* Element: An element is a pure substance consisting of only one type of atom. While an atom is the smallest unit of an element, it may not exhibit all the properties of the element in its pure form.

* Ion: An ion is an atom or molecule that has gained or lost electrons, giving it a net electrical charge. It's not the smallest unit that retains all the properties of a substance.

* Molecule: A molecule is formed when two or more atoms bond together. It's the smallest unit of a compound and retains the chemical and physical properties of that compound.

Example: Water (H₂O) is a compound. The smallest unit of water that retains its properties is a water molecule, consisting of two hydrogen atoms bonded to one oxygen atom.