The mass of an element is expressed in three ways: relative atomic mass average atomic mass and mass number. Relative atomic mass is the average mass per atom of the naturally occurring form of an element to the mass of an hydrogen atom (old) or one-twelfth the mass of an atom of carbon-12 (moderen system).
Each isotope has a different actual atomic mass because of a difference in neutrons. The average atomic mass is the weighted average of all the isotopes. Multiply the mass of each isotope by its percentage abundance and you will get the atomic mass that you see in the periodic table.
The mass number is a whole number and represents the total number of protons and neutrons in a given isotope.