What is the difference between atomic mass and atomic weight? atomic mass is characterized by the number of neutrons in the nucleus, whereas atomic weight is the weight of an individual atom. atomic mass is the average of the atomic masses of an atom's naturally occurring isotopes, whereas atomic weight is the weight of an individual atom. atomic mass is the mass of an individual atom, whereas atomic weight is the average (weighted) of the atomic masses of an atom's naturally occurring isotopes. atomic weight = atomic mass × g. atomic mass is characterized by the number of protons in the nucleus, whereas atomic weight is the average (weighted) of the atomic masses of an atom's naturally occurring isotopes.