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[SOLVED] why the masses of protons and neutrons are slightly greater than 1u?

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Fractional-N

Fractional-N

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Hi,
1u,The unified atomic mass unit, is defined to be: "one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state"(from wikipedia.org)

OK, so C12 has 6 protons, 6 neutrons, and 6 electrons. so:
6 * 1.672621777e-27(mass of proton) + 6 * 1.674927351e-27(mass of neutron) + 6 * 9.10938291e-31(mass of electron) = 2.009076039e-26

Now, 1u is equal to 1.660538921e-27(Kg)
and calculating the mass of C12 this way gives: 1.660538921e-27 * 12 = 1.9926467052e-26

Why the two calculations didn't give the same result?
 

In atomic nuclei a little amount of mass have to be seen as the binding energy (defect mass).
Then if you sum each single component you will obtain a little bit higher value with respect to that of the nucleus.

In C12 the defect mass is then 2.009076039e-26/1.660538921e-27 -12u = 0.09893976945u

Conversion between mass and energy can be calculated using E=mc²
 
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