Ethene. Molecular model of the hydrocarbon ethene (ethylene, C2.H4). It has two carbon atoms (black) and four hydrogen atoms (white). The carbon atoms are joined by a double bond (grey), instead of the single bonds between the carbon atoms and hydrogen atoms. Hydrocarbons are chemicals containing just hydrogen and carbon. When a hydrocarbon contains a double bond it is called an alkene, and it is also called unsaturated, because a double bond is more reactive than a saturated molecule containing just single bonds. The double bond constrains ethene to form a planar (flat) molecule. Ethene, a gas made from oil, is used mainly in the plastics industry.