The fractions from crude oil are largely mixtures of hydrocarbons that are alkanes. Alkanes are members of a homologous series with the general formula CnH2n+2, where n is the number of carbon atoms. An alkane with 4 carbon atoms has 2 × 4 + 2 = 10 hydrogen atoms, so its formula is C4H10 and it is called butane.
The first four alkanes are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Each differs from the next by one carbon atom and two hydrogen atoms. Alkanes are saturated, which means all the bonds between their carbon atoms are single bonds.
Members of a homologous series have similar chemical properties, and their physical properties, such as boiling point, change gradually as the chain gets longer. A fraction from crude oil is not pure. It is a mixture of alkanes with chain lengths in a similar range, together with small amounts of other compounds.