I was studying about atomic masses and realized that even if we say that the atomic mass unit corresponds to 1/12 of the mass of a carbon atom. why is it that even particular isotopes of elements have atomic mass in decimal values. 1/12 of a carbon atoms mass should equal to the mass of a neutorn or a proton (the mass of an hydrogen atom). It isn't that im confused with the concepts of atomic mass and mass number. my question is what is it that is contributing to the mass of an atom such that the atomic mass is in decimal values?