The organic compounds are classified on the basic of functional group present in their molecules. The  Systematic  arrangement of the compound containing similar functional group gives a series of various molecules with their different molecular masses, which is termed as homologous series. Thus a homologous series is defined as a systematic order of arrangement of structurally similar organic compounds, containing the same functional group in a family in which the two  adjacent members differ in their molecule formula by a -ch2- unit.
Since the individual member of a series content the the same functional group, these members are called homeologous and the phenomenon is known as a homology.


Characteristics of a Homologous series:

  1.  All the members of homologous series contents in functional group.
  2.  The various members of a series can be assigned by a common formula which is also known as general formula.
  3. Due to the presence of same functional group, All the member of homologous series have almost similar chemical properties.
  4.  The individual members of a homologous series can be prepared by a similar method known as General method of preparation.
  5. There is a sequence of gradual increase or decrease in the physical properties like boiling point, density, melting point, solubility, etc. with rise in the molecular mass. 
  6. Two successive members of a series are different in their molecular formula by -CH2-  unit or 14 atomic mass unit.
  7. The first members of homologouslogos series may show different behaviour from the subsequent members