The term “elementary substance” is further used to distinguish a chemical substance that is composed of same-element atoms (atoms with the same number of protons in the atomic nucleus) from the chemical element itself. The latter includes all species of atoms (atoms, ions, radicals) that have the same number of protons in the atomic nucleus . It has been suggested that only this latter concept should be employed when referring to a chemical element: the term “element” should be reserved to collectively designate all atomic forms of an element . Then, any molecule-building or other combination of atomic forms of an element constitutes an elementary substance.
Example: The allotropes dioxygen (O2) and ozone (O3) are elemenary substances of the chemical element oxygen. Isotopically labelled dioxygen species such as [16O][17O] and other polyatomic species based merely on oxygen isotopes also are elementary substances. On the other hand, water (H2O) is a chemical compound, since it consists of atoms belonging to two different elements—oxygen and hydrogen.
Keywords: chemistry, chemical education, terminology, IUPAC definition, chemical element, chemical compound, homonuclear molecule.
 Gold Book: chemical element [goldbook.iupac.org/C01022.html].
 Rollie J. Myers: What Are Elements and Compounds? J. Chem. Educ. 2012, 89, pp. 832-833. DOI: 10.1021/ed200269e.