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The mass of proton is 1.672 × 10−27 kg while it is 1.675 × 10−27 kg. Both are made up of 3 quarks each. Then what makes proton lighter than a neutron?

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    $\begingroup$ This question does not have an answer. The "answers" to the duplicate are non-answers. $\endgroup$ – John Duffield Aug 14 '15 at 13:13
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Note that the combined rest masses of the quarks (~10 MeV/$c^2$) account for about 1% of the proton and neutron mass (~938 MeV/$c^2$), the main contribution to the mass are the gluons from the Strong Force. Since the composition of the proton and neutron are different, so is the force that binds them.

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