What does isomer mean?
An isomer is a chemical compound or molecular species that has the same molecular formula as another but differs in how its atoms are arranged. The distinction can arise because atoms are connected in a different order, forming a different structural or constitutional isomer, or because the same connections are arranged differently in space, producing a stereoisomer. Isomers can therefore have very different chemical properties despite sharing a formula; for example, glucose and fructose are isomers. The term is most common in chemistry and biochemistry, including the study of carbohydrates, hydrocarbons, and natural products. In general discussion, “isomer” is also used more broadly to mean two things that share a basic form or composition while differing in some particular respect, though this extended use is less precise. The word is especially useful for understanding why molecular formulas alone do not always identify a substance.
nounA chemical compound or molecular species that has the same molecular formula as another compound but a different arrangement of its atoms. The difference may involve the atoms’ connectivity or their spatial arrangement.
- A chemical compound or molecular species having the same molecular formula as another but a different arrangement of atoms, either in their connectivity or in their spatial configuration.
"The two compounds are isomers, so they have the same molecular formula but different chemical properties."
"Glucose and fructose are isomers because they have the same molecular formula but different structures."
"The researchers separated a mixture of isomers by chromatography."
Use the plural when referring to two or more molecular compounds or species that share a molecular formula but differ in structure.
"The chemist identified three isomers among the products of the reaction."
Isomers prove that a molecular formula is a recipe, not a blueprint: the same ingredients can be arranged into different compounds with different properties.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does isomer come from?
The word derives from Greek iso-, meaning “equal,” and -mer, meaning “part,” reflecting the equality of the parts in the molecular formulas of different compounds. It entered chemistry in the nineteenth century as scientists developed ways to distinguish compounds that shared a formula but possessed different structures. Related terms such as isometric, isometry, isotope, and isobar use the same Greek iso- element in different technical contexts.
Related word forms
How isomer is actually used
In chemistry, the term is usually used for substances with the same molecular formula but different atom arrangements. Isomers may have substantially different physical, chemical, or biological properties.
Easily confused with isomer
An isomer is a molecular compound with the same formula as another but a different arrangement of its atoms, whereas an isotope is an atom of an element with a different number of neutrons.
An isobar is a nuclide with the same mass number as another but a different atomic number, while an isomer concerns molecular structure rather than nuclear composition.