The simplest hydrocarbon is composed of one carbon atom and four hydrogen atoms, CH 4 . This molecule is called methane and is the principal component of natural gas. As more carbon atoms are added to the methane molecule, a carbon chain, C-C, starts to form. The next molecule in the series is ethane which has two carbons and six hydrogen atoms, C 2 H 6 . Ethane is a very important chemical because it is used to make ethylene.
Ethylene is composed of two carbon atoms but only four hydrogen atoms.
The difference between these two molecules is that in the case of ethane the carbon atoms are tied together with a single atomic bond while in the case of ethylene the carbon atoms are joined by a double bond. Compounds consisting solely of carbon and hydrogen atoms joined together by single bonds are called saturated hydrocarbons. Those that are tied together by double bonds are called unsaturated . Ethylene is used as an important chemical building block to make plastics such as polyvinyl chloride for PVC piping, ethylene glycol for automotive antifreeze, and acetic acid for medicine. As more carbon atoms are added, the molecule's weight and properties change.