Which statement describes structural isomerism?

• A. Same molecular formula and same structural formula
• B. Different molecular formula and different structure
• C. Different molecular formula but same connectivity
• D. Same molecular formula but different structural formula

Answer: (D)

Structural isomerism arises when compounds share the same molecular formula but differ in the way atoms are connected to each other. This means the atoms are arranged differently, giving a different structural formula, even though the total number of each type of atom is the same. The correct statement reflects this: the two molecules have the same molecular formula but a different structural formula, so they are connected differently. An example is butane and isobutane, both C4H10, but with different carbon skeletons.

Why the other ideas don’t fit: having both the same molecular and structural formulas would mean they are identical compounds, not isomers. Having different molecular formulas means they are not isomers at all. Having different formulas but the same connectivity would require the same arrangement of atoms, which is impossible if the formulas differ, so that cannot describe isomerism.