What defines isomers in chemistry?

Isomers are defined as compounds that have the same molecular formula but different structures. This means that while the isomers share the same number and types of atoms, the arrangement of these atoms in space can vary, leading to distinct properties and classifications.

For example, consider the molecular formula C4H10, which represents two separate structural isomers: butane and isobutane. Even though both compounds consist of the same number of carbon and hydrogen atoms, their different arrangements lead to variations in physical properties such as boiling points and chemical reactivity.

The other options do not correctly define isomers. For instance, sharing the same mass but differing in atomic numbers refers to isotopes, which involve variations in neutrons rather than structural differences. Multiple phases existing simultaneously pertain to complex states of matter rather than the structural comparison required for isomers. Lastly, compounds sharing the same chemical properties but differing in physical properties might refer to other classes of compounds but do not encapsulate the concept of isomerism accurately.

Thus, the defining characteristic of isomers lies in their identical molecular formula coupled with differing structural configurations, which allow them to exhibit a variety of unique physical and chemical behaviors.