An abundant nonmetallic, tetravalent element, carbon has several allotropic forms:

• 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).
• 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 : 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 : The most recently discovered allotrope and the hardest substance known to man.
• Lamp black : 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 fiber axis there is no orientation of the carbon planes. The result are fibers with a higher specific strength than steel.

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 10 million known compounds. When united with oxygen it forms carbon dioxide which is absolutely 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.

Carbon is a remarkable element for many reasons. Its different forms include one of the softest (graphite) and one of the hardest (diamond) substances 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 in 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.

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• The isotope carbon-14 was discovered in February 27, 1940 and is used in radiocarbon dating.
• 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.
• Graphite carbon in a powdered, caked form is used as charcoal for cooking, artwork and other uses.
• Activated charcoal is used in medicine (as powder or compounded in tablets or capsules) to absorb toxins or poisons 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. Nanoparticles might however be toxic.