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Name: Ivan
Status: educator
Grade: 12
Country: Italy
Date: Spring 2012

I do not understand why the proton and the neutron have masses slightly more than 1.67 x 10 -24 kg, while the AMU is about 1.66 x 10 -24 kg and is defined as 1/12 of the mass of a carbon 12 atom (6 protons and 6 neutrons). I would expect to find an average between the two values, thus a value for the AMU higher than 1.67 x 10 -24 kg. But this is not the case. Can you help me?

When nucleons combine to form an atomic nucleus, their potential energy is lower than when they are separated. Since E = mc^2, that means that their mass is also less than when they are separated. Neat, huh?

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy

Hi Ivan,

The phenomenon you describe may be due to the differences in the method of measurement.

The masses of the individual proton and neutron are measured by acceleration at high speeds and deflected by a magnetic field. Therefore, they are also under the influence of the measurement proper which may introduce energy effects. Those energy effects translate to mass by m = E/c2.

For the C 12 measurement: The masses of nuclear protons and neutrons are measured at rest, unbound, and in the ground state divided by 12. Essentially, this is an average of the masses.

This discrepancy is one reason why the mass chosen to be used in calculations is selected by the experimental design of the researcher.

For most experiments in general or organic chemistry and in classical physics, the difference may be ignored.

Hope this helps! Peter E. Hughes, Ph.D. Milford, NH


At the level of atoms, mass does not just add together. Mass is a form of energy, based on Einstein’s E=mc^2. Energy must be added to a carbon atom to separate the six protons and six neutrons. IF you add this needed energy to the mass of a carbon nucleus, you will get the energy of six protons and six neutrons. This decrease of mass is what holds the nucleus together.

Dr. Ken Mellendorf Physics Instructor Illinois Central College

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