organic chemistry, branch of chemistry dealing with the compounds of carbon. While it is only the fourteenth most common element on earth, carbon forms by far the greatest number of different compounds. Organic chemistry is of vital importance to the petrochemical, pharmaceutical, and textile industries, where a prime concern is the synthesis of new organic molecules and polymers. Compounds containing only hydrogen and carbon, of which there are many thousands, are called hydrocarbons; the simplest is methane (CH₄). In general, a particular type of organic compound, such as an alcohol, aldehyde, ether, or ketone, is identified by the presence of a characteristic functional group of atoms. The functional group is the part of the molecule most responsible for its particular chemical nature. Organic compounds containing nitrogen are of great importance in biochemistry. They generally contain the amine group (NH₂). Molecules containing both the NH₂ and COOH groups are called amino acids and are the building blocks of proteins.