Carbon
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Carbon is a chemical element with the symbol C and atomic number 6. It is a non-metal with six protons in its nucleus. Carbon is located in group 14 of the periodic table, meaning that each carbon atom can bond with four electrons from other elements.
Carbon is significant because of its ability to form long chains and rings, bonding with other carbon atoms and with hydrogen, oxygen, nitrogen, and other elements. This creates an enormous number of molecules. Consequently, all known life is based on carbon chemistry, which is called organic chemistry, the study of carbon compounds.
Allotropes of elemental carbon
Elemental carbon exists in different forms called allotropes: substances made of the same element but with different structures. Diamond is known for extreme hardness, because each atom is bonded uniformly to four other atoms. Graphite consists of layered sheets that slide easily over one another, and is therefore used in pencil lead and lubrication. Newer forms discovered in the 20th and 21st centuries include fullerenes and graphene.
Carbon in nature
Carbon in the atmosphere occurs mainly as carbon dioxide (CO₂), a colorless gas. Plants take CO₂ from the air using energy from sunlight (photosynthesis), converting it into sugars and other compounds. Animals and bacteria release CO₂ back into the atmosphere when they respire or when organisms decay. This cycle is called the carbon cycle.
Large amounts of carbon are also stored in carbonate rocks, oceans, soil, and fossil fuels underground, including oil and gas. Burning fossil fuels releases additional CO₂ into the atmosphere, linked to rising global temperatures, because CO₂ traps outgoing heat.
Isotopes and scientific use
Carbon has isotopes, which are atoms with the same number of protons but different numbers of neutrons. The most common are carbon-12 and carbon-14. Carbon-14 is radioactive and decays at a constant rate; this is the basis for radiocarbon dating, used to determine the age of organic remains in the archaeological and historical record.