What fraction or percentage of carbon atoms has a mass of 12.011 amu?

No carbon atom has a mass of 12.011 amu

Explanation:

The atomic mass of elements that we read in the periodic table is actually the average mass of the atoms of said elements. In the case of carbon, this would mean that most carbon atoms in the universe have a mass of 12 amu (with 6 protons and 6 neutrons), while a smaller percentage has a mass of 13 amu (with 7 neutrons). There are other carbon isotopes, with a mass of 11 or 14, however they are less stable.

Zero fraction or percentage of carbon atom has a mass 12.011 amu.

Due to isotopes of carbon, no carbon atom has the mass 12.011 amu. This mass is not the exact mass, but it is the average mass of carbon atoms. Since, the weight of isotopes is different thus, the atomic weight of the carbon in periodic table is written by taking average of atomic weight of isotopes of carbon atoms by also considering their abundance.