Which scenario best describes isomerism?

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but differ in the arrangement of atoms within their structures. This results in distinct compounds with different physical and chemical properties. The correct choice illustrates this concept, highlighting that although these molecules share the same composition of atoms, their structural variations lead to differences in their characteristics.

For instance, consider the isomers of butane (C₄H₁₀), which can exist as either normal butane or isobutane. Even though both compounds consist of the same number of carbon and hydrogen atoms, their structures are different, resulting in different boiling points and reactivity.

The other choices emphasize aspects that do not align with the definition of isomerism. For example, having different molecular formulas would indicate entirely different compounds rather than isomers. Reacting at different rates pertains more to the kinetics of chemical reactions than structural differences. Identical boiling points suggest that the substances in question could be the same compound or conform to similar structural characteristics, further distancing them from the definition of isomerism.