Organic chemistry is the chemistry of carbon, an element that forms strong chemical bonds to other carbon atoms as well as to many other elements like hydrogen, oxygen, nitrogen, and the halogens. Because of its versatility in forming covalent bonds, more than a million carbon compounds are known. Many are composed of only carbon and hydrogen, collectively called hydrocarbons. Most hydrocarbons are obtained from petroleum. Carbon always forms four covalent bonds (four shared pairs of electrons) that may be present as four single bonds per atom, or two single bonds and one double bond, or one single bond and one triple bond. With the ability of carbon to bond in different ways, an important part of organic chemistry concerns the structure of compounds. For example, three organic compounds have the identical molecular formula, C 5H 12, but they are different because each connects the five carbon atoms together in a different arrangement. Compounds with the same formula but different structures are called isomers. In organic chemistry, both structure and composition are of prime importance.
Organic chemistry is important because the vital biological molecules in living systems are largely organic compounds. Nearly all commonly used plastics are prepared from organic compounds.