Nuclear binding energy and mass excess are two related but distinct concepts used in nuclear physics.
Nuclear binding energy is the amount of energy required to completely separate the nucleons (protons and neutrons) in a nucleus into their individual components. It represents the energy that is released when nucleons come together to form a nucleus. Nuclear binding energy is typically expressed in units of electron volts (eV) or mega-electron volts (MeV). It is a measure of the strength of the nuclear force, which holds the nucleus together.
Mass excess, on the other hand, is the difference between the mass of a nucleus and the sum of the masses of its individual nucleons (protons and neutrons) when they are at rest. Mass excess is also typically expressed in units of electron volts (eV) or mega-electron volts (MeV). It represents the difference between the actual mass of a nucleus and the mass that would be expected based on the masses of its individual nucleons. Mass excess is a measure of the binding energy of a nucleus, and it is related to the nuclear binding energy through the equation E = mc^2, where E is energy, m is mass, and c is the speed of light.
In summary, nuclear binding energy is the energy required to completely separate the nucleons in a nucleus, while mass excess is the difference between the mass of a nucleus and the sum of the masses of its individual nucleons. Both of these concepts are important in understanding the properties of atomic nuclei and how they are affected by nuclear reactions.