The periodic table lists an element's atomic mass. In other words, it lists the weighted average of all of the isotopic masses for a given element. Isotopes are atoms of the same element with a different number of neutrons in the nucleus; therefore, these atoms have a different mass and a different mass number. Remember, the mass numbers for all isotopes are NOT on the periodic table. If they were, it would look like a very complicated nuclide's chart. The mass number is always a whole number because it is merely the total of neutrons and protons. It would be impossible to have a fraction of a neutron or proton. The atomic mass is listed on the periodic table and always has many decimal places because it represents an average of all of the isotopic masses for an element.