A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by $\mathrm{CH}_2$ group.Example : Alkanes with general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$Characteristics :(i) All the members of a homologous series can be represented by the same general formula and they have same functional group.(ii) Any two adjacent homologues differ by 1 carbon atom and 2 hydrogen atoms in their molecular fomulae.(iii) The difference in the molecular masses of any two adjacent homologues is $14\,\mathrm{u}$.