AC-HPAT Chemistry Practice Test

Isomerism describes how the same set of atoms can be arranged to make different compounds. The key idea here is that structural (constitutional) isomers differ in how atoms are connected to each other — the connectivity changes, so you get different substances (like straight-chain butane versus branched isobutane). Stereoisomers, on the other hand, keep the same connections between atoms, but the 3D arrangement in space differs, so the molecules have different shapes or orientations without changing which atoms are bonded.

That makes the statement you’re looking at the best fit: one type changes connectivity, while the other keeps connectivity but varies spatial arrangement. Mass isn’t changed by these distinctions, since both kinds share the same molecular formula, and the elements involved stay the same. The other options mix up these ideas—structural isomers do not have the same connectivity, stereoisomers do not differ in mass, and they don’t involve changing which elements are present.