... (1) A vacuum (a space empty of matter, which has no substance), (2) A particle (electron, proton, neutron), (3) An element (a basic substance that can't be simplified; hydrogen, oxygen, etc.), (4) An atom (the smallest unit of an element, having all the characteristics of that element and consisting of a very small and dense central nucleus containing protons and neutrons, surrounded by one or more shells of orbiting electrons; atoms remain undivided in chemical reactions except for the donation, acceptance, or exchange of valence electrons), (5) A nuclide (which may be isotopic nuclides, with the same atomic number Z, isobaric nuclides, with the same mass number M or isotonic nuclides, with the same difference between the mass number and the atomic number, M-Z), (6) A molecule (which has no more properties of its constituent atomic parts), (7) A compound (a molecule that contains more than one element), and (8) An ion (it is an element, atom, nuclide, molecule or compound, in which the total number of electrons is not equal to the total number of protons, giving the element, atom or molecule a net positive or negative electrical charge). ...