Why do we have an avagadro constant and an atomic weight constant if the density of the elements fluxuates?

Elements high and low weigh the same by density up to four times different from the periodic tables density, but each element is about 200 pico meters and of simular chemical density. It would be pointless to do refractory mining. In fact, how could they have have had pure iron before carbon iron, or other impurities for that matter?