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Carbon

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                 6                boron ← carbon → nitrogen
                 -
                ↑
                C
                ↓
                Si

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                         Name, Symbol, Number carbon, C, 6
                                                 Chemical series nonmetals
                                             Group, Period, Block 14, 2, p
                                               Appearance black (graphite)
                                                       colorless (diamond)
                                             Atomic mass 12.0107 (8) g/mol
                                     Electron configuration 1s^2 2s^2 2p^2
                                                  Electrons per shell 2, 4
                                                       Physical properties
                                                               Phase solid
                           Density (near r.t.) (graphite) 2.267 g·cm^−3
                            Density (near r.t.) (diamond) 3.513 g·cm^−3
                                 Melting point  ? triple point, ca. 10 MPa
                                                      and (4300–4700)  K
                                                        (4027–4427 ° C,
                                                         7280–8000 ° F)
                                         Boiling point  ? subl. ca. 4000 K
                                                    (3727 ° C, 6740 ° F)
                              Heat of fusion (graphite) ? 100 kJ·mol^−1
                               Heat of fusion (diamond) ? 120 kJ·mol^−1
                                Heat of vaporization  ? 355.8 kJ·mol^−1
                                         Heat capacity (25 °C) (graphite)
                                                 8.517 J·mol^−1·K^−1
                                          Heat capacity (25 °C) (diamond)
                                                 6.115 J·mol^−1·K^−1

   CAPTION: Vapor pressure (graphite)

                                         P/Pa  1  10  100  1 k  10 k 100 k
                                        at T/K   2839 3048 3289 3572 3908

                                                         Atomic properties
                                               Crystal structure hexagonal
                                                     Oxidation states 4, 2
                                                     (mildly acidic oxide)
                                    Electronegativity 2.55 (Pauling scale)
                                                       Ionization energies
                                          ( more) 1st: 1086.5 kJ·mol^−1
                                                  2nd: 2352.6 kJ·mol^−1
                                                  3rd: 4620.5 kJ·mol^−1
                                                       Atomic radius 70 pm
                                               Atomic radius (calc.) 67 pm
                                                     Covalent radius 77 pm
                                               Van der Waals radius 170 pm
                                                             Miscellaneous
                                             Magnetic ordering diamagnetic
                                   Thermal conductivity (300 K) (graphite)
                                             (119–165) W·m^−1·K^−1
                                    Thermal conductivity (300 K) (diamond)
                                            (900–2320) W·m^−1·K^−1
                                     Thermal diffusivity (300 K) (diamond)
                                                      (503–1300) mm²/ s
                                              Mohs hardness (graphite) 1-2
                                              Mohs hardness (diamond) 10.0
                                             CAS registry number 7440-44-0
                                                         Selected isotopes

                  CAPTION: Main article: Isotopes of carbon

                                iso   NA   half-life   DM   DE ( MeV)  DP
                                ^12C 98.9% C is stable with 6 neutrons
                                ^13C 1.1%  C is stable with 7 neutrons
                                ^14C trace 5730 y    beta^- 0.156     ^14N

                                                                References

   Carbon ( IPA: /ˈkɑːbən/) is a chemical element in the periodic table
   that has the symbol C and atomic number 6. An abundant nonmetallic,
   tetravalent element, carbon has several allotropic forms.

Overview

   Carbon occurs in all organic life and is the basis of organic
   chemistry. This nonmetal also has the interesting chemical property of
   being able to bond with itself and a wide variety of other elements,
   forming nearly ten million known compounds. When united with oxygen it
   forms carbon dioxide, which is vital to plant growth. When united with
   hydrogen, it forms various compounds called hydrocarbons which are
   essential to industry in the form of fossil fuels. When combined with
   both oxygen and hydrogen it can form many groups of compounds including
   fatty acids, which are essential to life, and esters, which give flavor
   to many fruits. The isotope carbon-14 is commonly used in radioactive
   dating.

Notable characteristics

   Carbon is a remarkable element for many reasons. Its 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 its small size makes it capable of forming multiple bonds.
   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, regardless of its
   allotropic form.

   Carbon was not created during the Big Bang due to the fact that it
   needs a triple collision of alpha particles (helium nuclei) to be
   produced. The universe initially expanded and cooled too fast for that
   to be possible. It is produced, however, in the interior of stars in
   the horizontal branch, where stars transform a helium core into carbon
   by means of the triple-alpha process. It was also created in a
   multi-atomic state.

Applications

   Carbon is a very important component of all known living systems, and
   without it life as we know it could not exist (see alternative
   biochemistry). The major economic use of carbon 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 the boaring industry.
     * Graphite is combined with clays to form the 'lead' used in pencils.
     * Diamond is used for decorative purposes, and also as drill bits and
       other applications making use of its hardness.
     * Carbon is added to iron to make steel.
     * Carbon is used as a neutron moderator in nuclear reactors.
     * Carbon fibre, 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 adsorb toxins, poisons, or gases from the
       digestive system.

   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

   Carbon 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, 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 well-known carbon-nitrogen cycle.

   The name of Carbon comes from Latin carbo, whence comes French
   charbone, meaning charcoal. In German and Dutch, the names for carbon
   are Kohlenstoff and koolstof respectively, both literally meaning
   "coal-stuff".

Allotropes

   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.
   Enlarge
   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 back to the alpha form
   when it is heated above 1000 ° C.

   Because of the delocalization of the pi-cloud, graphite conducts
   electricity. The material 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 is so slow as to be
   unnoticeable. 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. All the names of
   fullerenes are after Buckminster Fuller, developer of the geodesic
   dome, which mimics the structure of "buckyballs".

   A nanofoam allotrope has been discovered which is ferromagnetic.
   Eight allotropes of carbon: diamond, graphite, lonsdaleite,C60, C540,
   C70, amorphous carbon and a carbon nanotube.
   Enlarge
   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 molecules 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.
     * Aggregated diamond nanorods (synthesised in 2005): The most
       recently discovered allotrope and the hardest substance known to
       man.
     * 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 fibre 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.

   Between diamond and graphite:
     * Graphite is soft and is used in pencils
     * Diamond is the hardest mineral known to man (although aggregated
       diamond nanorods are now believed to be even harder), but graphite
       is one of the softest.
     * Diamond is the ultimate abrasive, but graphite is a very good
       lubricant.
     * Diamond is an excellent electrical insulator, but graphite is a
       conductor of electricity.
     * Diamond is an excellent thermal conductor, but some forms of
       graphite are used for thermal insulation (i.e. firebreaks and
       heatshields)
     * Diamond is usually transparent, but graphite is opaque.
     * Diamond crystallizes in the cubic system but graphite crystallizes
       in the hexagonal system.

   Between amorphous carbon and nanotubes:
     * Amorphous carbon is among the easiest materials to synthesize, but
       carbon nanotubes are extremely expensive to make.
     * Amorphous carbon is completely isotropic, but carbon nanotubes are
       among the most anisotropic materials ever produced.

Occurrence

   There are nearly ten million carbon compounds known to science. Many
   thousands of these are vital to life processes. They are also many
   organic-based reactions of economic importance.

   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 "reserves" (not "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 Massachussets Institute of Tecnology, an
   estimate of the global carbon budget is:

   Biosphere, oceans, atmosphere.......3,7 x 10^18 moles

   Crust
   Organic Carbon ...............................1100 x 10^18 moles
   Carbonates......................................5200 x 10^18 moles

   Earth´s Mantle.............................100000 x 10^18 moles

Organic compounds

   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[2]CO[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[3]O[2], the uncommon dicarbon monoxide, C[2]O 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 a lot 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[2]^2- to form alloys with very 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 chains

   Carbon has the ability to form long chains with interconnecting C-C
   bonds. This property is called catenation. Carbon-carbon bonds are
   fairly strong, and abnormally stable. This property is important as it
   allows carbon to form a huge number of compounds; in fact, there are
   more known carbon-containing compounds than all the compounds of the
   other chemical elements combined.

   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.

Carbon cycle

   Under terrestrial conditions, conversion of one isotope 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, dispose of it somewhere. The paths that carbon follows in
   the environment are called 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 sedimentary rock, so forth.

Isotopes

   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

   Carbon is relatively safe. Inhalation of fine soot in large quantities
   can be dangerous. Carbon may spawn flames at very high temperatures and
   burn vigorously and brightly (as in the Windscale fire).

   There are a tremendous number of carbon compounds; some are lethally
   poisonous ( cyanide, CN^-), and some are essential to life ( glucose).
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