What defines an isomer?

• A. Different molecular formula
• B. Same molecular formula, different structural formula
• C. Same arrangement of atoms
• D. Identical properties in all respects

Answer: (B)

Isomerism happens when molecules have the same kinds and numbers of atoms but are connected or arranged differently, so they share the same molecular formula while forming different substances. The description that fits this idea is: same molecular formula, different structural formula. That means the atoms are present in equal amounts, but the way they are linked together (and sometimes how they’re arranged in space) is different, leading to distinct compounds.

For example, C4H10 can be arranged as n-butane or as 2-methylpropane; both have the same formula but different connectivity, so they are isomers.

The other possibilities don’t fit because a different molecular formula would not be an isomer, same arrangement of atoms would mean the same molecule, and identical properties in all respects would not hold for isomers, which can have different physical or chemical properties.