With what was discussed in the previous point, we are surprised to see that the mass of carbon is 12,011 amu, since a standard mass of 12 amu was assigned to it. The reason for this apparent inconsistency is the presence of isotopes: carbon-12, carbon-13 and carbon-14. The atomic mass of carbon is obtained by means of the weighted average of the masses of each isotope, that is, multiplying the abundance of each isotope by its mass and adding this result for all the isotopes.