homologous series

Homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series differ by the number of repeating units they contain.

For instance, the homologous series of straight-chained alkanes begins methane (\[\ce{CH4}\]), ethane (\[\ce{C2H6}\]), propane (\[\ce{C3H8}\]), butane (\[\ce{C4H10}\]), and pentane (\[\ce{C5H12}\]). In that series, successive members differ in mass by an extra methylene bridge (\[\ce{-CH2-}\] unit) inserted in the chain.