An isomer is defined as?

• A. Molecules with the same molecular formula but different structural formulas
• B. Molecules with different molecular formulas
• C. The same molecule in a different phase
• D. Different isotopes

Answer: (A)

Isomerism happens when compounds have the same atoms of the same elements but are connected or arranged differently, giving different properties. The correct idea is that isomers are molecules with the same molecular formula but different structural formulas. This can mean different connectivity of atoms (structural isomers) or different spatial arrangement (stereoisomers) while keeping the same formula. The other statements describe different concepts: a different molecular formula means a different substance; the same molecule in a different phase is a physical change, not a different arrangement of atoms; and different isotopes involve atoms with different neutron numbers, which are not what is meant by isomers (those are isotopologues).