Atomic mass represents the average weight of the atoms of a given element and depends on the number of protons and neutrons present in the atom but the number of electrons present in those atoms does not have any notable impact on the mass of an element.Different atoms of the same elements can have different numbers of the neutrons present in them, thus having different atomic masses. Such versions of an element with a different number of neutrons having different masses are called isotopes.While in any mass calculation involving elements and compounds, One should prefer using the average atomic mass found on the periodic table.