What defines a homologous series in organic chemistry?

A homologous series in organic chemistry is defined as a group of compounds that differ from each other by a specific structural unit, which is typically a -CH₂- group. This characteristic means that each successive compound in the series has a similar chemical structure and functional group, leading to analogous properties, while the physical properties (such as boiling and melting points) tend to change in a predictable manner with increasing chain length.

For example, in the alkane series, each member (like methane, ethane, propane, etc.) differs from the previous one by a single -CH₂- unit, which results in clear trends in physical properties, while maintaining the same functional group (in this case, the alkane group). This is key to understanding the behavior of organic compounds within a series and allows for predictions regarding their reactivity and properties.

The other options do not accurately convey the defining feature of a homologous series. They either refer to differing functional groups, specific types of bonding, or reactivity amongst the compounds, which do not capture the essence of what a homologous series truly represents in organic chemistry.