Carbon

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98.9 %

1.1%

< 10-9 %

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Carbon ( from PGmc. Kulo (s), coal ') or Carbon (from Latin carbo, charcoal ', Latinized Carboneum ) is a chemical element with the element symbol C and atomic number 6 in the periodic table it is in the fourth main group or carbon group and the second period.

It occurs in nature both in a tasteful (pure ) form ( diamond, graphite ) and chemically bound ( eg in the form of carbonates, carbon dioxide, oil, natural gas and coal) before. Due to its special electron configuration ( half-filled L-shell ) it possesses the ability to form complex molecules and has all the chemical elements of the greatest variety of chemical compounds. Carbon compounds form the molecular basis of all earthly life.

• 4.1 modifications 4.1.1 graphite
• 4.1.2 Diamond
• 4.1.3 lonsdaleite
• 4.1.4 fullerenes

• 4.2.1 Amorphous carbon
• 4.2.2 carbon fibers
• 4.2.3 glassy carbon
• 4.2.4 graphs
• 4.2.5 Activated Carbon
• 4.2.6 soot
• 4.2.7 Carbon Nanotubes
• 4.2.8 Carbon nanobuds
• 4.2.9 carbon nano foam
• 4.2.10 Aero graphite

• 6.1 Isotope ratio to determine the age of organic materials

Occurrence

Carbon is an essential element of the biosphere, it is in all living things - after oxygen (water) - the weight according to the most important element. All living tissue is made up of ( organic ) carbon compounds.

Geologically, however, it is not among the most common elements. One can find carbon in the inanimate nature of both elementary ( diamond, graphite ) and in compounds. The main localities of diamond are Africa ( esp. South Africa and the Democratic Republic of Congo) and Russia. Diamonds are often found in volcanic rocks such as kimberlite. Graphite is relatively rare in carbon-rich metamorphic rocks. The major deposits are in India and China.

Most often you will find carbon in the form of inorganic Carbonatgesteins (about 2.8 × 1016 t). Carbonate rocks are widespread and form part of the mountains. A well-known example of carbonate rock are the Dolomites in Italy. The most important minerals are calcium carbonate ( modifications: limestone, chalk, marble) CaCO3, calcium magnesium carbonate ( dolomite) CaCO3 · MgCO3, iron (II ) carbonate ( siderite ) and zinc carbonate FeCO3 ( calamine ) ZnCO3.

Known carbon deposits are the fossil fuels coal, oil and natural gas. These are not pure carbon compounds, but mixtures of many different organic compounds. They were created by transformation of plant (coal) and animal (oil, gas ) remains under high pressure. Major coal deposits are located in the United States, China and Russia, a well-known German in the Ruhr. The most important oil reserves lie in the Arabian Peninsula (Iraq, Saudi Arabia). Other major oil deposits are found in the Gulf of Mexico and the North Sea. About solid methane hydrate in the deep sea still little is known.

Carbon is still in the air before as carbon dioxide ( short carbon dioxide). It is involved in the composition of air is about 0.04 %. Carbon dioxide from the burning of carbon-containing compounds, respiration, and volcanic and is utilized by plant photosynthesis. Even in water, CO2 is dissolved (about 0.01 % by weight in the sea).

Quantitatively, the major part of the carbon in the rock shell ( lithosphere ) is stored. All other reserves account quantitatively for only about 1/1000 of total carbon.

Properties

Carbon exists in several allotropic modifications. All solids carbon-based can be traced back to the two basic types of diamond and graphite.

In diamond carbon is three-dimensional covalently bonded. Diamond is an insulator and transparent. It is the hardest known material and is used as a natural abrasives.

In graphite, the covalent bonding within the basal planes is stronger than that in diamond, while the levels are loosely bound through van der Waals forces. The free π - electrons are responsible for the deep black color, the slight cleavage and the high conductivity along the basal planes. Graphite is used as a high temperature resistant gasket material and lubricants, and as a raw material for pencil leads.

Contrary to popular opinion, however, the well-known lubricant properties of graphite are not a property of graphite per se, but are found only in the presence of traces of moisture. In vacuums or very dry atmosphere, the friction coefficient of graphite therefore increases dramatically.

At atmospheric pressure and temperatures below 4000 K, graphite is the thermodynamically stable form of carbon, see phase diagram. Because of the high activation energy also diamond is stable at room temperature and converts only above 500 ° C markedly in graphite to. Conversely, requires the transformation of graphite into diamond at a pressure of at least 20,000 bar ( 2 GPa ). For a sufficiently rapid reaction, the temperature should be above 1500 ° C, at a pressure of 60,000 bar in accordance with the phase diagram.

Carbon has the highest temperature resistance of all known materials. He sublimates at atmospheric pressure at 3915 K ( 3642 ° C) without losing previously to strength. The triple point is at 10.8 ± 0.2 MPa and 4600 ± 300 K.

Carbon is diamagnetic. Pyrolytic graphite has a large secluded anisotropy in the magnetic susceptibility (parallel: = -85 · 10-6; vertical: = -450 · 10-6 ), diamond, however, is isotropic ( = -22 · 10-6).

In its various modifications carbon shows very different properties. Carbon is the hardest element: as a crystalline diamond is the absolute maximum value of 90 GPa is reached on the Knoop hardness scale. In the form of graphite or carbon is according to rubidium, and cesium at 0.12 GPa drittweichste the element. Carbon also possesses the highest thermal conductivity, W / m at room temperature is more than 2000.

Atomic model of carbon

The model of the atomic and molecular orbitals illustrated as it comes to the expression of different forms of carbon.

Carbon has six electrons. According to the shell model two electrons occupy the inner 1s shell. The 2s level of the second shell also accepts two electrons, two more the 2px and 2py level. Only the four outer electrons in the second shell occur chemically in appearance. The probability of the electrons in the S- range is spherical. In a p- level, it is anisotropic. The electrons populate an hourglass-shaped room, one Sanduhrhälfte left and right of the center along the x - axis, if one imagines placed in the center of a Cartesian coordinate system, the atom. Perpendicular to stand the py and pz orbital ( according to y - and z -axis).

Diamond structure (SP3)

The 2s level can with the 3 2p levels hybridize form four energetically equivalent sp3 orbitals. This can be intuitively explained by the fact that one of the s- electron is lifted into the previously empty p orbital and thereby align the orbital energies of all four orbitals of the second stage. The emerging orbitals have an elongated, asymmetrical teardrop shape. Were arranged point-symmetrically to the center of the shapes of the p orbitals, they now appear club-shaped in one direction increases. The image illustrates the main lobes, the side lobes were for clarity omitted. The four sp3 orbitals are oriented to each other with the greatest possible distance symmetrically in space, they show in the corners of an imaginary tetrahedron.

If they overlap, the sp3 orbitals of atoms, they can form strong covalent bonds, which then reflect the tetrahedral structure. They form the backbone of the diamond lattice (see crystal structure there.)

Graphite structure ( SP2)

If only 2 of the 3 p orbitals involved in hybridization, caused the so-called sp2 orbitals. The sp2 orbitals are directed from two-dimensional ( as a surface or plane); above and below this area, the remaining p orbital forming a respective lobes. Is, for example, the p- orbital perpendicular to the xy plane, the sp2 orbitals are trigonal in the xy plane. You have to each other the same angle of 120 °. The left picture illustrates the situation. The unhybridized p orbital is omitted for clarity.

Sp2 carbon atoms may together form covalent bonds, which then lie in one plane. Its structure is trigonal, this is the basic structure of the Planarebenen of graphite ( see the crystal lattice structure therein). The remaining p- orbitals also interact with each other. They form the pi- bond with significantly lower binding energy than the sigma - bonds of sp2 or sp3 orbitals to form the top and bottom of the sigma - bond layer a so-called electron gas in the form of atomic core independent ( " delocalized " ) pi- electrons.

Chemically, it is called a double bond. The notation C = C neglects the different character of the two bonds. The binding energy of the diamond-like sp3 tetrahedral single bond "C -C ' is 350 kJ / mol, the higher the graphite-like sp2 trigonal double bond C = C, only 260 kJ / mol. In a carbon hexagonal ring of six carbon atoms, the pi bond by delocalization of the electrons within the ring stabilizes ( for more details see benzene ).

Triple bond ( sp1 )

If only one p orbital hybridizes with the s- orbital, there are two linearly arranged pi bond lobes. We orient it along the x axis, the remaining p- orbitals lie on the y -and z- axes. Two sp hybridized carbon atoms may form a triple bond. One example is the gas ethyne ( acetylene) HC ≡ CH. During sp3 bonds form three-dimensional structures, two-dimensional and SP2, SP1 form bonds at most one-dimensional (linear) chain, such as H -C ≡ C -C ≡ C -H.

Manifestations of carbon

Elemental carbon exists in three modifications, based on the binding structures sp3 and sp2: diamond, graphite, and fullerene.

In addition to these three modifications, there are other different forms of elemental carbon.

Modifications

Graphite

The hexagonal sp2 covalently bonded carbon atoms form high-strength levels. The levels are only loosely bound to each other through van der Waals forces. Macroscopically dominates the cleavage along the Planarebenen. As the levels are so thin, her extraordinary strength does not appear in graphite.

Because of this structure, graphite behaves very anisotropic: along the crystal planes of graphite is thermally and electrically very conductive heat transfer or charge transfer from crystal to crystal plane level functions, however, relatively poor.

Diamond

The sp3 tetragonal covalently bonded carbon atoms have no free electrons. The material is an insulator with a band gap of 5.45 eV, which does not absorb visible light. Addition of an impurity produced states in the band gap, thereby altering the electrical and optical properties. Thus the yellowish tone of many natural diamonds is due to nitrogen, while boron- doped diamond look bluish and are semiconducting. The diamond converts to the absence of air at temperatures around 1500 ° C in graphite to. It burns even at about 700-800 ° C to form carbon dioxide.

Diamond applies under standard conditions ( 1 bar, 25 ° C), commonly known as the metastable form of carbon. Based on recent research, this is no longer safe because

Lonsdaleite

Lonsdaleite, also called hexagonal diamond, is a very rare modification of the diamond. It occurs when graphite by shock events, ie high pressure and high temperature such as by impact events, is converted into diamond. Here, the nature of the hexagonal crystal structure is maintained, however each carbon atom covalently bound to four other unlike graphite.

Fullerenes

A regular hexagonal honeycomb pattern as forming the C- atoms in the basal planes of the graphite is planar. By replacing some hexagons pentagons occur curved surfaces that are " rolled up " in certain relative locations of the five-and six-membered rings to closed bodies. In the fullerenes such structures are realized. The sp2 bonds no longer lie in a plane, but form a spatially closed structure. The smallest possible structure consists only of pentagons and requires 20 carbon atoms, the corresponding body is a pentagon - dodecahedron. This simplest fullerene has been but so far only detected by mass spectrometry. One of the most stable fullerenes consists of 60 carbon atoms and contains only hexagons pentagons that have a common edge with any other pentagon. The resulting pattern ( truncated icosahedron, an Archimedean body ) is similar to the pattern on a ( old-fashioned ) Football. It is designated in honor of Richard Buckminster Fuller Buckminster fullerene. The molecular " bullets " of fullerenes are bonded to each other through relatively weak van der Waals interactions, similar to the basal planes in graphite. Meanwhile, a number of fullerenes of different sizes have been isolated and partially crystallized; they can therefore be considered as a real modification (s). Fullerenes are probably present in all carbon blacks, such as in the soot over candle flames.

Other forms of carbon

Amorphous carbon

In amorphous carbon (aC ), the atoms are linked without long-range order. The material can be combined with almost any sp2: produce sp3 hybridization conditions, the material properties over flow smoothly from those of graphite to those of diamond. In the industry, the term diamond-like coating or diamond-like carbon (DLC) is often used in this case. At a proportion of sp3 hybridization of over 70% is referred to as tetrahedral amorphous carbon (ta -C). This material is characterized by high electrical resistance, extreme hardness and optical transparency. The preparation can be carried out by means of PVD or PECVD techniques. The material will be deposited as a layer ( amorphous carbon layer ).

Carbon fibers

Carbon fibers are made of graphite-like sp2 -bonded carbon. Isotropic fibers behave similarly to polycrystalline graphite and have little strength. Fiber mats and bundles are used for heat seals. By stretching in the manufacture, it is possible to orient the basal planes along the fiber axis. This gives high strength fibers with properties which are close to the theoretical values ​​of graphite along the basal planes. Anisotropic carbon fibers are light, extremely stiff and strong and be used in composites.

Glassy carbon

Glassy carbon ( " glassy carbon " ) is a high-tech material made ​​of pure carbon, glassy and ceramic properties is united with those of graphite. In contrast to graphite glassy carbon has a fullerene-like microstructure. This results in a wide variety of positive material properties. The conductivity is for example lower than that of graphite.

Graphs

As a graph is called a graphite basal plane of sp2 -hybridized carbon. The thin layers obtained by chemical cleavage of graphite. Nestled in plastics, it is suitable as a raw material for new composite materials or for studies of two-dimensional crystals.

Activated carbon

Gently graphitization of organic materials such as coconut shells, resulting in a porous carbon. The cavities are like a sponge with each other and form a very large internal surface area. Activated carbon filters solutes at low concentration of liquids and gases can absorb.

Soot

Carbon black is also made of carbon, graphite-based. The purer the soot, the more clearly emerge the properties of graphite. Lamp or candle soot is heavily contaminated with organic compounds that prevent the formation of larger graphite associations.

Carbon nanotubes

Another form of carbon are cylindrically arranged, sp2 -hybridized carbon atoms. Their geometry is formed from a planar layer of graphite rolled up into a cylinder. The resulting tube may additionally be rotated, thereby changing the electrical properties. Several single-walled tubes are concentric with each other, so that one speaks of multiwalled carbon nanotubes ( MWCNT ), in contrast to single-walled carbon nanotubes ( SWCNT ).

Carbon nanobuds

Carbon nanobuds combine the properties of carbon nanotubes and fullerenes.

Carbon nano- foam

Carbon nano foam, an airgel, is a randomly oriented, net-like arrangement of carbon graphite planes. It is similar to glassy carbon, only with much larger networked cavities. Their average diameter is six to nine nanometers.

Must be distinguished from carbon airgel, which consists of coalesced nanoparticles. Its density is of 200-1000 kg/m3.

Aero graphite

Aero graphite consists of a network of porous carbon tubes, and has a density of 0.2 milligrams per cubic centimeter of the lightest solid in the world. Aero graphite can be compressed by up to 95 % and pull apart again in the original form.

Compounds

Carbon is the element for hydrogen form most of the compounds of all elements may be ( hydrogen in the first place, since most compounds of carbon and hydrogen containing ). Peculiarities of carbon are to form chains and rings with itself and other elements, as well as double and triple bonds involving π orbitals. Due to its moderately strong electronegativity he has to electropositive a good binding capacity as well as to more electronegative elements. All oxidation states of IV to IV occur in nature in inorganic or organic compounds.

Carbon compounds are traditionally seen a few exceptions to organic chemistry, this is also sometimes referred to as the chemistry of carbon. Comprises the organic chemistry, due to the ability of the carbon to form long chains and form covalent bonds with other atoms, more connections than the entire inorganic chemistry. And biochemistry is a part of the organic carbon chemistry. The simplest organic compounds include alkanes, methane and ethane.

Only relatively few carbon compounds are traditionally made to the inorganic compounds, including quantitatively most important oxygen - compounds:

• Carbides, carbon element compounds ExCy type in which the carbon of the reactant is electronegative. Many metals can form carbides, which are sometimes very hard and for cutting tools are used (eg tungsten carbide).

• Carbon monoxide CO is a highly toxic gas which acts strongly reducing and in the primary metal (eg iron ) plays an important role.

• Carbon dioxide CO2 is a by the release during the combustion of fossil carbon stocks (coal, oil, natural gas) arising in large quantities of greenhouse gases. It is exhaled by most organisms and plant used in the photosynthesis. Carbon dioxide is now to about 0.038 % component of the atmosphere, in the pre-industrial era, the proportion was 0.028%.

• Carbonic acid H2CO3 is a metastable product of water and in the water dissolved CO2; a moderately strong acid, but which is usually summarized with respect to the permanent conversion between carbonic acid and dissolved CO2 with the CO2.

• Suboxides as Trikohlenstoffdioxid ( malonic, C3O2 ) Tetrakohlenstoffdioxid ( C4O2 ), Pentakohlenstoffdioxid ( C5O2 ), oxalic ( C4O6 ) and Mellitsäureanhydrid ( C12O9 ).

• Hydrogen carbonates or bicarbonates HCO3-, the most well known sodium bicarbonate is used among other things as a leavening agent e.

• Carbonate CO32 - E2 are the divalent salts of carbonic acid. The two most carbonates are sodium carbonate, common name soda, an important raw material for the manufacture of glass and calcium carbonate from the eg mussels, snails build their shells and sequester the stony corals. The calcium carbonate by them and by other processes formed over long periods of time makes all the mountains today (see: limestone). Calcium carbonate is also an important building material.

• Carbon-sulfur compounds, of which the best known compound carbon disulfide ( carbon disulfide, CS2), a very toxic liquid.

• Carbon -nitrogen compounds, such as cyanides, the most well known potassium cyanide a very strong, respiratory blocking, poison is. Many other cyanides are similar toxic.

Isotopes

Carbon has two stable isotopes, 12C and 13C. 12C comes to 98.9 % in nature, 13C to 1.1%. 12C is by definition the reference point for the unit of atomic mass. 13C can be detected in NMR spectroscopic studies, since it, unlike 12C, has a magnetic moment.

Besides these two stable isotopes, there are several unstable isotopes. The best-known unstable isotope is 14C with a half-life of 5730 years. It is produced by natural nuclear reactions in the atmosphere of 14N.

Isotope ratio to determine the age of organic materials

Organic material which takes part in the carbon cycle, showing the same ratio of 14 C to the stable isotopes of carbon, such as in the atmosphere. After the end of the metabolism, so for example, after the precipitation of a tree, this ratio decreased gradually by the decay. Therefore, the determination of the ratio of 14C to stable isotopes allowed the determination of age to objects made ​​of organic material ( radiocarbon method), which is used in archeology in particular.