Question

The atomic mass of an element is the weighted average of what?

Answer 1

...Of the mass of the individual isotopes.

Explanation:

Most elements have a number of isotopes; i.e. elements with common `Z" (the number of nuclear protons")`, but different numbers of neutrons, massive nuclear particles. Of course `Z` defines the identity of the element.

Even for hydrogen (`Z=1`) there are several isotopes available and routinely utilized: `""^1H`, protium; `""^2H`, deuterium;`""^3H`, trittium. For all these isotopes, `Z=1`, but there are `0,1,2` neutrons in the nucleus. The weighted average of the masses of the isotopes is the atomic mass reported on the Periodic Table.