Where does an atomic number come from ?
The atomic mass or relative isotopic mass of each isotope and nuclide of a chemical element is therefore a number that can in principle be measured to a very great precision , since every specimen of such a nuclide is expected to be exactly identical to every other specimen , as all atoms of a given type in the same energy state , and every specimen of a particular nuclide , are expected to be exactly identical in mass to every other specimen of that nuclide .