Why is the atomic mass of an element an average mass?

Answer 1

The atomic mass in a period table is a weighted average that accounts for the different isotopes (atoms with the same number of protons, but a different number of neutrons) of a particular compound.

For example, hydrogen's atomic mass is 1.0079, which accounts for protium (1 proton, 0 neutron), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons). Because of the closeness of 1.0079 to 1.00 (the expected mass of protium), you know that deuterium and tritium are uncommon in nature.

Answer 2

It is an average of the various isototpes of the element that exist in nature. For example, Carbon is mostly C-12. But there is also a small amount of C-14 (this isotope is radioactive and used in carbon dating of old items). The average mass of carbon is therefore shown as 12.011.