What defines an isomer?

An isomer is defined as a compound that has the same molecular formula as another compound but differs in the arrangement of atoms within that molecule. This difference in arrangement can lead to distinct physical and chemical properties, even though the compounds are made up of the same types of atoms in the same quantities.

For instance, consider two isomers of butane (C4H10): one is n-butane, which has a linear structure, while the other is isobutane, which has a branched structure. Both compounds contain four carbon atoms and ten hydrogen atoms, but their different arrangements result in unique properties.

The other choices do not accurately capture the definition of isomers. A compound containing only one element does not pertain to isomers, since isomers must involve multiple types of elements. The characterization of a compound that cannot exist in multiple forms is contrary to the concept of isomers, which can exist in various forms. Lastly, the notion of having a fixed structure and formula contradicts the essence of isomerism, where the same molecular formula can give rise to multiple structural variations.