Carbon is a chemical element with the atomic number 6 and the symbol C. It has three naturally occurring isotopes:
- 12 C and 13C which are stable;
- 14C which is radioactive with a half-life of 5,730 years which makes it possible to date elements using carbon for their structure.
| Symbol | C |
| Name | Carbon |
| Atomic number | 6 |
| Group | 14 |
| Period | 2nd period |
| Block | Block p |
| Element family | Non-metal |
| Electronic configuration | [He] 2 s 2 2 p 2 |
| Electrons by energy level | 2, 4 |
| Atomic mass | 12.010 74 ± 0.000 8 u, |
| Atomic radius (calc) | 70 pm (67 pm) |
| Covalence radius | SP3 76 ± 1 pm SP2 73 ± 2 pm SP 69 ± 1 pm |
| Van der Waals radius | 150 pm |
| Oxidation state | -4, 0, +4, +2 |
| Electronegativity (Pauling) | 2,55 |
| Oxide | Weak acid |
| 1re: 11,260 30 eV | 2nd: 24.383 3 eV |
| 3rd: 47.887 8 eV | 4th: 64.493 9 eV |
| 5th: 392.087 eV | 6e: 489,993 34 eV |
| Ordinary state | Diamagnetic solid |
| Allotrope in the standard state | Graphite |
| Other allotropes | Diamond, graphene, nanotubes, fullerenes, amorphous carbon |
| Density | 1.8 to 2.1 g·cm-3 (amorphous), 1.9 to 2.3 g·cm-3 (graphite),3.15 to 3.53 g·cm-3 (diamond), 3,513 g·cm-3 (gem diamond, 25 °C) |
| Crystal system | Hexagonal (graphite)Cubic diamond (diamond) |
| Hardness (Mohs) | 0,5 |
| Color | Black (graphite) |
| Boiling point | 3,825 °C (sublimation) |
| Vaporization energy | 355.8 kJ·mol-1 |
| Triple point | 4 489 °C, 10 800 kPa |
| Molar volume | 5.29×10-6 m3·mol-1 |
| Speed of sound | 18,350 m·s-1 at 20 °C |
| Mass heat | 710 J·kg-1· K-1 |
| Electrical conductivity | 61×103 S·m-1 |
| Thermal conductivity | 129 W·m-1· K-1 |
| CAS No. | 7440-44-0 |
| No ECHA | 100.028.321 |
| SI & CNTP units, unless otherwise stated. |
Carbon is the lightest element in group 14 of the periodic table. The simple carbon body has several allotropic forms, mainly graphite and diamond. This element forms various inorganic compounds such as carbon dioxide CO2, and a wide variety of organic compounds and polymers. It is the basic element of all known life forms.
Carbon is the 4th most abundant element in the universe and the 15th most abundant in the Earth’s crust. It is present on Earth as a simple body (coal and diamonds), inorganic compounds (CO2) and organic compounds (biomass, oil and natural gas). Many carbon-based structures have also been synthesized: activated carbon, carbon black, fibers, nanotubes, fullerenes and graphene.
The burning of the element in all its forms has been the foundation of technological development since prehistoric times. Carbon-based materials have applications in many other fields: composite materials, lithium-ion batteries, air and water pollution control, electrodes for arc furnaces or aluminum synthesis, etc.
History and Etymology of Carbon
The name carbon comes from the Latin carbo, carbōnis (“coal”). The manufacture of this element in the form of charcoal by pyrolysis of wood under a layer of earth was also known to the Romans. Carbon in its diamond form has been known since ancient times in Asia, it is also mentioned in the Old Testament. Its name also comes from the Roman adámas, adámantis (“hard steel”).
The notion of carbon element appears when René Antoine Ferchault de Réaumur studies the formation of steel from iron, he notes that this transformation corresponds to the absorption of an element by iron. In 1772, Antoine Lavoisier then studied the combustion of coal and diamonds, he noted the notable formation of carbon dioxide but did not detect the formation of water. He proves that these two materials are formed only of this element.
Natural graphite had been known since ancient times, but its nature was not understood because it was confused with molybdenite and believed to be a form of lead. In 1779, Carl Wilhelm Scheele demonstrated, also by oxidation of graphite, that it is composed mainly of carbon. In 1787, Louis-Bernard Guyton-Morveau’s Chemical Nomenclature devoted an article to him defining the element as the pure form of coal.
The name “carbon” does not appear in the dictionary of the Académie française until its 6th edition (1832-5). The nineteenth century corresponds to the rise of carbon for energy production. For example, in 1865, Antoine César Becquerel published the carbon content of the main forms of energy wood purchased at the time in Paris:
- 1 stere of hardwood (oak, elm, hornbeam, beech and ash): 140 kg;
- 1 stere of white wood (birch, aspen, poplar and softwood): 87 kg;
- 1 stere of wood with bundles and cotrets: 122 kg.
History is then marked by the increased importance of carbon, we can cite for example:
- 1828: discovery of organic compounds and organic chemistry (see article Friedrich Wöhler);
- 1842: with the strength of materials, August Wöhler lays the foundations for the future “science of materials”;
- 1985: Discovery of fullerenes by Robert Curl, Harold Kroto and Richard Smalley;
- 2004: discovery of graphene by Andre Geim, composed of a single layer of graphite.
Element
Formation
The element carbon did not come directly from the Big Bang (primordial nucleosynthesis), because the conditions for its formation were not met (the expansion and cooling of the universe was too fast). Carbon, on the other hand, is mass-produced in the core of very massive stars, known as the horizontal branch, where three helium nuclei merge (triple alpha reaction).
Carbon has been present on Earth since its formation. It exists in the form of sediments, coal, oil, and also in its pure graphite, diamond form. Natural diamonds can be found in the kimberlite of the chimneys of ancient volcanoes, especially in South Africa and Arkansas. Microscopic diamonds can sometimes be found in some meteorites.
Isotopes and atomic mass of Carbon
Carbon has two isotopes that are stable in nature:
- 12C (abundance = 98.93%) which was chosen as the single reference nuclide for atomic mass 12, after several proposals (formerly hydrogen, then together with oxygen for chemists);
- 13C (abundance = 1.07%).
The atomic mass of carbon, 12.010 7, is slightly above 12 due to the presence of the isotope, 13C.
Carbon also has two radioisotopes:
- 14C: Radioactive half-life of 5,730 years commonly used for dating archaeological objects up to 50,000 years. It will be of no use to the archaeologists of tomorrow, interested in the treasures of today’s civilization, because thermonuclear explosions, carried out in the atmosphere from the 1960s onwards, have created considerable excesses;
- 11C has a period of 20 minutes. This short period and the relative ease of substituting an 11C atom for a 12 C carbon atom (stable) make it an isotope used in nuclear medicine, especially positron emission tomography. The most widely used radiotracers to date are 11 C-Raclopride which preferentially binds to dopaminergic D2 receptors, and 11C-Acetate used in cardiac imaging.
Electronic structure
Carbon with six electrons adopts a 1s 2 2s 2 2p2 ground state electronic configuration. It has four electrons on its valence shell, which allows it to form four covalent bonds, including type σ bonds (first bond with an atom) or type π (second or third bond). Type π links are always accompanied by a type σ link. The overlap of electronic functions in a π link is lower. These bonds are therefore less “strong”.
Single body
Solid State
Carbon occurs in nature in two main allotropic forms:
- graphite, a stack of hexagonal and monoplane crystalline structures (graphene), and gray in color. It is the stable form at room temperature and pressure;
- The diamond, of tetrahedral crystalline structure (structure type “diamond”) is transparent. It is the stable form at high temperature and high pressure, metastable at temperature and ambient pressure.
Under normal pressure conditions, carbon is in graphite form, in which each atom is bonded to three others in a layer of fused hexagonal rings, like those of hydrocarbon aromatic compounds. Through the delocalization of π orbitals, graphite conducts electricity. Graphite is soft because the chemical bonds between the planes are weak (2% of those of the planes) and the layers, therefore, slide easily relative to each other.
Under very high pressure, the element crystallizes in a face-centered cubic system called diamond, in which each atom is bonded to four others (interatomic distance of 136 pm). Diamond, thanks to the strength of carbon-carbon bonds, is, along with boron nitride, the hardest material to scratch. At room temperature, metamorphosis to graphite is so slow that it is undetectable. Under certain conditions, carbon crystallizes into lonsdaleite, a shape similar to diamond but hexagonal. Of all the gemstones, the diamond is the only one to burn completely.
In addition to graphite (pure sp2) and diamond (pure sp3), carbon exists in amorphous and highly disordered form (a-C). These amorphous forms of the element are a mixture of sites with three graphite-type bonds or four diamond-type bonds. Many methods are used to make A-C: sputtering, electron beam evaporation, electric arc deposition, laser ablation, etc. In 2019, the cyclic molecule C 18 (pure sp1) was synthesized by removing CO groups in oxide C24O6.
Carbon onions are structures based on a fullerene-like structure, but whose wall consists of several layers of the element.
The cylindrical shapes of carbon are called nanotubes (carbon nanotube, abbreviation: NTC). They were discovered in the pellet forming at the cathode of the electric arc during fullerenes synthesis. These objects of nanometric diameter and length sometimes reaching the millimeter are presented as planes of the element of monatomic thickness (or graphene) wound on themselves and forming a tube of nanometric diameter). Nanotubes whose wall consists of only one carbon plane are called “monosheets”. The nanotubes manufactured by the electric arc method are almost all “multilayered”.
Graphene consists of a single plane of carbon of monatomic thickness. Graphene can be obtained simply by taking a single carbon plane from a graphite crystal.
Together with these structures, a large number of polyhedral nanoparticles are observed. Like onions and multilayer nanotubes, observations in high-resolution Transmission Electron Microscopy (HRTEM) reveal that these carbon nanoparticles consist of several layers of graphene, closed, leaving a nanometric cavity at their center.
Liquid and gas
At atmospheric pressure, carbon (graphite) sublimates at 4,100 K. In gaseous form, it usually forms into small chains of atoms called carbynes. Cooled very slowly, these fuses to form the irregular and deformed graphitic sheets that make up the soot. Among the latter, we find in particular, the spherical monosheet shape C60 called fullerene, or more precisely buckminsterfullerene, and its varieties C n (20 ≤ n ≤ 100), which form extremely rigid structures.
Liquid carbon is formed only above the pressure and temperature of the triple point, and therefore above 10.8 ± 0.2 MPa (about 100 times atmospheric pressure) and 4,600 ± 300 K.
Compounds
Carbon is the essential component of organic compounds, which frequently contain at least one carbon-hydrogen bond. However, the element also exists in nature in inorganic form, mainly in the form of carbon dioxide, and in mineral form.
Organic carbon
Carbon chemistry is essentially covalent. Carbon is the basis of a multitude of compounds that can contain a large number of atoms, in association with hydrogen, oxygen, nitrogen, halogens, phosphorus, sulfur, and metals, in single, double or triple bonds. The study and synthesis of these compounds constitute organic chemistry. The main organic compounds of the element are the “hydrocarbons” of molecules associating carbon and hydrogen. Hydrocarbons are classified into three families:
- alkanes, where carbon forms sp3 (“simple”) bonds: methane CH4, ethane C2H6, etc.;
- alkenes, where at least one carbon forms bonds (“double”) (carbons sp 2): ethene (ethylene) C2 H 4, propene C3H6, etc.;
- alkynes, where at least one carbon forms bonds (“triple”) (sp carbons): ethyne (acetylene) C 2 H2, propyne C3H4, etc.
Depending on the number of carbon atoms, the suffix -ane, -ene or -yne is preceded:
- Meth-
- éth-
- Prop-
- goal-
- Pent-
- Hex-
- hept-
- Oct-
- No-
- Dec-
Rotation is free around single carbon-carbon bonds. On the other hand, double or triple bonds are rigid: the double bond is planar, the bond angles around the carbon atoms are 120°. This leads to the formation of diastereomers, i.e. compounds with the same chemical formula but a different arrangement of atoms in space. The triple bond is linear.
In addition, carbon sp3 can form chiral compounds (from the Greek kheir (ἣ χείρ), the hand). The simplest case is a compound having 4 different substituents around a carbon atom. Depending on the arrangement in space of these substituents, we obtain two molecules that are different: they are not superposable, it is a pair of enantiomers. Enantiomers are the image of each other in a mirror (like our two hands).
In aromatic hydrocarbons, carbon atoms form rings or nuclei stabilized by π delocalized bonds.
Inorganic carbon
This type of carbon atom is relatively rare in terms of variety compared to organic and mineral carbons. It is most often in the form of inorganic or organo-metallic complexes that integrate a bare carbon atom or a CO or CO2 molecule, in their coordination spheres. Like what:
- C in [Fe5 C(CO)15] and [Ru6C(CO)17];
- CO in the many complexes of the Ni(CO)4 or Fe(CO)5 type;
- CO2 in the complex [Ni(CO 2){P(C 6 H 11)3}2]0,75C6 H5Me.
Mineral carbon
The carbon dioxide molecule CO2 exists in a gaseous state in the Earth’s atmosphere. A certain amount of this CO2 dissolves in the ocean and inland waters, and part of the dissolved CO2 reacts with the water molecule to form carbonic acid H2CO3 following the reaction: H2O + CO2 (dissolved) ⇔ H 2CO3.
Then H2CO3 (dihydrogen carbonate, or carbonic acid), being a diacid, yields its two protons in the measurement of the acidity constants of the acid-base couples (H2CO3/HCO3−) and (HCO3–/CO32−) and the initial composition in acid-base solutes of water according to the equations: H2CO3 + H2O ⇔ HCO3–(hydrogen carbonate ion, or bicarbonate) + H3O+ (hydronium ion, or hydrated proton) and: HCO3− + H2O ⇔ CO32− (carbonate ion) + H3O+.
However, it turns out that in seawater, this carbonate system is present in large quantities and in such proportions that it plays a fundamental buffer role in the acidity of ocean water (pH 8.1-8.4) which it makes very stable. This rate of carbonates (and borates, to be exact) is called alkalinity or full alkalimetric strength (TAC, measured in French degrees, or kH measured in German degrees; there are other units. The best is to speak in ppm, or parts per million).
This pH allowed “geological” quantities of planktonic protozoan limestone tests to form calcareous sedimentary rocks consisting essentially of a crystal of calcium carbonate and magnesium (a mixture called limestone): Paris stone, marble, etc. All this chemistry is traditionally included in inorganic, i.e. mineral, chemistry, although there are obviously many points on which this is not justified. Thus, the element contained in carbon dioxide, carbonic acid, hydrogen carbonate and carbonate can be described as inorganic carbon. This is also true for diamond carbon and other allotropic varieties of carbon crystal.
Dangers of carbon and its compounds
Pure carbon has low toxicity to humans and can be safely handled and even ingested in the form of graphite or charcoal. It is resistant to dissolution or chemical attack, even in the acidic contents of the digestive tract, for example. Charcoal from coconuts is also used medicinally.
In contrast, carbon disulfide CS2, although structurally similar to carbon dioxide, is a highly toxic liquid used as a solvent (vulcanization of rubber).
Other oxides of carbon are carbon monoxide CO, and the less common carbon suboxide C3O2. Carbon monoxide is a colorless, odorless gas formed by incomplete combustion of organic compounds or pure carbon (coal). Carbon monoxide binds more strongly than oxygen to blood hemoglobin to form carboxyhemoglobin, a stable compound. The result of this reaction is the poisoning of hemoglobin molecules, which can be fatal (see entry in question).
The cyanide ion CN− has a chemical behavior similar to a halide ion. Salts containing the cyanide ion are highly toxic. Cyanogen, a gas of composition (CN)2 is also close to halogens.
With metals, the element forms C4− carbides or C2-2− acetylides. Whatever happens, with an electronegativity of 2.5, carbon prefers to form covalent bonds. Some carbides are covalent meshes, such as silicon carbide, SiC, which resembles diamond, and is also used for the cutting of these.
The toxicity of new allotropic forms of carbon (fullerenes, nanotubes, graphene) is now widely studied. In their native state, these nanostructures remain difficult to filter in the air and could constitute a hazard that needs to be assessed. Note that as part of their use, these compounds are generally dispersed in a solvent, or fixed on a solid substrate.
References (sources)
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