A homologous series may be defined as follows:

A group of organic compounds containing a particular functional group is termed a homologous series. A member of any homologous series is called a homologue.

Characteristics of a homologous series: A homologous series shows the following characteristics:

(i) All the members of homologous series can be described by a common general formula. For example, all alkanes can be described by the general formula CnH2n+2.


(ii) Each member of a homologous series differs from its higher and lowers neighboring members by a common difference of CH2.

(iii) All the members of a homologous series show similar chemical properties.

(iv) Physical properties in a homologous series show a regular variation with a increase in molecular mass.