What characteristics define a homologous series?

A homologous series is defined by its members having the same general formula, similar chemical properties, and a gradual change in physical properties.

A homologous series is a group of organic compounds that share a similar structure and follow a simple numerical pattern. The members of a homologous series all have the same general formula and exhibit similar chemical properties due to the presence of the same functional group. A functional group is a group of atoms responsible for the characteristic reactions of a particular compound. For example, in the alcohol homologous series, the functional group is -OH.

In addition to having the same general formula and similar chemical properties, members of a homologous series also show a gradual change in physical properties. This means that as the molecular size or mass increases, the physical properties such as boiling point, melting point, and viscosity also increase. This is due to the increase in the strength of intermolecular forces, specifically van der Waals forces, as the size of the molecules increases.

Furthermore, each successive member of a homologous series differs by a -CH2- unit (or one methylene group) and 14 units of molecular mass. This regular increment is another defining characteristic of a homologous series. For instance, in the alkane series (methane, ethane, propane, etc.), each subsequent member has one more carbon and two more hydrogen atoms.

In summary, a homologous series is defined by its members having the same general formula, similar chemical properties due to the same functional group, a gradual change in physical properties, and a regular increment in molecular mass. Understanding these characteristics is crucial for predicting the properties and reactions of organic compounds.