What defines isomers in organic chemistry?

Isomers in organic chemistry are defined as compounds that share the same molecular formula but have different structural formulas. This distinction is crucial because isomers can exhibit vastly different chemical and physical properties despite having the same types and numbers of atoms.

For example, butane and isobutane (methylpropane) both have the molecular formula C4H10, yet they have different structural arrangements of their carbon and hydrogen atoms, leading to different boiling points, densities, and reactivities. This concept of having the same atoms arranged differently underlies the fundamental nature of isomerism.

Other options presented do not accurately define isomers. Some may suggest variations such as differences in molecular formulas or the ability to interconvert, which would not align with the fundamental definition that isomers must have identical molecular formulas yet distinct structural methods of arrangement. Thus, the option highlighting the same molecular formula with differing structural formulas encapsulates the essence of isomerism in organic chemistry.