One of the most important properties of carbon is that it can make strong electron-pair bonds with other carbon atoms. A C-C bond is nearly as strong as a C-H bond ( 83 kcal/mole versus 99 kcal/mole ). Virtually endless carbon chains thus are possible, as well as branched chains of the type shown on the right. Straight- and branched chain compounds of C and H are called hydrocarbons, of which propane gas, gasoline, motor oil, paraffin wax, and polyethylene plastic are familiar examples. Chapters 19 through 21 will be devoted entirely to carbon compounds, and will be the bridge into the chemistry of living organisms. These simple hydrocarbons are built from chains of linked carbon tetrahedra, with the general formula CH₃-CH₂-CH₂-----CH₂-CH₃. The smaller hydrocarbons such as propane, CH₃-CH₂-CH₃, have such weak van der Waals forces between molecules that they are gases at room temperature. Gasolines, with six to nine carbon atoms in a chain, have sufficiently large surface areas and strong enough van der Waals attractions that they are liquids. Paraffin waxes with 20 or more carbons per chain are solids, and at the extreme limit of several thousand carbon atoms per chain we find the tough and chemically unreactive polyethylene plastic, familiar to us in lightweight water bottles for hikers and acid-resistant laboratory beakers.