Nuclear binding energy is the energy required to split a nucleus of an atom into its component parts. The component parts are neutrons and protons, which are collectively callednucleons. The binding energy of nuclei is always a positive number, since all nuclei require net energy to separate them into individual protons and neutrons. Thus, the mass of an atom's nucleus is always less than the sum of the individual masses of the constituent protons and neutrons when separated. This notable difference is a measure of the nuclear binding energy, which is a result of forces that hold the nucleus together. Because these forces result in the removal of energy when the nucleus is formed, and this energy has mass, mass is removed from the total mass of the original particles, and the mass is missing in the resulting nucleus. This missing mass is known as the mass defect, and represents the energy released when the nucleus is formed.
Nuclear binding energy can also be considered to apply to situations when the nucleus is split into fragments composed of more than one nucleon, and in this case the binding energies for the fragments (as compared to the whole) may be either positive or negative, depending on where the parent nucleus and the daughter fragments fall on the nuclear binding energy curve (see below). If new binding energy is available when light nuclei fuse, or when heavy nuclei split, either of these processes will result in a release of the binding energy, and this energy is available as nuclear energy and can be used for production of nuclear power or for the construction of nuclear weapons. When a large nucleus splits into pieces, excess energy is emitted as photons (gamma rays) and as kinetic energy of a number of different ejected particles (fission products, see nuclear fission).
Total mass is conserved throughout all such processes so long as the system is isolated. During each nuclear transmutation, the "mass defect" mass is relocated to, or carried away by, other particles which are no longer a part of the original nucleus.
The mass defect of a nucleus represents the mass of the energy of binding of the nucleus, and is the difference between the mass of a nucleus and the sum of the masses of thenucleons of which it is composed. Determining the relevant nuclear binding energy encompasses three steps of calculation, which involves the creation of mass defect by removing the mass as released energy.