The smallest unit of a compound is a

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

Answer: (B)

A compound is made from two or more different elements bonded together, and the smallest piece that still has the properties of that compound is a molecule. A molecule is a stable group of atoms held together by chemical bonds, and it represents the compound as a single unit—like a water molecule, H2O. An atom, on its own, is the basic unit of an element, not a compound. An element is a pure substance composed of one type of atom. An ion is a charged particle formed when electrons are gained or lost, not the basic unit of a compound. Note that for some ionic compounds the smallest repeating unit in a crystal is described as a formula unit, but in common teaching the smallest unit of a compound is treated as a molecule.