What's the Difference between Mass Number and Atomic Mass?

To see all my Chemistry videos, check out http://socratic.org/chemistry What's the difference between mass number and atomic mass? Mass number is the number of protons and neutrons in an atom, and it tells us about the mass of the atom in amu, or atomic mass units. Atomic mass 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.