The process of breaking of a heavy unstable nucleus into two lighter nuclei is called nuclear fission. E.g. when a fast moving neutron strikes a uranium nucleus, Krypton and Barium are formed as shown in the equation.
0n1 + 92U235 → 36Kr92 + 56Ba141 + 30n1 + energy
This is represented in the picture as below:
Large amount of energy (about 200 Mev) is released during this process. If this energy is used by a nuclear reactor it can be used for various useful purposes. If the energy is released without control, it becomes an atomic bomb.
Bohr wheeler theory of nuclear fission:
Bohr and Wheeler compared the atomic nucleus with a liquid drop which is spherical in shape due to surface tension. When a neutron strikes the nucleus with certain velocity, the nucleus absorbs the neutron. The K.E. of neutron sets up vibration within the nucleus and the spherical shape distorts into ellipsoidal shape. If the vibration energy is not large enough the nucleus regains its spherical shape. The excess amount of energy is released as γ-rays.
If the vibration energy is large enough the shape further distorts into dumbbell shape and breaks into two nuclei o about same sizes. Some excess neutrons may be produced during this process.
The process of combining two lighter nuclei to make up larger nucleus is known as nuclear fusion. For e.g. the deuterium nuclei combine to give helium nucleus and energy.
1H2 + 1H3 → 2He4 + 0n1 + energy
This reaction is represented in a picture below:
Energy of about 24 meV is produced per fusion which is about 6meV per nucleus. Fusion process is not artificially possible because it require a temperature of about 107 Kelvin. Energy in the sun and stars are believed to be produced by this process.