See also: Carbon (homonymy)
The carbon is a chemical element of the family of the Cristallogène S, of symbol C , Atomic number 6 and of Atomic mass 12,0107.
History and etymology
Carbon was discovered as of prehistory and was produced by the incomplete combustion of the organic matters producing of the coal. It was also known in its form Diamant, natural diamonds being able to be in the Kimberlite chimneys of old volcanos, in particular in South Africa and Arkansas. One can sometimes find diamonds microscopic in certain meteorites.
The name carbon comes from Latin carbo , carbōnis meaning “coal”. It appears in the dictionary of the French Academy, only with its 6th edition (1832-5)
Nucleosynthesis
The element carbon is not directly resulting from the Big Bang (paramount Nucléosynthèse), because the conditions of its formation were not met (the dilation and the cooling of the Univers were too fast).
Carbon on the other hand is product bulk in the heart of the very massive star S, said connects horizontal, where three cores of Hélium amalgamate (Réaction triples alpha).
See also: Nucleosynthesis
Chemistry
Pure carbon is present in nature in two current forms:
- the Graphite, of crystalline Structure hexagonal and gray color, it is the most current form by far,
- the Diamant, of tetrahedral crystalline structure and transparent color.
Electronic structure
Carbon comprises 4 electron S on its external layer ((K) 2 (L) 4) (2s 2 , 2p 2 ) and can form a connection and three connections : it is thus about a tetravalent element (except in the case of certain intermediate reactions or in the family of the carbenes , with divalent carbon, the two electrons 2s not taking part in the connection). It has a great affinity for others Atome S light, of which itself, and its small size enables him to form multiple connections by hybridization of its orbital (sp or “ triple liaison ” linear, sp 2 or “ double liaison ” planar with angles of 120° , sp 3 or “ simple liaison ” tetrahedral) : that explains the multitude of made up containing carbon whose study constitutes the Organic chemistry.
Current forms
The most current shape of carbon is the Carbon dioxide CO, which one of the minority components of the atmosphere Ground stre produced and is metabolized by the alive beings . CO is a majority compound of the atmosphere of another planets like Venus.
In water, it forms very small quantities of Dioxide carbon, HCO, which produces Ion S Carbonate CO 2 or Hydrogénocarbonate ( bicarbonate ) HCO - . Many minerals are carbonates, in particular the various limestone shapes (Calcite, Craie, Marbre…). One knows the therapeutic properties of bicarbonate NaHCO sodium.
Dangers of carbon and its compounds
(Di) the Carbon bisulphide CS, though of structure similar to carbon dioxide, is a highly toxic liquid used like Solvant (Vulcanization of rubber).
Other carbon monoxide are the Carbon monoxide CO, and the carbon suboxyde CO, less common. The Carbon monoxide is a colorless and odorless gas, formed by incomplete Combustion of the organic compounds or pure carbon (coal). It has a structure semirésonante, the length of its connection (111 pm) indicating a hybrid nature, which results in representing it by the formule :
The Carbon monoxide sticks, preferentially with oxygen, the blood Hémoglobine to form carboxyhaemoglobin, a stable compound. The result of this reaction is the poisoning of the molecules of Hémoglobine, which can be mortal (see the entry in question).
The ion Cyanide CN - and a chemical behavior similar to an ion Halide. Salts containing the ion cyanide are highly toxic. The cyanogen, a gas of composition (CN) is also close to the Halogène S.
With metals, carbon forms carbides C 4 or of the acétylures C 2 . No matter what it arrives, with an electronegativity of 2,5, carbon prefers to form covalent bonds. Some carbides are covalent lattices, as the Carbide silicon, SiC, which resembles the Diamant, and is used besides for the size of those.
The Nanotechnologie S make it possible to produce particular molecules, of which fullerenes, and nanofils or nanotubes difficult to filter in the air, and who can pass through the mucous membranes and bore the cellular envelopes. Some fear differed effects as one found some for the Amiante.
Hydrocarbons
One calls Hydrocarbons molecules associating carbon and Hydrogen. One classifies hydrocarbons into three familles :
- alkanes, where carbon forms connections sp 3 (“ simples ”): CH methane, CH ethane, etc;
- olefinic hydrocarbons, where at least a carbon forms connections sp 2 (“ double ”) : CH ethylene, CH propene, etc;
- alkyne compounds, where at least a carbon forms connections sp (“ triple ”) : CH acetylene, CH propyne, etc
According to the number of carbon atoms, one makes precede the suffix - ass, - ene or - yne par :
- méth-
- éth-
- m.
- goal
- pent-
- hex-
- hept-
- Oct.
- not
- DEC
Only carbon sp 3 give place to mobile connections. Other connections, sp or sp 2 are rigides : the connection doubles sp 2 is planar, each of the two other connections forming an angle of 120°, the triple connection is linear. That led to the formation of enantiomer S, i.e. of body having the same chemical formula but different space conformations.
Moreover, carbon sp 3 can form asymmetrical compounds, it appears then a phenomenon of chirality (of the Greek ̔ η χείρ, the hand, the molecules are symmetrical one of the other according to a symmetry plane, like our two hands)
In the aromatic hydrocarbons, the carbon atoms form cycles or cores stabilized by delocalized connections π. They mix in the various pure carbon shapes. There are even more hydrocarbon derivatives, such as for example the Halogénure S, the alcohols and the carboxylic acid.
Allotropes
Under the normal conditions of Pressure, carbon is in the form Graphite, in which each atom is related to three others in a layer of amalgamated hexagonal rings, like those of made up the Aromatique S hydrocarbon. Thanks to the delocalization of orbital the , graphite leads the electricity. Graphite is soft, because the chemical bonds between the plans are weak (2 % of those of the plans) and the layers thus slip easily the ones compared to the others.
Under very high pressure, carbon crystallizes in a cubic system with centered face named Diamant, in which each atom is related to four others (interatomic distance from 136 pm). The Diamond, thanks to the resistance of the connections carbon-carbon, is, with the Boron nitride, the matter hardest to stripe. With room temperature, the metamorphosis in Graphite is so slow that it appears invisible. Under certain conditions, carbon crystallizes in Lonsdaleite, a form similar to diamond but hexagonal. Let us note that, of all the invaluable Pierres, the Diamant is only the Consumer completely.
In addition to graphite (pure sp 2 ) and of diamond (pure sp 3 ), carbon exists in amorphous and highly disordered form (ac). These amorphous shapes of carbon are a mixture of sites to three connections of the graphite type or four connections of the diamond type. Many methods are used to manufacture a-C : pulverization, evaporation by beam of electron S, deposit with the electric arc, laser ablation…
Carbon sublimates with: 4,100 K. In gas form, it usually constitutes in small chains of atoms called Carbynes. Cooled very slowly, those amalgamate to form the irregular graphitic sheets and deformations which compose the Suie. In particular, among these last, one finds forms where the sheets are folded in a form stable and closed like a Sphère or a tube, called Fullerène S, like buckminsterfullerene, C. Some of these forms are also known under the name of “ footballène ”, and properties have which all were not analyzed yet, but seem extremely rigid structures.
The cylindrical shapes of carbon are called Nanotube S and were discovered in the base being formed with the Cathode electric arc lasting the synthesis of Fullerène S. These objects of diameter nanometric and length reaching the millimetre sometimes are presented in the form of layers of graphene rolled up on themselves.
The nanotubes manufactured by the method of the electric arc are almost all “ multifeuillets ”, jointly with these nanotubes one observes a great number of Nanoparticule S polyhedric. Observations in electronic Microscopy in transmission high-resolution (HRTEM : High-resolution Transmission Electron Microscopy ) reveals that these nanoparticules carbon consist of several layers of graphene, are closed, leaving a nanometric cavity in their center.
And finally the onions of carbon are at the base of multi-layer fullerenes.
Isotopes
Carbon has two Isotope S stable in nature, 12 C (98,89 %) and 13 C (1,11 %).
The Radioisotope 14 A.c. a period of 5 730 years and for the dating of archaeological objects until 50 is usually used; 000 years. It will not be of any utility for the archeologists of tomorrow interested by the treasures of current civilization because the thermonuclear explosions carried out in the atmosphere starting from the Années 1960 created considerable excesses.
The Radioisotope 11 A.c. a period of 20 minutes. This short period and relative facility to substitute an atom of 11 C with a carbon atom 12 C (stable) make of it an isotope used in Nuclear medicine, in particular in Tomographie with emission of positon. The radiotraceurs most used to date are it 11 C-Raclopride which is fixed preferentially on the dopaminergique receivers D2, and it 11 C-acetate used in cardiac imagery.
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