In organic chemistry, an alkane, or paraffin (a historical name that also has other meanings), is a saturated hydrocarbon. Alkanes consist only of hydrogen and carbon atoms and all bonds are single bonds. Alkanes (technically, always acyclic or open-chain compounds) have the general chemical formula C_nH_2n+2. For example, methane is CH₄, in which n = 1 (n being the number of carbon atoms). Alkanes belong to a homologous series of organic compounds in which the members differ by a molecular mass of 14.03 u (mass of a methylene group, —CH₂—, one carbon atom of mass 12.01 u, and two hydrogen atoms of mass ~1.01 u each). There are two main commercial sources: petroleum (crude oil) and natural gas.