How does the average atomic mass of an element differ from the mass of an individual atom of the same element?

In periodic tables, the relative atomic mass is usually not a whole number, even though the mass of both protons and neutrons is 1. This is due to isotopes, which are atoms of the same element, but they have different amounts of neutrons in their nuclei. So the relative mass of an individual atom is always a whole number, and the relative atomic mass is an average of all of the isotopes of the element.