The probability of getting a carbon atom with mass 12.011 is zero.
The average mass of a carbon atom is 12.011 this is due to isotopes of carbon. No carbon atom has the mass 12.011 amu because this is not the exact mass, but this is the average mass of isotopes of carbon. Since, different isotopes have different masses, the atomic mass of the carbon in periodic table is written by taking average of atomic masses of its isotopes in accordance to their abundance.
