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Carbon (from Latin: carbo, meaning "coal") is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent, meaning that its atoms are able to form up to four covalent bonds. 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.

The Japanese word for carbon, tanso (炭素), literally translates to "charcoal element," reflecting the ancient human relationship with this element's most accessible form. Carbon's unique ability to form stable bonds with itself and with other elements in virtually limitless configurations makes it the basis of organic chemistry and, by extension, all known life.

Carbon is remarkable among the elements for the sheer variety of its allotropic forms -- distinct structural arrangements of the same atoms that yield dramatically different physical properties. From the hardest known natural material to one of the softest, carbon's allotropes span an extraordinary range.

Diamond

In diamond, each carbon atom is bonded to four others in a tetrahedral arrangement, forming an extremely rigid three-dimensional network. This sp³ hybridization gives diamond its unparalleled hardness (10 on the Mohs scale), exceptional thermal conductivity, and optical brilliance. Natural diamonds form at pressures exceeding 4.5 GPa and temperatures above 1000°C.

Graphite

Graphite consists of stacked layers of graphene sheets, held together by weak van der Waals forces. Each layer is a hexagonal lattice of sp²-bonded carbon atoms. The layers slide easily over one another, making graphite an excellent lubricant. The delocalized electrons within each layer give graphite its metallic lustre and electrical conductivity.

Graphene

A single layer of graphite -- a one-atom-thick sheet of carbon atoms arranged in a hexagonal lattice -- constitutes graphene. Isolated experimentally in 2004 by Geim and Novoselov (Nobel Prize, 2010), graphene exhibits extraordinary mechanical strength (tensile strength of 130 GPa), exceptional electrical conductivity, and near-perfect optical transparency.

The carbon cycle describes the biogeochemical exchange of carbon between the Earth's atmosphere, oceans, lithosphere, and biosphere. It operates on timescales ranging from days (respiration) to millions of years (geological sequestration) and is the fundamental regulatory mechanism for Earth's climate.

Photosynthesis fixes approximately 120 gigatonnes of carbon per year from the atmosphere into organic matter. Respiration and decomposition return a roughly equal amount. The oceans absorb approximately 2.5 Gt C per year as a net sink, while anthropogenic emissions from fossil fuel combustion add approximately 10 Gt C annually to the atmosphere.

Carbon forms more compounds than any other element, with the possible exception of hydrogen. Nearly ten million carbon compounds are known, and thousands of new ones are synthesized each year. The field of organic chemistry is defined as the study of carbon compounds.

Carbon dioxide (CO₂) is the most climatically significant carbon compound, absorbing infrared radiation and contributing to the greenhouse effect. Methane (CH₄) is 80 times more potent as a greenhouse gas over a 20-year period. Carbon monoxide (CO) is a toxic gas produced by incomplete combustion.

Carbon's applications span virtually every domain of human technology. As fuel, carbon compounds (coal, oil, natural gas) supply approximately 80% of global primary energy. As a structural material, carbon fiber composites are essential in aerospace and automotive engineering. As an electronic material, carbon nanotubes and graphene promise to transform semiconductor technology.

Activated carbon is used extensively in water purification, air filtration, and medical applications. Carbon black serves as a pigment and reinforcing filler in tires and rubber products. Diamond, both natural and synthetic, is indispensable in cutting, drilling, and polishing applications.