The atomic weight reported in the periodic table of elements for Carbon is the average weight of all the known isotopes of Carbon occurring naturally. There are 3: Carbon-12, Carbon-13 and Carbon-14. The numbers represent the mass numbers, or the sum of neutrons and protons in the nucleus. Each of the individual masses are multiplied with their individual percentage abundances. Then, when you add them up, it will yield the average atomic weight equal to 12.011 amu or grams/mole C.
