The smallest unit which a chemical compound can be divided without losing its identity is a ___

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

Answer: (B)

The essential idea is that a compound is a distinct combination of atoms bonded in a fixed ratio, and the smallest piece that still behaves like that compound is a molecule. A molecule is the smallest cluster of atoms that retains the properties that define the compound. For example, water’s identity comes from the water molecule H2O; splitting it into separate hydrogen and oxygen atoms removes the compound’s characteristic properties, so you no longer have water.

Atoms are the basic units of elements, not compounds, because a compound contains two or more elements. An ion is a charged particle and, while ionic compounds involve ions in a lattice, breaking the compound down to a single ion does not preserve the compound’s overall identity. A crystal describes the solid’s orderly arrangement, not the smallest unit that defines a compound’s identity. Thus, the molecule is the best description of the smallest unit that keeps the compound’s identity.