Functional groups: It may be defined as an atom or group of atoms joined  in a specific manner which is responsible for the characteristic chemical properties of the organic compounds.

All the compounds having a particular functional group behave alike. For example all compounds containing -OH (hydroxyl) group undergo similar reactions.

The functional group present in the following molecules are encircled.

Common functional groups and their families of compound

Homologous Series:

Homologous Series: A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues.

They exhibit  similar properties. For example, the compounds given below belong to the alcohol family.

CH₃OH methyl alcohol (methanol)

CH₃CH₂CH₂OH propyl alcohol (1-propanol)

CH₃CH₂.CH₂CH₂OH butyl alcohol (1-butanol)

Characteristics of a Homologous Series

All members of a homologous series exhibit some common characteristics. They are:

1. All the members of a homologous series can be represented by a common general formula, as they have the same functional group. For example, alkanes can be represented by the formula C_nH_2n+2.

CH₄                                          

Methane

C₂H₅

 Ethane

C₃H₈

Propane

C₄H₁₀

Butane

Each member of a homologous series has a common difference of -CH₂ from the next higher or lower member.

2. Common general methods of preparation exist for all members of the series.
3. All members exhibit similar chemical behavior.

An increase in molecular mass of members within a homologous series show a similar regular gradation of the physical properties, such as, physical state, melting and boiling points etc.