What small particle of substance retains the characteristics of the substance?
If it is an element, then the smallest particle that retains the characteristics of the element is an atom. If it is a diatomic element, then the smallest particle that retains the characteristics of the element is a molecule. If it is a molecular compound, then the smallest particle that retains the characteristics of the compound is a molecule. If it is an ionic compound, the smallest particle that retains characteristics of the compound is a formula unit.