For #Na#, #Z#, the atomic number #=# #11#; there are thus 11 massive, positively charged particles in each #Na# nucleus. For #Mg#, there are #12# such particles. Of course, the common isotopes of each metal have different number of neutrons, massive, neutrally charged particles in the nucleus.
What distinguishes one element from another, is always #Z#, the atomic number. A given selection of nuclei of the same element, NECESSARILY have, the same #Z#, but they might have different number of neutrons. The protium nucleus, #""^1H#, is mightily common, but there are hydrogen nuclei with a neutron in the nucleus, to give the deuterium isotope, #""^2H#. #""^1H# and #""^2H#, and even #""^3H" (tritium)"#, for instance, are isotopes of elemental hydrogen.
