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any of numerous groups of chemical compounds in each of which the difference between successive members is a simple structural unit. Such series are most common among organic compounds, the structural difference being a methylene group, as in the paraffin hydrocarbons, or alkanes; the normal primary alcohols, or 1-alkanols; and the normal carboxylic acids, or alkanoic acids. In each of these homologous series the number of methylene groups-designated by the subscript n-may have the successive values 0, 1, 2, 3, etc.