StellarIsotopes
Atoms of each element can have different numbers of neutrons. For example, some neon atoms have 10 neutrons, others have 11 neutrons, etc. Atoms with differing amounts of neutrons but the same number of protons are called isotopes. This means that they are chemically the same element but have different masses. Different isotopes usually have different abundances. Neon-20 (neon atom with atomic mass of 20 and 10 neutrons) has an abundance of 90.5%, Neon-21 has an abundance of 0.27%, and Neon-22 has an abundance of 9.25%. The average atomic mass of an element is the weighted average of the atomic masses of each isotope, and it is the number you see under each element on the periodic table.
Ex: 20 amu * 90.5% + 21 amu * 0.27% + 22 amu * 9.25% = 20.1917 amu