What is a structural (constitutional) isomer?

A structural (constitutional) isomer is characterized by having the same molecular formula but a different arrangement of atoms within that formula. This means that while the isomers share the same number and types of atoms, they differ in how those atoms are connected or bonded to each other. For instance, two structural isomers may differ in the connectivity of carbon chains, functional groups, or even the position of elements within the molecular structure.

This understanding is crucial because structural isomers can exhibit vastly different physical and chemical properties, even though they contain the same atoms. The distinction between the arrangements of atoms leads to different molecular architectures, influencing their reactivity, boiling points, solubility, and overall behavior in chemical reactions.

In contrast to the other options provided, isomers with the same bonding sequence would not be properly classified as structural isomers, since they would not demonstrate the necessary variance in connectivity. Isomers that differ only in their three-dimensional arrangement describe stereoisomers, which is a different category. Lastly, isomers with identical molecular weights do not necessarily indicate different connectivity or structure, thus they may not be classified solely as structural isomers.