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Isomerism refers to the phenomenon where two or more chemical compounds have the same molecular formula but differ in the arrangement of their atoms in space. These different compounds, known as isomers, possess distinct physical and chemical properties. The concept of isomerism is fundamental to understanding the diversity of molecular structures and their impact on reactivity and function.

There are two main categories of isomerism: structural isomerism and stereoisomerism. Structural isomers, also called constitutional isomers, have different connectivity between atoms, meaning the atoms are bonded in a different order. This can lead to variations in functional groups or carbon skeletons. For example, butane and isobutane are structural isomers, both with the formula C4H10, but with different branching of the carbon chain.

Stereoisomers, on the other hand, have the same connectivity but differ in the spatial arrangement of their atoms. This category is further divided into enantiomers and diastereomers. Enantiomers are stereoisomers that are non-superimposable mirror images of each other, much like left and right hands. Diastereomers are stereoisomers that are not mirror images of each other. Geometric isomerism, a type of diastereomerism, arises from restricted rotation around a double bond or within a ring structure, leading to different spatial orientations of substituents. Understanding isomerism is crucial in fields such as organic chemistry, biochemistry, and pharmacology, where the specific three-dimensional structure of a molecule can dictate its biological activity and therapeutic efficacy.