the average atomic mass of the element is the weighted average atomic mass of element with respect to the natural abundance of all the isotopes.
isotopes are atoms of the same elements with different mass numbers , in other words same number of protons but different numbers of neutrons.
to find the average atomic mass we need to know the atomic masses of all the isotopes in addition to the natural abundance percentages of the isotopes.
answer is we need to know the atomic masses of the isotopes as well
