Organic compounds contain a structural backbone of carbon atoms, referred to as the carbon skeleton. A carbon atom is tetravalent, so it can make four covalent bonds. It often bonds with atoms such as hydrogen, nitrogen, and oxygen. The simplest hydrocarbon, methane, has a single carbon atom bonded to four hydrogen atoms. Longer carbon chains create new molecules. The skeleton can either remain linear and unbranched, such as in hexane or it can have branched side chains, as in isohexane. Molecules with same number of atoms in different arrangements are called structural isomers. Hexane and isohexane share the same chemical formula, C6H14 even though hexane is linear and isohexane is branched at the second carbon atom. These structures can contain double bonds at different positions, forming hexenes. They can even be arranged in rings, as seen in cyclohexene.