Atomic Mass Units: Discrepancy Explained?

[bsjams]

Been many years since I took general and org chem and am currently reading general chem textbook as refresher. Came across defintion of amu (atomic mass unit). Text says that the mass of the carbon-12 atom is defined to be exactly 12 amu. However, a couple of pages over, I read that the mass of the proton is about 1.007 amu, and the mass of the neutron is about 1.009 amu. Add up the masses of 6 protons and 6 neutrons for the carbon-12 atom, and you come up with more than 12 amu. I can't resolve this apparent discrepancy. I've consulted other texts, etc. Does some type of relativistic effect bear on this situation? In other words, does, for example, an isolated proton have a mass of 1.007 amu, but then when part of an atom have a slightly different mass because of some relativistic effect? HELP!

 

[Bystander]

The mass difference is the "binding energy" for ¹²C.

 

[ravenprp]

The discrepancy between the mass of an individual proton or neutron and the mass of the same particle within an atom can be explained by the concept of binding energy. In an atom, the protons and neutrons are held together by strong nuclear forces, which require energy to overcome. This binding energy reduces the overall mass of the atom, making it slightly less than the sum of the individual masses of its constituent particles.

This phenomenon is known as mass defect and is described by Einstein's famous equation E=mc^2. The binding energy is converted into mass, resulting in a decrease in the overall mass of the atom.

In the case of carbon-12, the mass defect is relatively small, but it becomes more significant for larger atoms with more protons and neutrons. This is why the mass of an individual proton or neutron is not exactly the same as the mass of the same particle within an atom.

So, the explanation for the discrepancy between the mass of an individual proton or neutron and the mass of the same particle within an atom is due to the binding energy and its conversion to mass. This is a fundamental concept in nuclear chemistry and is important to understand in order to fully grasp the concept of atomic mass units.