What is an isomer?

• A. Compound with same atoms, but arranged differently
• B. Compound with different atoms arranged the same
• C. Compound with same arrangement
• D. Compound that is a polymer

Answer: (A)

An isomer is a compound that has the same atoms and the same molecular formula as another compound, but the atoms are connected or arranged differently. This difference in how the atoms are linked or oriented in space gives distinct properties even though the formula is the same. So the idea is: same atoms, arranged differently. The description that fits this best is a compound with the same atoms, but arranged differently. A polymer is a long chain made of repeating units, which is a different concept, and the other options describe either different atoms or the same arrangement, which wouldn’t be isomers.