What is the smallest particle of a compound?
See the explanation.
If the compound is covalent (contains only nonmetals), the smallest particle would be a molecule. A molecule is formed when atoms share electrons. The sharing of electrons forms a covalent bond.
If the compound is ionic (contains a metal cation and a nonmetal anion, or a polyatomic ion), the smallest particle would be a formula unit, which represents the smallest whole number ratio between the elements. This is because ionic compounds form crystal lattices, and there is no discrete smallest particle like a molecule.