Atomic mass is the average masses of all the isotopes known for that element as they occur in earth's nature.
Atomic number is the element's identity, the number of protons the nucleus has, will never change.
Mass number is [NOT just another way to say atomic mass.]
Mass number is equal to the number of protons AND neutrons, that is of ONE particular isotope of an element. This is a round figure, not an average like atomic mass.