The phenomenon you're referring to is called the mass defect in nuclear physics. When nucleons (protons and neutrons) come together to form a nucleus, there is a release of energy due to the strong nuclear force binding them together. This release of energy is equivalent to a loss of mass according to Einstein's mass-energy equivalence principle (E=mc²), where energy and mass are interchangeable.
The mass defect in nuclear binding arises because the total mass of the individual nucleons is slightly greater than the mass of the nucleus they form. This difference in mass is attributed to the binding energy that holds the nucleons together. The binding energy is a manifestation of the strong nuclear force, which is responsible for overcoming the electromagnetic repulsion between positively charged protons in the nucleus.
Now, let's consider the quarks within nucleons. Quarks are bound together by the strong force to form protons and neutrons. However, the mass of a proton or a neutron is greater than the sum of the masses of its constituent quarks. This difference in mass is again attributed to the binding energy associated with the strong force.
In both cases, whether it's the binding of nucleons or quarks, the mass difference arises due to the energy associated with the strong force holding the particles together. The key distinction lies in the energy scales involved. The binding energy of nucleons in a nucleus is relatively small compared to the individual nucleon masses, resulting in a mass loss. On the other hand, the binding energy of quarks within nucleons is much larger compared to the individual quark masses, resulting in a mass increase.
So, while both scenarios involve the binding energy of particles, the difference in mass change arises due to the different energy scales involved in the respective processes.