Which option best defines a molecule?

• A. The smallest unit of an element
• B. The smallest unit that can be divided into atoms
• C. The smallest particle of a substance
• D. The smallest unit that retains the identity of a chemical compound

Answer: (D)

A molecule is the smallest unit of a substance that keeps its chemical properties, formed when atoms are bonded together. This means it can represent a compound’s identity, and even elements can exist as molecules (like O2) when they naturally form discrete bonded groups. So the best description is the one that states the smallest unit that retains the identity of a chemical compound, because it captures the idea that the substance’s distinct properties are seen in the molecule as a whole. The other statements are less precise: one implies the absolute smallest unit of an element (an atom), another sounds like it covers anything that can be divided into atoms without tying to the substance’s identity, and the last is too vague about what a “smallest particle” means.