And while all neutral atoms of say, oxygen, always contain 8 protons and 8 electrons they do not all necessarily have the same number of neutrons. Atoms of the same element with different numbers of neutrons are known as isotopes.

Some oxygen atoms may have 8 neutrons while others may have 7 or 9. Chemically these atoms are essentially the same and often come mixed in nature. Thus the average atomic masses (which are the other numbers on the periodic table) are weighted averages of the mass numbers (A, from the German Atomgewichte or "atomic weight") of the different isotopes that occur naturally. The mass numbers themselves are simply the sum of the masses of the protons and neutrons on the atomic mass scale---a scale on which each particle is counted as 1.