Final answer:
The smallest unit in which most substances naturally exist is the molecule, which is a combination of two or more atoms bonded together, retaining the substance's properties.
Step-by-step explanation:
For most substances, the representative particle, or smallest unit in which the substance naturally exists, is the molecule. Molecules are multi-atom combinations that are held together so strongly they act as a single particle. A molecule retains the physical and chemical properties of the substance it represents and is composed of two or more atoms bonded together. In contrast, an atom is the smallest particle of an element that has the properties of that element and can enter into chemical combinations. A compound, on the other hand, is a substance consisting of two or more elements combined in fixed proportions, with the smallest particle of most compounds also being a molecule.