.7 kJ/mol, are much higher than those of the heavier
group-14 elements. The electronegativity of carbon is 2.5,
significantly higher than the heavier group-14 elements (1.8-1.9), but
close to most of the nearby nonmetals, as well as some of the second-
and third-row transition metals. Carbon's covalent radii are normally
taken as 77.2 pm (C−C), 66.7 pm (C=C) and 60.3 pm (C≡C), although
these may vary depending on coordination number and what the carbon is
bonded to. In general, covalent radius decreases with lower
coordination number and higher bond order.
Chemical
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Graphite is much more reactive than diamond at standard conditions,
despite being more thermodynamically stable, as its delocalised pi
system is much more vulnerable to attack. For example, graphite can be
oxidised by hot concentrated nitric acid at standard conditions to
mellitic acid, C6(CO2H)6, which preserves the hexagonal units of
graphite while breaking up the larger structure.
Carbon-based compounds form the basis of all known life on Earth, and
the carbon-nitrogen-oxygen cycle provides a small portion of the
energy produced by the Sun, and most of the energy in larger stars
(e.g. Sirius). Although it forms an extraordinary 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. It does not react with sulfuric acid,
hydrochloric acid, chlorine or any alkalis. At elevated temperatures,
carbon reacts with oxygen to form carbon oxides and will rob oxygen
from metal oxides to leave the elemental metal. This exothermic
reaction is used in the iron and steel industry to smelt iron and to
control the carbon content of steel:
: + 4 C + 2 → 3 Fe + 4 .
Carbon reacts with sulfur to form carbon disulfide, and it reacts with
steam in the coal-gas reaction used in coal gasification:
:C + HO → CO + H.
Carbon combines with some metals at high temperatures to form metallic
carbides, such as the iron carbide cementite in steel and tungsten
carbide, widely used as an abrasive and for making hard tips for
cutting tools.
Allotropes
============
Atomic carbon is a very short-lived species and, therefore, carbon is
stabilized in various multi-atomic structures with diverse molecular
configurations called allotropes. The three relatively well-known
allotropes of carbon are amorphous carbon, graphite, and diamond. Once
considered exotic, fullerenes are nowadays commonly synthesized and
used in research; they include buckyballs, carbon nanotubes, carbon
nanobuds and nanofibers. Several other exotic allotropes have also
been discovered, such as lonsdaleite, glassy carbon, carbon nanofoam
and linear acetylenic carbon (carbyne).
The system of carbon allotropes spans a range of extremes:
|Graphite is one of the softest materials known. |Synthetic
nanocrystalline diamond is the hardest material known.
|Graphite is a very good lubricant, displaying superlubricity.
|Diamond is the ultimate abrasive.
|Graphite is a conductor of electricity. |Diamond is an excellent
electrical insulator, and has the highest breakdown electric field of
any known material.
|Some forms of graphite are used for thermal insulation (i.e.
firebreaks and heat shields), but some other forms are good thermal
conductors. |Diamond is the best known naturally occurring thermal
conductor.
|Graphite is opaque. |Diamond is highly transparent.
|Graphite crystallizes in the hexagonal system. |Diamond crystallizes
in the cubic system.
|Amorphous carbon is completely isotropic. |Carbon nanotubes are
among the most anisotropic materials known.
Graphene is a two-dimensional sheet of carbon with the atoms arranged
in a hexagonal lattice. As of 2009, graphene appears to be the
strongest material ever tested.
* The process of separating it from graphite will require some
further technological development before it is economical for
industrial processes. If successful, graphene could be used in the
construction of a space elevator. It could also be used to safely
store hydrogen for use in a hydrogen based engine in cars.
At very high pressures, carbon forms the more compact allotrope,
diamond, having nearly twice the density of graphite. Here, each atom
is bonded tetrahedrally to four others, forming a 3-dimensional
network of puckered six-membered rings of atoms. Diamond has the same
cubic structure as silicon and germanium, and because of the strength
of the carbon-carbon bonds, it is the hardest naturally occurring
substance measured by resistance to scratching. Contrary to the
popular belief that '"diamonds are forever"', they are
thermodynamically unstable (Δf'G'°(diamond, 298 K) = 2.9 kJ/mol) under
normal conditions (298 K, 105 Pa) and should theoretically transform
into graphite. But due to a high activation energy barrier, the
transition into graphite is so slow at normal temperature that it is
unnoticeable. However, at very high temperatures diamond will turn
into graphite, and diamonds can burn up in a house fire. The bottom
left corner of the phase diagram for carbon has not been scrutinized
experimentally. Although a computational study employing density
functional theory methods reached the conclusion that as and ,
diamond becomes more stable than graphite by approximately 1.1 kJ/mol,
more recent and definitive experimental and computational studies show
that graphite is more stable than diamond for , without applied
pressure, by 2.7 kJ/mol at 'T' = 0 K and 3.2 kJ/mol at 'T' = 298.15 K.
Under some conditions, carbon crystallizes as lonsdaleite, a hexagonal
crystal lattice with all atoms covalently bonded and properties
similar to those of diamond.
Fullerenes are a synthetic crystalline formation with a graphite-like
structure, but in place of flat hexagonal cells only, some of the
cells of which fullerenes are formed may be pentagons, nonplanar
hexagons, or even heptagons of carbon atoms. The sheets are thus
warped into spheres, ellipses, or cylinders. The properties of
fullerenes (split into buckyballs, buckytubes, and nanobuds) have not
yet been fully analyzed and represent an intense area of research in
nanomaterials. The names 'fullerene' and 'buckyball' are given after
Richard Buckminster Fuller, popularizer of geodesic domes, which
resemble the structure of fullerenes. The buckyballs are fairly large
molecules formed completely of carbon bonded trigonally, forming
spheroids (the best-known and simplest is the soccerball-shaped C
buckminsterfullerene). Carbon nanotubes (buckytubes) are structurally
similar to buckyballs, except that each atom is bonded trigonally in a
curved sheet that forms a hollow cylinder. Nanobuds were first
reported in 2007 and are hybrid buckytube/buckyball materials
(buckyballs are covalently bonded to the outer wall of a nanotube)
that combine the properties of both in a single structure.
Of the other discovered allotropes, carbon nanofoam is a ferromagnetic
allotrope discovered in 1997. It consists of a low-density
cluster-assembly of carbon atoms strung together in a loose
three-dimensional web, in which the atoms are bonded trigonally in
six- and seven-membered rings. It is among the lightest known solids,
with a density of about 2 kg/m(3). Similarly, glassy carbon contains a
high proportion of closed porosity, but contrary to normal graphite,
the graphitic layers are not stacked like pages in a book, but have a
more random arrangement. Linear acetylenic carbon has the chemical
structure −(C≡C)− . Carbon in this modification is linear with 'sp'
orbital hybridization, and is a polymer with alternating single and
triple bonds. This carbyne is of considerable interest to
nanotechnology as its Young's modulus is 40 times that of the hardest
known material - diamond.
In 2015, a team at the North Carolina State University announced the
development of another allotrope they have dubbed Q-carbon, created by
a high-energy low-duration laser pulse on amorphous carbon dust.
Q-carbon is reported to exhibit ferromagnetism, fluorescence, and a
hardness superior to diamonds.
Occurrence
============
Carbon is the fourth most abundant chemical element in the observable
universe by mass after hydrogen, helium, and oxygen. 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. Microscopic
diamonds may also be formed by the intense pressure and high
temperature at the sites of meteorite impacts.
In 2014 NASA announced a [
http://www.astrochem.org/pahdb/ greatly
upgraded database] for tracking polycyclic aromatic hydrocarbons
(PAHs) in the universe. More than 20% of the carbon in the universe
may be associated with PAHs, complex compounds of carbon and hydrogen
without oxygen. These compounds figure in the PAH world hypothesis
where they are hypothesized to have a role in abiogenesis and
formation of life. PAHs seem to have been formed "a couple of billion
years" after the Big Bang, are widespread throughout the universe, and
are associated with new stars and exoplanets.
It has been estimated that the solid earth as a whole contains 730 ppm
of carbon, with 2000 ppm in the core and 120 ppm in the combined
mantle and crust. Since the mass of the earth is , this would imply
4360 million gigatonnes of carbon. This is much more than the amount
of carbon in the oceans or atmosphere (below).
In combination with oxygen in carbon dioxide, carbon is found in the
Earth's atmosphere (approximately 900 gigatonnes of carbon -- each ppm
corresponds to 2.13 Gt) and dissolved in all water bodies
(approximately 36,000 gigatonnes of carbon). Carbon in the biosphere
has been estimated at 550 gigatonnes but with a large uncertainty, due
mostly to a huge uncertainty in the amount of terrestrial deep
subsurface bacteria. Hydrocarbons (such as coal, petroleum, and
natural gas) contain carbon as well. Coal "reserves" (not "resources")
amount to around 900 gigatonnes with perhaps 18,000 Gt of resources.
Oil reserves are around 150 gigatonnes. Proven sources of natural gas
are about (containing about 105 gigatonnes of carbon), but studies
estimate another of "unconventional" deposits such as shale gas,
representing about 540 gigatonnes of carbon.
Carbon is also found in methane hydrates in polar regions and under
the seas. Various estimates put this carbon between 500, 2500, or
3,000 Gt.
According to one source, in the period from 1751 to 2008 about 347
gigatonnes of carbon were released as carbon dioxide to the atmosphere
from burning of fossil fuels. Another source puts the amount added to
the atmosphere for the period since 1750 at 879 Gt, and the total
going to the atmosphere, sea, and land (such as peat bogs) at almost
2,000 Gt.
Carbon is a constituent (about 12% by mass) of the very large masses
of carbonate rock (limestone, dolomite, marble, and others). Coal is
very rich in carbon (anthracite contains 92-98%) and is the largest
commercial source of mineral carbon, accounting for 4,000 gigatonnes
or 80% of fossil fuel.
As for individual carbon allotropes, graphite is found in large
quantities in China, Russia, Mexico, Canada, and India. Natural
diamonds occur in the rock 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
Angola. Diamond deposits have also been found in Arkansas, Canada, the
Russian Arctic, Brazil, and in Northern and Western Australia.
Diamonds are found naturally, but about 90% of all industrial diamonds
used in the U.S. are now manufactured.
Carbon-14 is formed in upper layers of the troposphere and the
stratosphere at altitudes of 9-15 km by a reaction that is
precipitated by cosmic rays. Thermal neutrons are produced that
collide with the nuclei of nitrogen-14, forming carbon-14 and a
proton. As such, of atmospheric carbon dioxide contains carbon-14.
Carbon-rich asteroids are relatively preponderant in the outer parts
of the asteroid belt in the Solar System. These asteroids have not yet
been directly sampled by scientists. The asteroids can be used in
hypothetical space-based carbon mining, which may be possible in the
future, but is currently technologically impossible.
Isotopes
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Isotopes of carbon are atomic nuclei that contain six protons plus a
number of neutrons (varying from 2 to 16). Carbon has two stable,
naturally occurring isotopes. The isotope carbon-12 ((12)C) forms
98.93% of the carbon on Earth, while carbon-13 ((13)C) forms the
remaining 1.07%. The concentration of (12)C is further increased in
biological materials because biochemical reactions discriminate
against (13)C. In 1961, the International Union of Pure and Applied
Chemistry (IUPAC) adopted the isotope carbon-12 as the basis for
atomic weights. Identification of carbon in nuclear magnetic resonance
(NMR) experiments is done with the isotope (13)C.
Carbon-14 ((14)C) is a naturally occurring radioisotope, created in
the upper atmosphere (lower stratosphere and upper troposphere) by
interaction of nitrogen with cosmic rays. It is found in trace amounts
on Earth of 1 part per trillion (0.0000000001%) or more, mostly
confined to the atmosphere and superficial deposits, particularly of
peat and other organic materials. This isotope decays by 0.158 MeV
β(−) emission. Because of its relatively short half-life of years,
(14)C is virtually absent in ancient rocks. The amount of (14)C in the
atmosphere and in living organisms is almost constant, but decreases
predictably in their bodies after death. This principle is used in
radiocarbon dating, invented in 1949, which has been used extensively
to determine the age of carbonaceous materials with ages up to about
40,000 years.
There are 15 known isotopes of carbon and the shortest-lived of these
is (8)C which decays through proton emission and has a half-life of
3.5 s. The exotic (19)C exhibits a nuclear halo, which means its
radius is appreciably larger than would be expected if the nucleus
were a sphere of constant density.
Formation in stars
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Formation of the carbon atomic nucleus occurs within a giant or
supergiant star through the triple-alpha process. This requires a
nearly simultaneous collision of three alpha particles (helium
nuclei), as the products of further nuclear fusion reactions of helium
with hydrogen or another helium nucleus produce lithium-5 and
beryllium-8 respectively, both of which are highly unstable and decay
almost instantly back into smaller nuclei. The triple-alpha process
happens in conditions of temperatures over 100 megakelvins and helium
concentration that the rapid expansion and cooling of the early
universe prohibited, and therefore no significant carbon was created
during the Big Bang.
According to current physical cosmology theory, carbon is formed in
the interiors of stars on the horizontal branch. When massive stars
die as supernova, the carbon is scattered into space as dust. This
dust becomes component material for the formation of the
next-generation star systems with accreted planets. The Solar System
is one such star system with an abundance of carbon, enabling the
existence of life as we know it. It is the opinion of most scholars
that all the carbon in the Solar System and the Milky Way comes from
dying stars.
The CNO cycle is an additional hydrogen fusion mechanism that powers
stars, wherein carbon operates as a catalyst.
Rotational transitions of various isotopic forms of carbon monoxide
(for example, (12)CO, (13)CO, and (18)CO) are detectable in the
submillimeter wavelength range, and are used in the study of newly
forming stars in molecular clouds.
Carbon cycle
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Under terrestrial conditions, conversion of one element to another is
very rare. Therefore, the amount of carbon on Earth is effectively
constant. Thus, processes that use carbon must obtain it from
somewhere and dispose of it somewhere else. The paths of carbon in the
environment form the carbon cycle. For example, photosynthetic plants
draw carbon dioxide from the atmosphere (or seawater) and build it
into biomass, as in the Calvin cycle, a process of carbon fixation.
Some of this biomass is eaten by animals, while some carbon is exhaled
by animals as carbon dioxide. The carbon cycle is considerably more
complicated than this short loop; for example, some carbon dioxide is
dissolved in the oceans; if bacteria do not consume it, dead plant or
animal matter may become petroleum or coal, which releases carbon when
burned.
Organic compounds
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Carbon can form very long chains of interconnecting carbon-carbon
bonds, a property that is called catenation. Carbon-carbon bonds are
strong and stable. Through [p[catenation, carbon forms a countless
number of compounds. A tally of unique compounds shows that more
contain carbon than do not.
The simplest form of an organic molecule is the hydrocarbon--a large
family of organic molecules that are composed of hydrogen atoms bonded
to a chain of carbon atoms. A hydrocarbon backbone can be substituted
by other atoms, known as heteroatoms. Common heteroatoms that appear
in organic compounds include oxygen, nitrogen, sulfur, phosphorus, and
the nonradioactive halogens, as well as the metals lithium and
magnesium. Organic compounds containing bonds to metal are known as
organometallic compounds ('see below'). Certain groupings of atoms,
often including heteroatoms, recur in large numbers of organic
compounds. These collections, known as 'functional groups', confer
common reactivity patterns and allow for the systematic study and
categorization of organic compounds. Chain length, shape and
functional groups all affect the properties of organic molecules.
In most stable compounds of carbon (and nearly all stable 'organic'
compounds), carbon obeys the octet rule and is 'tetravalent', meaning
that a carbon atom forms a total of four covalent bonds (which may
include double and triple bonds). Exceptions include a small number of
stabilized 'carbocations' (three bonds, positive charge), 'radicals'
(three bonds, neutral), 'carbanions' (three bonds, negative charge)
and 'carbenes' (two bonds, neutral), although these species are much
more likely to be encountered as unstable, reactive intermediates.
Carbon occurs in all known organic life and is the basis of organic
chemistry. When united with hydrogen, it forms various hydrocarbons
that are important to industry as refrigerants, lubricants, solvents,
as chemical feedstock for the manufacture of plastics and
petrochemicals, and as fossil fuels.
When combined with oxygen and hydrogen, carbon can form many groups of
important biological compounds including sugars, lignans, chitins,
alcohols, fats, aromatic esters, carotenoids and terpenes. With
nitrogen, it forms alkaloids, and with the addition of sulfur also it
forms antibiotics, amino acids, and rubber products. With the addition
of phosphorus to these other elements, it forms DNA and RNA, the
chemical-code carriers of life, and adenosine triphosphate (ATP), the
most important energy-transfer molecule in all living cells. Norman
Horowitz, head of the Mariner and Viking missions to Mars (1965-1976),
considered that the unique characteristics of carbon made it unlikely
that any other element could replace carbon, even on another planet,
to generate the biochemistry necessary for life.
Inorganic compounds
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Commonly carbon-containing compounds which are associated with
minerals or which do not contain bonds to the other carbon atoms,
halogens, or hydrogen, are treated separately from classical organic
compounds; the definition is not rigid, and the classification of some
compounds can vary from author to author (see reference articles
above). Among these are the simple oxides of carbon. The most
prominent oxide is carbon dioxide (). This was once the principal
constituent of the paleoatmosphere, but is a minor component of the
Earth's atmosphere today. Dissolved in water, it forms carbonic acid
(), 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 () is
similar.
The other common oxide is carbon monoxide (CO). It 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, but behaves much like a halide ion (pseudohalogen).
For example, it can form the nitride cyanogen molecule ((CN)), similar
to diatomic halides. Likewise, the heavier analog of cyanide, cyaphide
(CP(−)), is also considered inorganic, though most simple derivatives
are highly unstable. Other uncommon oxides are carbon suboxide (), the
unstable dicarbon monoxide (CO), carbon trioxide (CO),
cyclopentanepentone (CO), cyclohexanehexone (CO), and mellitic
anhydride (CO). However, mellitic anhydride is the triple acyl
anhydride of mellitic acid; moreover, it contains a benzene ring.
Thus, many chemists consider it to be organic.
With reactive metals, such as tungsten, carbon forms either carbides
(C(4−)) or acetylides () to form alloys with high melting points.
These anions are also associated with methane and acetylene, both very
weak acids. With an electronegativity of 2.5, carbon prefers to form
covalent bonds. A few carbides are covalent lattices, like carborundum
(SiC), which resembles diamond. Nevertheless, even the most polar and
salt-like of carbides are not completely ionic compounds.
Organometallic compounds
==========================
Organometallic compounds by definition contain at least one
carbon-metal covalent bond. A wide range of such compounds exist;
major classes include simple alkyl-metal compounds (for example,
tetraethyllead), η(2)-alkene compounds (for example, Zeise's salt),
and η(3)-allyl compounds (for example, allylpalladium chloride dimer);
metallocenes containing cyclopentadienyl ligands (for example,
ferrocene); and transition metal carbene complexes. Many metal
carbonyls and metal cyanides exist (for example, tetracarbonylnickel
and potassium ferricyanide); some workers consider metal carbonyl and
cyanide complexes without other carbon ligands to be purely inorganic,
and not organometallic. However, most organometallic chemists consider
metal complexes with any carbon ligand, even 'inorganic carbon' (e.g.,
carbonyls, cyanides, and certain types of carbides and acetylides) to
be organometallic in nature. Metal complexes containing organic
ligands without a carbon-metal covalent bond (e.g., metal
carboxylates) are termed 'metalorganic' compounds.
While carbon is understood to strongly prefer formation of four
covalent bonds, other exotic bonding schemes are also known.
Carboranes are highly stable dodecahedral derivatives of the
[B12H12]2- unit, with one BH replaced with a CH+. Thus, the carbon is
bonded to five boron atoms and one hydrogen atom. The cation
[(PhPAu)C](2+) contains an octahedral carbon bound to six
phosphine-gold fragments. This phenomenon has been attributed to the
aurophilicity of the gold ligands, which provide additional
stabilization of an otherwise labile species. In nature, the
iron-molybdenum cofactor (FeMoco) responsible for microbial nitrogen
fixation likewise has an octahedral carbon center (formally a carbide,
C(-IV)) bonded to six iron atoms. In 2016, it was confirmed that, in
line with earlier theoretical predictions, the hexamethylbenzene
dication contains a carbon atom with six bonds. More specifically, the
dication could be described structurally by the formulation
[MeC(η5-C5Me5)]2+, making it an "organic metallocene" in which a MeC3+
fragment is bonded to a η5-C5Me5− fragment through all five of the
carbons of the ring.
It is important to note that in the cases above, each of the bonds to
carbon contain less than two formal electron pairs. Thus, the formal
electron count of these species does not exceed an octet. This makes
them hypercoordinate but not hypervalent. Even in cases of alleged
10-C-5 species (that is, a carbon with five ligands and a formal
electron count of ten), as reported by Akiba and co-workers,
electronic structure calculations conclude that the electron
population around carbon is still less than eight, as is true for
other compounds featuring four-electron three-center bonding.
History and etymology
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The English name 'carbon' comes from the Latin 'carbo' for coal and
charcoal, whence also comes the French 'charbon', meaning charcoal. In
German, Dutch and Danish, the names for carbon are 'Kohlenstoff',
'koolstof', and 'kulstof' respectively, all literally meaning
coal-substance.
Carbon was discovered in prehistory and was known in the forms of soot
and charcoal to the earliest human civilizations. Diamonds were known
probably as early as 2500 BCE in China, while carbon in the form of
charcoal was made by the same chemistry as it is today, by heating
wood in a pyramid covered with clay to exclude air.
A new allotrope of carbon, fullerene, that was discovered in 1985
includes nanostructured forms such as buckyballs and nanotubes. Their
discoverers - Robert Curl, Harold Kroto, and Richard Smalley -
received the Nobel Prize in Chemistry in 1996. The resulting renewed
interest in new forms led to the discovery of further exotic
allotropes, including glassy carbon, and the realization that
"amorphous carbon" is not strictly amorphous.
Graphite
==========
Commercially viable natural deposits of graphite occur in many parts
of the world, but the most important sources economically are in
China, India, Brazil, and North Korea. Graphite deposits are of
metamorphic origin, found in association with quartz, mica, and
feldspars in schists, gneisses, and metamorphosed sandstones and
limestone as lenses or veins, sometimes of a metre or more in
thickness. Deposits of graphite in Borrowdale, Cumberland, England
were at first of sufficient size and purity that, until the 19th
century, pencils were made by sawing blocks of natural graphite into
strips before encasing the strips in wood. Today, smaller deposits of
graphite are obtained by crushing the parent rock and floating the
lighter graphite out on water.
There are three types of natural graphite--amorphous, flake or
crystalline flake, and vein or lump. Amorphous graphite is the lowest
quality and most abundant. Contrary to science, in industry
"amorphous" refers to very small crystal size rather than complete
lack of crystal structure. Amorphous is used for lower value graphite
products and is the lowest priced graphite. Large amorphous graphite
deposits are found in China, Europe, Mexico and the United States.
Flake graphite is less common and of higher quality than amorphous; it
occurs as separate plates that crystallized in metamorphic rock. Flake
graphite can be four times the price of amorphous. Good quality flakes
can be processed into expandable graphite for many uses, such as flame
retardants. The foremost deposits are found in Austria, Brazil,
Canada, China, Germany and Madagascar. Vein or lump graphite is the
rarest, most valuable, and highest quality type of natural graphite.
It occurs in veins along intrusive contacts in solid lumps, and it is
only commercially mined in Sri Lanka.
According to the USGS, world production of natural graphite was 1.1
million tonnes in 2010, to which China contributed 800,000 t, India
130,000 t, Brazil 76,000 t, North Korea 30,000 t and Canada 25,000 t.
No natural graphite was reported mined in the United States, but
118,000 t of synthetic graphite with an estimated value of $998
million was produced in 2009.
Diamond
=========
The diamond supply chain is controlled by a limited number of powerful
businesses, and is also highly concentrated in a small number of
locations around the world (see figure).
Only a very small fraction of the diamond ore consists of actual
diamonds. The ore is crushed, during which care has to be taken in
order to prevent larger diamonds from being destroyed in this process
and subsequently the particles are sorted by density. Today, diamonds
are located in the diamond-rich density fraction with the help of
X-ray fluorescence, after which the final sorting steps are done by
hand. Before the use of X-rays became commonplace, the separation was
done with grease belts; diamonds have a stronger tendency to stick to
grease than the other minerals in the ore.
Historically diamonds were known to be found only in alluvial deposits
in southern India. India led the world in diamond production from the
time of their discovery in approximately the 9th century BC to the
mid-18th century AD, but the commercial potential of these sources had
been exhausted by the late 18th century and at that time India was
eclipsed by Brazil where the first non-Indian diamonds were found in
1725.
Diamond production of primary deposits (kimberlites and lamproites)
only started in the 1870s after the discovery of the diamond fields in
South Africa. Production has increased over time and an accumulated
total of over 4.5 billion carats have been mined since that date. Most
commercially viable diamond deposits were in Russia, Botswana,
Australia and the Democratic Republic of Congo. By 2005, Russia
produced almost one-fifth of the global diamond output (mostly in
Yakutia territory; for example, Mir pipe and Udachnaya pipe) but the
Argyle mine in Australia became the single largest source, producing
14 million carats in 2018. New finds, the Canadian mines at Diavik and
Ekati, are expected to become even more valuable owing to their
production of gem quality stones.
In the United States, diamonds have been found in Arkansas, Colorado,
and Montana. In 2004, a startling discovery of a microscopic diamond
in the United States led to the January 2008 bulk-sampling of
kimberlite pipes in a remote part of Montana.
While natural diamonds form over time deep within the Earth, synthetic
diamonds are created in laboratories through a variety of methods. The
original method uses high pressure and high temperature (HPHT) and is
still widely used because of its relatively low cost. The process
involves large presses that can weigh hundreds of tons to produce a
pressure of at . The second method, using chemical vapor deposition
(CVD), creates a carbon plasma over a substrate onto which the carbon
atoms deposit to form diamond. Other methods include explosive
formation (forming detonation nanodiamonds) and sonication of graphite
solutions.
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 other than food and wood is in the form
of hydrocarbons, most notably the fossil fuel methane gas and crude
oil (petroleum). Crude oil is distilled in refineries by the
petrochemical industry to produce gasoline, kerosene, and other
products. Cellulose is a natural, carbon-containing polymer produced
by plants in the form of wood, cotton, linen, and hemp. Cellulose is
used primarily for maintaining structure in plants. Commercially
valuable carbon polymers of animal origin include wool, cashmere, and
silk. Plastics are made from synthetic carbon polymers, often with
oxygen and nitrogen atoms included at regular intervals in the main
polymer chain. The raw materials for many of these synthetic
substances come from crude oil and coal.
The uses of carbon and its compounds are extremely varied. It can form
alloys with iron, of which the most common is carbon steel. Graphite
is combined with clays to form the 'lead' used in pencils used for
writing and drawing. It is also used as a lubricant and a pigment, as
a moulding material in glass manufacture, in electrodes for dry
batteries and in electroplating and electroforming, in brushes for
electric motors, and as a neutron moderator in nuclear reactors.
Charcoal is used as a drawing material in artwork, barbecue grilling,
iron smelting, and in many other applications. Wood, coal and oil are
used as fuel for production of energy and heating. Gem quality
diamond is used in jewelry, and industrial diamonds are used in
drilling, cutting and polishing tools for machining metals and stone.
Carbon fiber, which is produced by pyrolyzing synthetic polyester
fibers, is used to reinforce plastics, creating advanced, lightweight
composite materials.
Carbon fiber is made by pyrolysis of extruded and stretched filaments
of polyacrylonitrile (PAN) and other organic substances. The
crystallographic structure and mechanical properties of the fiber
depend on the type of starting material, and on the subsequent
processing. Carbon fibers made from PAN have structure resembling
narrow filaments of graphite, but thermal processing may re-order the
structure into a continuous rolled sheet. The result is fibers with
higher specific tensile strength than steel.
Carbon black is used as the black pigment in printing ink, artist's
oil paint, and water colours, carbon paper, automotive finishes, India
ink and laser printer toner. Carbon black is also used as a filler in
rubber products such as tyres and in plastic compounds. Activated
charcoal is used as an absorbent and adsorbent in filter material in
applications as diverse as gas masks, water purification, and kitchen
extractor hoods, and in medicine to absorb toxins, poisons, or gases
from the digestive system. Carbon is used in chemical reduction at
high temperatures. Coke is used to reduce iron ore into iron
(smelting). Case hardening of steel is achieved by heating finished
steel components in carbon powder. Carbides of silicon, tungsten,
boron, and titanium are among the hardest known materials, and are
used as abrasives in cutting and grinding tools. Carbon compounds make
up most of the materials used in clothing, such as natural and
synthetic textiles and leather, and almost all of the interior
surfaces in the built environment other than glass, stone, drywall,
and metal.
Diamonds
==========
The diamond industry falls into two categories: one dealing with
gem-grade diamonds and the other, with industrial-grade diamonds.
While a large trade in both types of diamonds exists, the two markets
function dramatically differently. Unlike precious metals such as gold
or platinum, gem diamonds do not trade as a commodity. There is a
substantial mark-up in the sale of diamonds, and there is not a very
active market for resale of diamonds.
Industrial diamonds are valued mostly for their hardness and heat
conductivity, with the gemological qualities of clarity and color
being mostly irrelevant. About 80% of mined diamonds (equal to about
100 million carats or 20 tonnes annually) are unsuitable for use as
gemstones and relegated for industrial use (known as 'bort)'.
Synthetic diamonds, invented in the 1950s, found almost immediate
industrial applications; 3 billion carats (600 tonnes) of synthetic
diamond is produced annually.
The dominant industrial use of diamond is in cutting, drilling,
grinding, and polishing. Most of these applications do not require
large diamonds; in fact, most diamonds of gem-quality except for their
small size can be used industrially. Diamonds are embedded in drill
tips or saw blades, or ground into a powder for use in grinding and
polishing applications. Specialized applications include use in
laboratories as containment for high-pressure experiments (see diamond
anvil cell), high-performance bearings, and limited use in specialized
windows. With the continuing advances in the production of synthetic
diamonds, new applications are becoming feasible. Garnering much
excitement is the possible use of diamond as a semiconductor suitable
for microchips, and because of its exceptional heat conductance
property, as a heat sink in electronics.
Precautions
======================================================================
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Pure carbon has extremely low toxicity to humans and can be handled
safely in the form of graphite or charcoal. It is resistant to
dissolution or chemical attack, even in the acidic contents of the
digestive tract. Consequently, once it enters into the body's tissues
it is likely to remain there indefinitely. Carbon black was probably
one of the first pigments to be used for tattooing, and Ötzi the
Iceman was found to have carbon tattoos that survived during his life
and for 5200 years after his death. Inhalation of coal dust or soot
(carbon black) in large quantities can be dangerous, irritating lung
tissues and causing the congestive lung disease, coalworker's
pneumoconiosis. Diamond dust used as an abrasive can be harmful if
ingested or inhaled. Microparticles of carbon are produced in diesel
engine exhaust fumes, and may accumulate in the lungs. However, in
these examples, the harm may result from contaminants (e.g., organic
chemicals, heavy metals) rather than from the carbon itself.
Carbon may burn vigorously and brightly in the presence of air at high
temperatures. Large accumulations of coal, which have remained inert
for hundreds of millions of years in the absence of oxygen, may
spontaneously combust when exposed to air in coal mine waste tips,
ship cargo holds and coal bunkers, and storage dumps.
In nuclear applications where graphite is used as a neutron moderator,
accumulation of Wigner energy followed by a sudden, spontaneous
release may occur. Annealing to at least 250 °C can release the energy
safely, although in the Windscale fire the procedure went wrong,
causing other reactor materials to combust.
The great variety of carbon compounds include such lethal poisons as
tetrodotoxin, the lectin ricin from seeds of the castor oil plant
'Ricinus communis', cyanide (CN(−)), and carbon monoxide; and such
essentials to life as glucose and protein.
See also
======================================================================
* Carbon chauvinism
* Carbon detonation
* Carbon footprint
* Carbon star
* Carbon planet
* Low-carbon economy
* Timeline of carbon nanotubes
External links
======================================================================
*
* [
http://www.periodicvideos.com/videos/006.htm Carbon] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.britannica.com/eb/article-80956/carbon-group-element
Carbon on Britannica]
*
[
https://web.archive.org/web/20100618165649/http://invsee.asu.edu/nmodules/Carbonmod/everywhere.html
Extensive Carbon page at asu.edu] (archived 18 June 2010)
*
[
https://web.archive.org/web/20011109080742/http://electrochem.cwru.edu/ed/encycl/art-c01-carbon.htm
Electrochemical uses of carbon] (archived 9 November 2001)
*
[
https://web.archive.org/web/20121109012854/http://www.forskning.no/Artikler/2006/juni/1149432180.36
Carbon--Super Stuff. Animation with sound and interactive 3D-models.]
(archived 9 November 2012)
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Carbon
