A double bond is a type of chemical bond in which two electron pairs are shared between two atoms. This type of bond involves four bonding electrons between atoms, rather than the usual two bonding electrons involved in a single bond. Because of the large number of electrons, double bonds tend to be reactive. Double bonds are shorter and stronger than single bonds.
Double bonds are drawn as two parallel lines in chemical structure diagrams. The equal sign is used to indicate a double bond in a formula. Russian chemist Alexander Butlerov introduced double bonds in structural formulas in the mid-19th century.
Ethylene (C2H4) is a hydrocarbon with a double bond between the two carbon atoms. Other alkenes also contain double bonds. Double bonds are seen in imine (C=N), sulfoxides (S=O), and azo compounds (N=N).