Which statement best defines a homologous series?

• A. Similar chemical properties
• B. Trend in physical properties
• C. Same functional group
• D. Members differ by CH2

Answer: (D)

A homologous series is a family of compounds that share the same functional group and, from one member to the next, differ by one CH2 unit. That repeating CH2 unit is the defining pattern, so each successive member gets a little larger while keeping the same reactive feature, which is why their chemical behavior is similar and their physical properties change gradually. This is why the statement about differing by CH2 is the best definition: it captures the essential incremental building block that creates the series. While having similar chemical properties and showing trends in physical properties are true features of such series, they describe consequences rather than the defining pattern itself. The same functional group is necessary, but by itself it doesn’t specify the regular CH2 progression that creates the whole series.