izomer

An izomer, typically spelled isomer in English, is a compound that has the same molecular formula as another compound but a different arrangement of atoms or a different spatial orientation. Isomers can exhibit distinct physical and chemical properties despite sharing the same formula.

Isomerism is divided into structural (constitutional) isomerism, where the connectivity of atoms differs, and stereoisomerism, where the connectivity is the same but the three‑dimensional arrangement varies. A further distinction within stereoisomerism identifies enantiomers (mirror-image isomers) and diastereomers (non-mirror-image stereoisomers). Geometric isomerism, a subset of stereoisomerism, arises from restricted rotation around double bonds or within ring structures, producing cis and trans forms.

Examples of structural isomers include butane and isobutane (both C4H10), which differ in how the carbon skeleton is connected. In stereoisomerism, enantiomers are non-superimposable mirror images and can have different interactions with polarized light and biological systems; diastereomers include pairs like certain glucose and galactose configurations that are not mirror images.

Geometric isomers are illustrated by cis-2-butene and trans-2-butene (C4H8), which differ in the relative positions of substituents around a double bond. Functional isomerism, another subtype, occurs when isomers have the same formula but different functional groups, such as ethanol and dimethyl ether (both C2H6O).

Isomer concepts are foundational in chemistry, influencing reactivity, spectroscopy, patenting, and pharmacology.