The total number of protons and neutrons in the nucleus of an atom is called the mass number.

The average mass of atoms of the element is called the atomic mass or atomic weight. This is the weighted average of the isotopes, based on their relative abundance. For example, there are two isotopes of lithium:

7.42% of lithium is Li-6 with a mass of 6.015 atomic mass units (u) and 92.55% is Li-7 with a mass of 7.061 atomic mass units. We would expect the average mass to be much closer to 7u than 6u, since most lithium atoms exist as Li-7. Multiplying each mass by its percent abundance and adding those numbers together gives an average mass of 6.94 atomic mass units.

Another description of an element's mass is the molar mass. This is the mass of one mole of atoms, expressed in grams/mole, which will be equivalent to the number in atomic mass units.