Carbon (from Latin: carbo "coal") is a chemical element with symbol C and atomic number 6. On the periodic table, it is the first (row 2) of six elements in column (group) 14, which have in common the composition of their outer electron shell. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.
Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all forms of carbon-based life, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life.
The atoms of carbon can be bonded together in different ways: allotropes of carbon. The best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form. For example, graphite is opaque and black, while diamond is highly transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek word "γράφω" which means "to write"), while diamond is the hardest naturally-occurring material known. Graphite is a very good conductor, while diamond has a very low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen.
The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates. Carbon forms a vast number of compounds, more than any other element, with almost ten million compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.
^ Conventional Atomic Weights 2013. Commission on Isotopic Abundances and Atomic Weights
^ Standard Atomic Weights 2013. Commission on Isotopic Abundances and Atomic Weights
^ Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
^ Haaland, D (1976). "Graphite-liquid-vapor triple point pressure and the density of liquid carbon". Carbon 14 (6): 357. doi:10.1016/0008-6223(76)90010-5.
^ Savvatimskiy, A (2005). "Measurements of the melting point of graphite and the properties of liquid carbon (a review for 1963–2003)". Carbon 43 (6): 1115. doi:10.1016/j.carbon.2004.12.027.
^ "Fourier Transform Spectroscopy of the System of CP" (PDF). Retrieved 2007-12-06.
^ "Fourier Transform Spectroscopy of the Electronic Transition of the Jet-Cooled CCI Free Radical" (PDF). Retrieved 2007-12-06.
^ "Carbon: Binary compounds". Retrieved 2007-12-06.
^ a b c d e Properties of diamond, Ioffe Institute Database
^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
^ "History of Carbon and Carbon Materials - Center for Applied Energy Research - University of Kentucky". Retrieved 2008-09-12.
^ Senese, Fred (2000-09-09). "Who discovered carbon?". Frostburg State University. Retrieved 2007-11-24.
^ "Carbon – Naturally occurring isotopes". WebElements Periodic Table. Retrieved 2008-10-09.
^ "History of Carbon". Retrieved 2013-01-10.
^ "Biological Abundance of Elements". The Internet Encyclopedia of Science. Retrieved 2008-10-09.
^ "World of Carbon – Interactive Nano-visulisation in Science & Engineering Education (IN-VSEE)". Retrieved 2008-10-09.
^ Chemistry Operations (December 15, 2003). "Carbon". Los Alamos National Laboratory. Archived from the original on 2008-09-13. Retrieved 2008-10-09.

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