The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms.

• A. Atom
• B. Element
• C. Molecule
• D. Compound

Answer: (B)

When thinking about what is the smallest piece of a substance that still has its properties, the distinction between atoms and larger assemblies is key. A molecule is the smallest unit that can have the properties of a compound, and it is formed when two or more atoms bond together. An atom, by itself, is the basic unit of an element and doesn’t always carry the properties of a compound. An element is a pure substance consisting of one type of atom, while a compound is made from two or more elements in a fixed ratio; the smallest unit of a compound is a molecule. The description given—built from atoms and retaining the substance’s properties—points to a molecule, since that is the smallest unit that exhibits the characteristic properties of the substance as a whole.