Structural isomerism refers to

• A. Same molecular formula but different structural formula
• B. Different molecular formula but same structure
• C. Same structural formula but different mass
• D. Mirror-image isomerism only

Answer: (A)

Structural isomerism happens when compounds have the same molecular formula but the atoms are arranged differently, giving different connectivity or skeletons. That’s why the correct description is: same molecular formula but different structural formula. An example is butane and isobutane, both C4H10, but with different carbon frameworks.

The other statements don’t fit because: having a different molecular formula but the same structure would mean they aren’t the same composition, so they can’t be isomers; having the same structural formula but different mass is impossible since the mass is determined by the atoms present; mirror-image isomerism (enantiomerism) is a type of stereoisomerism, not structural isomerism, and there are other forms of structural isomerism beyond just mirror images.