The process of binding nucleons (protons and neutrons) to form a nucleus involves the release of energy, but it does not result in a loss of mass. In fact, the combined mass of the individual nucleons in a nucleus is slightly less than the mass of the resulting nucleus. This phenomenon is known as mass defect.
The mass of a nucleus is determined by the sum of the masses of its constituent protons and neutrons, as well as the binding energy that holds the nucleons together. The binding energy is a measure of the energy required to separate the nucleons from the nucleus. According to Einstein's mass-energy equivalence (E=mc²), the binding energy contributes to the total mass of the nucleus.
When nucleons come together to form a nucleus, they are subject to the strong nuclear force, which is responsible for binding the protons and neutrons together. This force acts over very short distances and is stronger than the electromagnetic repulsion between positively charged protons. As nucleons move closer, the strong force becomes dominant, overcoming the electrostatic repulsion and binding the nucleons together.
During the formation of a nucleus, some of the mass is converted into binding energy. This conversion is a result of the mass-energy equivalence mentioned earlier. The binding energy is released when nucleons come together and is responsible for the stability and cohesion of the nucleus.
The mass defect, which is the difference between the mass of the individual nucleons and the mass of the nucleus, represents the converted mass-energy. This energy is associated with the strong force that binds the nucleons together. The famous equation E=mc² tells us that this energy is equivalent to mass.
To summarize, the process of forming a nucleus involves the conversion of mass into binding energy. The mass of the nucleus is slightly less than the combined mass of its constituent nucleons due to this conversion, but the total mass-energy of the system remains conserved.