A carbon - 12 atom and a regular carbon atom would have the same number of protons which is 6. So a carbon - 12 atom would have 6 protons. Both, however, would differ in the number of neutrons. Carbon - 12 atom has 6 neutrons. To determine the mass defect of a carbon - 12 atom, we have to add the total mass of protons and the total mass of neutrons and subtract the known mass of a carbon - 12 atom. That would be like this.
6 (1.00728 amu) + 6 (1.00866 amu) = x
6.04368 amu + 6.05196 amu = x
12.09564 amu = x
Then subtract it with 12 amu to get the defect mass
12.09564 amu - 12.00000 amu = y
0.09564 amu = y
So the defect mass would be 0.09564 amu.
