The masses of protons and neutrons, despite both being composed of three quarks, are not exactly the same. This difference arises from several factors:
Quark masses: While the quarks in protons and neutrons have the same charge (up and down quarks), their masses are not equal. The up quark is slightly lighter than the down quark. This small mass difference contributes to the overall mass difference between protons and neutrons.
Binding energy: Protons and neutrons are held together by the strong nuclear force, which involves the exchange of particles called gluons. The binding energy, or the energy required to separate the quarks within a nucleon (proton or neutron), contributes to the mass of the nucleon. The strong force is a complex interaction, and its effects on the overall mass of the nucleon can lead to slight differences between protons and neutrons.
Electromagnetic effects: Protons have a positive electric charge, while neutrons are electrically neutral. This difference in charge results in electromagnetic interactions within the nucleons. The electromagnetic forces can influence the effective mass of the nucleons, leading to a small difference in their overall masses.
It's important to note that the masses of protons and neutrons are quite close to each other. The mass of a proton is approximately 1.0073 atomic mass units (u), while the mass of a neutron is about 1.0087 u. The difference in mass is around 0.0014 u, which is relatively small compared to the total mass of the nucleons.