What are compounds with identical molecular formulas but different structural formulas called?

Compounds with identical molecular formulas but different structural formulas are referred to as isomers. Isomerism occurs when molecules share the same chemical composition, meaning they have the same number and types of atoms, yet they are arranged differently in space. This difference in arrangement leads to distinct physical and chemical properties among the isomers.

For example, consider butane and isobutane. Both can be represented by the molecular formula C4H10, but their structural formulas differ significantly. The variation in their structures accounts for differences in boiling points, melting points, and even their reactivity.

The other terms listed do not fit this definition. Allotropes refer to different forms of the same element in the same physical state—like carbon can exist as graphite or diamond. Polymers are large molecules made up of repeating structural units (monomers) and represent a different concept entirely, primarily involving macromolecules. Mixtures are combinations of two or more substances that retain their individual properties and are not chemically bonded. Therefore, isomers stand out as the appropriate term for compounds with the same molecular formula but different structural arrangements.