Why isn't the mass of an electron used to determine the atomic mass of an element are averaged together . why are some isotopes more heavily weighted than others?

The Atomic Mass of an element is not based solely on the mass of electrons because the majority of an atom's mass comes from protons and neutrons in the nucleus. Isotopes of an element have different numbers of neutrons, which affects their mass. When calculating the average atomic mass, isotopes are weighted based on their natural abundance; those that occur more frequently contribute more significantly to the average than rarer isotopes. This results in a weighted average that reflects the relative stability and prevalence of each isotope.