Carbon forms more compounds than all other elements combined; several million are known. Each carbon atom forms four bonds (four single bonds, two single and one double bond, two double bonds, or one single and one triple bond) with up to four other atoms. A large number of chain, branched, ring, and three-dimensional structures can occur. The study of these carbon compounds and their properties and reactions is organic chemistry. With hydrogen, oxygen, nitrogen, and a few other elements whose small amounts belie their important roles, carbon forms the compounds that make up all living things: proteins, carbohydrates, lipids, and nucleic acids.
Organisms consume carbon and return it to the environment in the carbon cycle. Carbon dioxide
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