Which structural feature is common in compounds of a homologous series?

Compounds within a homologous series share a common structural feature, which is the same functional group. This characteristic is what defines the class of compounds and determines their chemical behavior. For instance, in alcohols, the functional group is the hydroxyl group (-OH). All alcohols contain this group, which imparts similar chemical properties to the members of this series.

The presence of the same functional group in each compound leads to patterns in their reactivity and properties, even though they may differ in physical characteristics like boiling point and solubility, which can vary due to the differing numbers of carbon and hydrogen atoms in the molecules. The systematic increase in these atoms contributes to the gradual change in physical properties, but it is the uniform functional group that binds the series together conceptually.

While variations in carbon and hydrogen atoms exist in compounds of a homologous series, they are not the defining commonality. Therefore, the shared functional group is the fundamental aspect that categorizes compounds into a homologous series and dictates their similar chemical behavior.