Crude oil is a finite resource found in rocks. It is the remains of ancient biomass, mainly plankton, that was buried in mud. It forms far more slowly than we use it, so it will run out.
Crude oil is a mixture of a very large number of compounds. Most of them are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only. Most of the hydrocarbons in crude oil are alkanes.
The alkanes form a homologous series with the general formula CnH2n+2, so the number of hydrogen atoms is twice the number of carbon atoms plus two. The first four members are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). Propane, for example, has three carbon atoms and eight hydrogen atoms.
To recognise an alkane, check that the molecule contains only carbon and hydrogen and that its formula fits CnH2n+2. In a displayed formula every bond in an alkane is a single bond. You only need to know the names of the first four alkanes.
