The sun and stars are seemingly inexhaustible sources of energy. That energy is the result of nuclear reactions, in which matter is converted to energy. We have been able to harness that mechanism and regularly use it to generate power. Presently, nuclear energy provides for approximately 16% of the world's electricity.
Nuclear Fission (the splitting of nuclei) and Nuclear Fusion (the joining of nuclei) are nuclear processes that both result in the release of energy that is no longer needed by the resulting nucleus, after Nuclear Fission or Nuclear Fusion has occurred.
The term “fission” means “splitting apart”, so in nuclear fission the splitting of atomic nuclei produces typically two or three smaller nuclei. We find that when nuclear fission occurs, the mass of the reaction products is less than the original mass of the nucleus or reacting particles, resulting in the release of the energy that was used to bind the original nucleus together. This is the case of elements with heavy nuclei (such as Uranium).In the same context, the term “fusion” means combining nuclei together. In nuclear fusion the total mass of the reaction product (also called the daughter nucleus) is still less than the original mass of the nucleus or reacting particles, even though two nuclei are now combined. This is because it takes less energy for atoms with lighter nuclei (from elements such as Helium) to exist fused together, rather than exist individually. Therefore, energy is released when fusion of lighter nuclei takes place. Nuclear fusion is more common that fission in nature and is most easily obtained using lighter elements such as Hydrogen, Helium and Carbon.
In general, if a nucleus is formed through “gluing” nucleons together, its mass is smaller than the mass of the original free nucleons. This effect is known as the mass defect.
The IS unit of energy, the Joule (J), is too large to measure the energy released by a single nucleus. By convention we use the MeV (million electron volt) for this, where 1MeV = 106eV and 1eV = 1.602177x10-19J.A nuclear process that releases a large amount of energy is the fission of a heavy nucleus. For example, when a single 235U nucleus undergoes fission, about 200MeV is released. This is a lot of energy as can be seen from some comparisons: