Given an element, why don't ALL the atoms of this element have the same mass?
The atomic mass depends on the number of protons, and neutrons, massive nuclear particles.
For a given element, the number of postively charged, massive nuclear particles, the number of protons, is fixed. Why? Because the number of protons determines
The nucleus can also contains neutrons, massive, neutrally charged nuclear particles. If we go to the simplest element, hydrogen, most hydrogen nuclei contain only the 1 massive particle, which is of course a proton, we represent this isotope as
For heavier elements, especially transition metals, the isotopic distribution becomes quite large, as there are a number of stable isotopes available. The atomic mass printed on the Periodic Table is the weighted average of the masses of the individual isotopes in their particular proportions.