There is a difference between the meanings of the chemistry terms atomic mass and mass number. ... Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes. Mass number is a count of the total number of protons and neutrons in an atom's nucleus.
Atomic weight is the average mass of all the isotopes of a certain type. It is a weighted average that takes into account the abundances of all of the different isotopes.