An homologous series is a series of related organic compounds that share similar properties and the same general formula, differing only by a single, unchanging collection of atoms. A notable example is the alkane hydrocarbon group, the general formula of which is C₂ H_{2n + 2}. Therefore each member of the group changes by a CH₂ group – methane (CH₄), ethane (C₂H₆), propane (C₃H₈), etc. Successive members of a homologous series are called homologues.