Hydrocarbons are naturally-occurring chains of carbon atoms surrounded by hydrogen atoms. Natural gas is 99+% methane (CH4), the smallest and simplest hydrocarbon, and the lightest one by molecular weight, having only one carbon atom. As the carbon number of a hydrocarbon chain increases to 6 or 7 and above, the molecules strongly prefer to be in the liquid form. Crude oil may average C20 or so, on up to C100 for very heavy oils.