Molecule definition.

• A. A pure substance composed of two or more elements chemically united in fixed proportions.
• B. The smallest particle of a substance that retains the physical and chemical properties of that substance (Ex: H2O or H2).
• C. The atom as a whole.
• D. A charged particle.

Answer: (B)

Understanding what a molecule is helps distinguish it from atoms, ions, and compounds. A molecule is the smallest unit of a substance that retains its physical and chemical properties; it can be made of two or more atoms bonded together, and those atoms can be the same element (as in H2 or O2) or different elements (as in H2O). This makes the statement with examples like H2O or H2 the best fit, because it captures that the molecule is the smallest piece that preserves the substance’s characteristic properties, regardless of whether it’s a diatomic element or a compound.

The first option describes a compound—two or more elements chemically united in fixed proportions—which is related but not universal to all molecules (elemental molecules like O2 exist). The atom as a whole refers to a single atom, not the bonded group that defines a substance’s properties. A charged particle is an ion, not a molecule.