Carbon (from Latin: carbo "coal") is a chemical element with
symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.
There are several allotropes of carbon of which the best
known are graphite,
diamond,
and amorphous carbon.[16]
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 word "γράφω"
which means "to write"), while diamond is the hardest
naturally-occurring material known. Graphite is a very good conductor, while diamond has a very low electrical conductivity. Under normal
conditions, diamond, carbon nanotubes, and graphene
have the highest thermal conductivities of all known materials.
Occurrence
Carbon
is the fourth
most abundant chemical element in the 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 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.
Allotropes
Atomic carbon is a very short-lived species and,
therefore, carbon is stabilized in various multi-atomic structures with
different 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 amorphous
form is an assortment of carbon atoms in a non-crystalline, irregular, glassy
state, which is essentially graphite but not held in a crystalline macrostructure. It is
present as a powder, and is the main constituent of substances such as charcoal,
lampblack
(soot) and activated
carbon. At normal pressures carbon takes the form of graphite,
in which each atom is bonded trigonally to three others in a plane composed of
fused hexagonal
rings, just like those in aromatic hydrocarbons. The resulting
network is 2-dimensional, and the resulting flat sheets are stacked and loosely
bonded through weak van der Waals forces. This gives graphite its
softness and its cleaving properties (the sheets slip easily
past one another). Because of the delocalization of one of the outer electrons
of each atom to form a π-cloud, graphite conducts electricity,
but only in the plane of each covalently
bonded sheet. This results in a lower bulk electrical conductivity for carbon than
for most metals.
The delocalization also accounts for the energetic stability of graphite over
diamond at room temperature.
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, thus making 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 in terms of
resistance to scratching. Contrary to the popular belief that "diamonds are forever",
they are in fact thermodynamically unstable under normal conditions and
transform into graphite.
However, due to a high activation energy barrier, the transition into graphite
is so extremely slow at room temperature as to be unnoticeable. Under some
conditions, carbon crystallizes as lonsdaleite.
This form has a hexagonal crystal lattice where all atoms are covalently bonded.
Therefore, all properties of lonsdaleite are close to those of diamond.
Compounds
Organic compounds
Correlation between the carbon
cycle and formation of organic compounds. In plants, carbon dioxide formed
by carbon fixation can join with water in photosynthesis (green) to form organic compounds,
which can be used and further converted by both plants and animals.
Carbon
has the ability to form very long chains of interconnecting C-C bonds. This
property is called catenation. Carbon-carbon bonds are strong, and stable. This property
allows carbon to form an almost 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 organic
compounds contain hydrogen as well).
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. Chain length, side
chains and functional groups all affect the properties of
organic molecules.
Carbon
occurs in all known organic life and is the basis of organic chemistry. When united with hydrogen, it forms various hydrocarbons which 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, and 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.
Inorganic compounds
Commonly
carbon-containing compounds which are associated with minerals or which do not
contain hydrogen or fluorine, are treated separately from classical organic compounds; however the definition is not
rigid (see reference articles above). Among these are the simple oxides of
carbon. The most prominent oxide is carbon dioxide (CO2). 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 (H2CO3), 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 (CS2) 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)2), similar to diatomic halides.
Other uncommon oxides are carbon suboxide (C3O2), the unstable dicarbon monoxide (C2O), carbon trioxide (CO3), cyclopentanepentone (C5O5) cyclohexanehexone (C6O6), and mellitic anhydride (C12O9).
With
reactive metals, such as tungsten, carbon forms either carbides (C4−), or acetylides (C2−2) 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.
Organometallic compounds
Organometallic
compounds by definition contain at least one carbon-metal bond. A wide range of
such compounds exist; major classes include simple alkyl-metal compounds (for
example, tetraethyllead), η2-alkene compounds
(for example, Zeise's salt), and η3-allyl compounds
(for example, allylpalladium
chloride dimer);
metallocenes containing cyclopentadienyl ligands (for example, ferrocene); and transition
metal carbene complexes. Many metal carbonyls exist (for example, tetracarbonylnickel); some workers consider the carbon monoxide ligand to be purely inorganic, and
not organometallic.
While
carbon is understood to exclusively form four bonds, an interesting compound
containing an octahedral hexacoordinated carbon atom has been reported. The
cation of the compound is [(Ph3PAu)6C]2+. This
phenomenon has been attributed to the aurophilicity of the gold ligands.
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