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

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.