The homologous series of straight-chained alkanes begins methane (CH), ethane (CH), propane (CH), butane (CH), and pentane (CH). In that series, successive members differ in mass by an extra methylene bridge (-CH- unit) inserted in the chain. Thus the molecular mass of each member differs by 14 atomic mass units. Adjacent members in such a series, such as methane and ethane, are known as "adjacent homologues".
Within that series, many physical properties such as boiling point gradually change with increasing mass. For example, ethane (CH), has a higher boiling point than methane (CH). This is because the London dispersion forces between ethane molecules are higher than that between methane molecules, resulting in stronger forces of intermolecular attraction, raising the boiling point.
Some important classes of organic molecules are derivatives of alkanes, such as the primary alcohols, aldehydes, and (mono)carboxylic acids form analogous series to the alkanes. The corresponding homologous series of primary straight-chained alcohols comprises methanol (CHO), ethanol (CHO), 1-propanol (CHO), 1-butanol, and so on. The single-ring cycloalkanes form another such series, starting with cyclopropane.
Biopolymers also form homologous series, for example the polymers of glucose such as cellulose oligomers starting with cellobiose, or the series of amylose oligomers starting with maltose, which are sometimes called maltooligomers. Homooligopeptides, oligopeptides made up of repetitions of only one amino acid can also be studied as homologous series.