Carbon is a chemical element with the symbol C and atomic number 6. A nonmetal belonging to group 14 of the periodic table, it occurs both as an uncombined element and in an extensive variety of compounds. Its ability to form strong bonds with itself and other elements makes it central to organic chemistry and the molecular structure of all known life. Elemental carbon has several structurally distinct forms, including graphite, diamond, and graphene, with markedly different physical properties. (periodic-table.rsc.org)
Atomic structure and bonding
A neutral carbon atom contains six protons and six electrons. Its ground-state electron configuration is 1s² 2s² 2p², leaving four electrons in its outer shell. Carbon commonly forms four bonds through the sharing of electrons in covalent bonds. This tetravalence is a foundational principle of carbon chemistry, although reactive intermediates and some unusual compounds do not follow the simplest four-bond description. (periodic-table.rsc.org)
Carbon atoms can bond to one another in long chains, branched structures, and rings. This capacity, called catenation, combines with the possibility of single, double, and triple carbon–carbon bonds to produce a large diversity of molecular structures. Carbon also bonds readily with elements such as hydrogen, oxygen, nitrogen, and phosphorus. Different arrangements of the same constituent atoms can produce isomers with different properties. (openstax.org)
The geometries of many carbon compounds are described using orbital hybridization:
- sp³: four bonding directions arranged approximately tetrahedrally, as in methane and diamond.
- sp²: three bonding directions in a plane, associated with double bonds and the hexagonal networks of graphite and graphene.
- sp: two opposed bonding directions, characteristic of linear structures containing carbon–carbon triple bonds.
These descriptions help relate molecular geometry and bonding to the properties of carbon compounds and materials. (openstax.org)
Isotopes and atomic mass
Carbon has two stable naturally occurring isotopes, carbon-12 and carbon-13. In normal terrestrial materials, carbon-12 accounts for approximately 98.84–99.04 percent of carbon atoms and carbon-13 for approximately 0.96–1.16 percent. Their proportions vary because physical, chemical, and biological processes fractionate isotopes. Accordingly, carbon’s standard atomic weight is expressed as the interval [12.0096, 12.0116]; 12.011 is a commonly used rounded value. (ciaaw.org)
Carbon-12 defines the unified atomic mass unit: one unit is one-twelfth of the mass of a free carbon-12 atom at rest in its ground state. Its atomic mass is therefore exactly 12 on this scale. (ciaaw.org)
Carbon-14 is radioactive, with a half-life of approximately 5,730 years. It is continually introduced into Earth’s near-surface environment, principally through cosmic-ray-induced nuclear reactions. Its abundance is too small to contribute appreciably to the standard atomic weight. (ciaaw.org)
Carbon-14 provides the basis for radiocarbon dating, a form of radiometric dating used for carbon-bearing remains and other suitable materials. Once exchange with an external carbon reservoir ceases, radioactive decay progressively reduces the carbon-14 content. The method generally applies to ages within roughly the past 50,000 years. Interpretation must account for factors such as contamination, changes in atmospheric isotope abundance, and the source of the sample’s carbon; a measured radiocarbon age is not automatically a calendar age. (pubs.usgs.gov)
Allotropes and carbon materials
Allotropes are different structural forms of the same element. Carbon’s contrasting allotropes demonstrate how atomic arrangement can determine a material’s hardness, conductivity, transparency, and other properties. (media.iupac.org)
Graphite consists of stacked sheets of carbon atoms arranged in hexagonal networks. Strong bonding holds each sheet together, while weaker interactions permit adjacent sheets to slide. Graphite is soft, opaque, and electrically conductive, with properties that make it useful in lubricants, electrical components, and high-temperature applications. (nobelprize.org)
Diamond has a three-dimensional network in which each carbon atom is covalently bonded to four neighbors. It is exceptionally hard and, when sufficiently pure, transparent. At atmospheric pressure it is metastable rather than the equilibrium form of carbon: it persists because transformation to graphite is kinetically hindered. High pressure favors diamond’s stability. (goldbook.iupac.org)
Graphene is a single atomic layer of carbon in a hexagonal lattice. Its electrical transport, mechanical strength, and thermal conductivity make it important in materials research. Graphene is also a useful structural reference for understanding graphite, carbon nanotubes, and fullerene cages. (nobelprize.org)
Fullerenes are closed-cage carbon molecules. The best-known example, C₆₀ or buckminsterfullerene, has 60 carbon atoms arranged in a cage containing pentagonal and hexagonal faces. Carbon nanotubes have cylindrical carbon networks related geometrically to rolled graphene sheets. These structures expanded the study of carbon beyond its familiar bulk crystalline forms. (publications.iupac.org)
Amorphous and disordered carbons lack the long-range crystalline order of ideal graphite or diamond. The category includes materials with varied local bonding and microstructure, rather than one uniform substance. Charcoal, coke, and soot are carbon-rich materials that may also contain other elements and impurities; they should not simply be equated with chemically pure carbon. (media.iupac.org)
Compounds and chemical reactions
Hydrocarbons contain only carbon and hydrogen. They range from the simple molecule methane, CH₄, to much larger chains and rings. Carbon compounds containing additional elements include alcohols, organic acids, sugars, and many other classes distinguished by their functional groups. A carbon skeleton and the groups attached to it jointly determine a compound’s chemical behavior. (openstax.org)
Carbon also occurs in compounds conventionally treated as inorganic, including carbon dioxide and carbonates. Carbonate minerals are important constituents of rocks such as limestone and dolomite. Thus, “carbon-containing” and “organic” are not interchangeable classifications. (openstax.org)
Carbon spans a range of oxidation states, including −4 in methane, 0 in the uncombined element, and +4 in carbon dioxide. Oxidation of carbon-bearing fuels is central to combustion and many biological energy transformations. The energy released by such reactions comes from the overall balance between breaking existing bonds and forming new ones, not from bond breaking alone. (periodic-table.rsc.org)
Origin and occurrence
Much of the universe’s carbon was produced by nucleosynthesis inside stars. During helium burning, the triple-alpha process combines three helium-4 nuclei to form carbon-12. Stellar mass loss and stellar explosions distribute newly formed elements into space, supplying material from which later stars and planetary systems form. (imagine.gsfc.nasa.gov)
On Earth, carbon occurs in native graphite and diamond, carbonate rocks, carbon-rich sediments, living organisms, soils, the ocean, and the atmosphere. It is also present in fossil fuels, including coal, petroleum, and natural gas. Most of Earth’s carbon is stored in rocks rather than in living organisms or the atmosphere. (periodic-table.rsc.org)
Biological role and the carbon cycle
Carbon forms the structural backbone of major biological molecules, including proteins, carbohydrates, lipids, and nucleic acids. Its bonding versatility allows the construction of molecules that store information, catalyze reactions, form cellular structures, and participate in energy transfer. (openstax.org)
During photosynthesis, plants and other photosynthetic organisms incorporate carbon from carbon dioxide into organic compounds using an external energy supply. Carbon subsequently passes through organisms and their environments. Respiration, decomposition, and combustion return organic carbon to oxidized forms, while some carbon becomes stored in sediments and rocks. (science.nasa.gov)
The carbon cycle encompasses exchanges among these reservoirs. Biological exchange and ocean–atmosphere exchange operate alongside slower geological processes such as weathering, sediment burial, and volcanic release. The amount of carbon in a reservoir and the rate at which carbon enters or leaves it are distinct quantities; a large reservoir need not exchange carbon rapidly. (science.nasa.gov)
History
The name carbon derives from the Latin carbo, meaning charcoal. Charcoal and soot were familiar in prehistoric times, so carbon has no single discovery date comparable to elements first isolated in modern laboratories. In 1797, Smithson Tennant demonstrated that diamond is a form of pure carbon. (ciaaw.org)
In 1858, August Kekulé and Archibald Couper independently proposed carbon’s tetravalence and helped establish the structural understanding of organic compounds. In 1874, Jacobus van ’t Hoff and Joseph Le Bel introduced the spatial interpretation of carbon’s four bonding directions, contributing to the development of stereochemistry. (openstax.org)
The discovery of fullerenes in 1985 was recognized by the 1996 Nobel Prize in Chemistry. Experiments on isolated graphene reported in 2004 subsequently established a major field of two-dimensional materials research and were recognized by the 2010 Nobel Prize in Physics. (nobelprize.org)
Applications and environmental significance
Carbon materials serve several distinct industrial functions:
- Graphite is used in lubricants, refractory materials, motor brushes, and electrodes. Natural and synthetic graphite are important negative-electrode materials in lithium-ion batteries. (usgs.gov)
- Diamond is used in cutting, drilling, and abrasive tools because of its hardness. (periodic-table.rsc.org)
- Activated carbon is used in filtration and purification, where its porous structure supports adsorption. (periodic-table.rsc.org)
- Carbon fibers provide strong, lightweight reinforcement in composite materials used in aircraft, sporting equipment, and other products. (periodic-table.rsc.org)
- Coke and charcoal are used in metal smelting, while hydrocarbons supply fuels and chemical feedstocks for products such as plastics, solvents, and synthetic fibers. (periodic-table.rsc.org)
Environmental effects depend on carbon’s chemical form and movement between reservoirs. Fossil-fuel combustion and land-use change alter the carbon cycle and increase atmospheric carbon dioxide. Carbon dioxide and methane are greenhouse gases involved in climate change, while ocean uptake of carbon dioxide contributes to ocean acidification. These effects concern particular compounds and processes, not elemental carbon as a single undifferentiated substance. (science.nasa.gov)
Measurements expressed as a mass of carbon must also be distinguished from measurements expressed as a mass of carbon dioxide. From their approximate molar masses, 1 tonne of carbon corresponds to 44/12, or about 3.67 tonnes, of carbon dioxide when fully converted to CO₂. This is a mass conversion, not a measure of the climate effect of every carbon-containing substance. (periodic-table.rsc.org)
References
- Carbon — Element information, properties and usesperiodic-table.rsc.org
- Atomic Weight of Carbonciaaw.org
- Standard Atomic Weightsciaaw.org
- Frequently Asked Questionsciaaw.org
- 3 Biological Molecules — Concepts of Biologyopenstax.org
- 3 Carbon — Biology 2eopenstax.org
- 4 Development of Chemical Bonding Theory — Organic Chemistryopenstax.org
- The Chemical Bondcen.acs.org
- The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phasedoi.org
- Graphene: Scientific Background on the Nobel Prize in Physics 2010nobelprize.org