Take a look at the table above
Methane has 1 carbon atom and 4 hydrogen atoms, and ethane has 2 carbon atoms and 6 hydrogen atoms.
If we remove one carbon and four hydrogen atoms (i.e., CH4) from ethane, we are left with one carbon and two hydrogen atoms.
In other words,
C2H6 (Ethane) − CH4 (methane) = CH2 unit.
C3H8 (Propane) − C2H6 (ethane) = CH2 unit.
C4H10 (Butane) − C3H8 (propane) = CH2 unit.
C5H12 (Pentane) − C4H10 (butane) = CH2 unit.
Clearly, each successive member of alkane family is differ by a CH2 unit. Such type of groups or series of organic compounds where adjacent members are differ by a CH2 unit, form a homologous series and the members of the series are called homologues.
A homologous series is defined as a group or series of organic compounds all the members of which contain same functional group and any two members of which differ by a -CH2 unit.
Examples : Alkanes, Alkenes, Alkynes etc.
Below are the examples of organic compounds containing functional groups forming a homologous series.
CH3−OH, CH3CH2−OH, CH3CH2CH2−OH .....
General formula : CnH2n+1OH
CH3−COOH, CH3CH2−COOH, CH3CH2CH2−COOH .....
General formula : CnH2n+1COOH