Isomer

An isomer is a chemical compound that has the same molecular formula as another compound but a different arrangement of atoms or different spatial orientation. Isomers can exhibit different physical and chemical properties despite sharing the same elemental composition.

Structural isomers, or constitutional isomers, differ in how their atoms are connected in the molecule. Examples include n-butane and isobutane (C4H10), which have the same formula but different carbon skeletons. Other forms include chain isomers and position isomers of substituted hydrocarbons.

Stereoisomers have the same bond connectivity but differ in the three-dimensional arrangement of atoms. They are divided into geometric isomers and optical isomers. Geometric isomers arise from restricted rotation about double bonds or ring systems, such as cis-2-butene versus trans-2-butene. Optical isomers, or enantiomers, are non-superimposable mirror images created by chiral centers; they can have different interactions with plane-polarized light and with biological systems. Common pairs include the enantiomeric forms of lactic acid or glyceraldehyde. Enantiomeric pairs may form racemic mixtures.

Diastereomers are stereoisomers that are not mirror images and can occur when there are multiple chiral centers. Tautomerism is another related phenomenon in which isomeric forms rapidly interconvert, as in keto-enol pairs.

In inorganic chemistry, additional types of isomerism are recognized, including coordination, linkage, and ionization isomers, especially in metal complexes and salts. Isomerism is a foundational concept across chemistry, influencing properties, reactivity, and biological activity.