A homologous series is a group of organic compounds that have:

A homologous series consists of organic compounds that have the same functional group and similar chemical properties. Members of a homologous series differ from each other by a constant unit, specifically a CH2 group. The physical properties of compounds in a homologous series show a gradual change as the molecular size increases. Examples of homologous series include alkanes, alkenes, and alkynes, where each member differs by a CH2 unit. This concept helps predict the properties and reactivity of organic compounds within the same series.