"Representative particles" is the term used for the smallest unit of a substance. For example, the representative particle in an element e.g. Sodium is a single sodium "atom". But if the element is diatomic, e.g , oxygen, then the representative particle in oxygen would be the "molecule" O2, not a single oxygen atom. If its an ionic compound, e.g. NaCl, the representative particle in ionic compound would be "formula unit".
