Not a fact anymore

Two substances with the same molecular formula must be the same compound.

What we know now

Different compounds can share a molecular formula while differing in atom connectivity or three-dimensional arrangement. These isomers can have very different physical, chemical, and biological properties.

Why it changed

Composition analysis initially encouraged chemists to identify a compound by the proportions of its elements. Discoveries such as silver cyanate and silver fulminate showed identical composition but sharply different properties, leading Berzelius to introduce the concept of isomerism.

Status: Overturned
Category: Chemistry
Accepted for: ≈43 years
Accepted approximately: Late 18th century–1820s
Changed approximately: 1820s–1830s

Isomers are compounds built from the same kinds and numbers of atoms but arranged differently—either connected in a different order or positioned differently in three-dimensional space. A molecular formula therefore still places strong constraints on possible substances, but it does not identify a unique structure.

The discovery of isomerism was a major step toward structural chemistry: knowing which atoms are present is not enough; their bonding and spatial arrangement also matter.

Sources and what they establish

Current evidence

IsomerInternational Union of Pure and Applied Chemistry
Defines isomers as species with the same molecular formula but different structures and properties.
A Review of IsomerismOpenStax
Explains constitutional and stereochemical forms of isomerism.

Historical context

IsomerismEncyclopædia Britannica
Reviews the early discoveries that forced chemists to distinguish composition from structure.