During nuclear fusion, atoms combine to form a larger atom, releasing energy in the process. The mass lost during fusion is converted into this energy according to Albert Einstein's famous equation, E=mc², where E represents energy, m represents mass, and c represents the speed of light.
The loss of mass during fusion is primarily a consequence of the binding energy of the atomic nucleus. The nucleus of an atom is made up of protons and neutrons, and these particles are held together by the strong nuclear force. However, the individual protons and neutrons themselves have slightly more mass when they are separate than when they are bound together in a nucleus. This difference in mass is known as the mass defect.
When lighter atomic nuclei fuse together to form a heavier nucleus, the resulting nucleus has a lower mass than the sum of the masses of the individual nuclei. The missing mass is converted into energy in accordance with Einstein's equation. This energy is released in the form of electromagnetic radiation, such as gamma rays.
It's important to note that the loss of mass during fusion is a small fraction of the total mass involved. However, since c² (the speed of light squared) is an enormous value, a small amount of mass can generate a significant amount of energy. This is why nuclear fusion is a potential source of vast amounts of energy, as demonstrated in the Sun and in experimental fusion reactors.