Define a homologue series in organic chemistry.

A homologue series in organic chemistry is defined as a series of compounds that differ by a single -CH2- unit. This characteristic allows for the systematic categorization of compounds based on their molecular structure. For example, alkanes form a homologue series where each successive compound has one additional -CH2- group compared to the previous one. This results in a predictable pattern in physical properties such as boiling points and solubility due to the incremental changes in molecular mass and structure.

Each member of the series shares similar chemical properties because they possess the same functional groups and general formula, but they also exhibit variations in physical properties as a result of the increasing carbon chain length. The systematic difference by -CH2- units helps in the understanding of trends across the series, aiding in both prediction of behavior and nomenclature.