i. A series of carbon compounds having the same functional group, similar chemical properties and the successive members differing from each other in their molecular formula by a -CH2- (methylene) unit is called homologous series.
ii. Homologous series of alkanes: All alkanes have same general formula CnH2n+2 (where n = 1, 2, 3, ....) and they show similar chemical properties. The first member of the series is methane (CH4). The next succeeding member of the series is ethane (C2H6) and it differs by a -CH2- unit. The first five members of homologous series of alkanes are listed in the following table: