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

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.

the electronic configuration of an atom with eight electrons is 1s²2s²2p⁴. the orbital diagrams are shown in the figure.

as per the hunds rule:

1. electrons tend to avoid being in the same orbital as per as practicable.

2. electrons in different orbitals of the same energy will have their spin parallel.

this can be illustrated from the multiplicity of the orbitals of the atom. as there are four electrons in the p orbital and p orbital has 3 sub-shells. the electrons can be fed in different ways as shown in the figure but the correct orbital diagram will be as per the hunds rule.

law of conservation of matter