Hydrocarbons are molecules that contain hydrogen and carbon. These molecules vary widely in terms of the numbers of carbons and hydrogens, the numbers of single and double bonds, and the structural orientation of each component. Therefore, it is possible to have extremely small hydrocarbons, such as methane (CH4), and extremely large hydrocarbons. One of the smaller, but biologically and commercially useful hydrocarbons is ethylene, and we will discuss its properties and uses below.