In nuclear physics, nuclear fusion is a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as the release of large amounts of energy. This difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is the process that powers active or "main sequence" stars, or other high magnitude stars.

Nuclear fission is caused by the impact of a neutron.

Nuclear fission is a process in which the nucleus of an atom is split into two smaller nuclei, along with the release of a large amount of energy. This process is typically initiated by the impact of a neutron on a heavy atomic nucleus. Let's understand why a neutron is the key particle in causing nuclear fission.

1. Neutrons and Atomic Nuclei:
- Neutrons are subatomic particles that carry no electrical charge, making them electrically neutral.
- Atomic nuclei are made up of protons and neutrons, with protons carrying a positive charge and neutrons carrying no charge.
- The strong nuclear force, which is one of the fundamental forces in nature, holds the nucleus together by overcoming the repulsive electromagnetic force between protons.

2. Neutron-induced Fission:
- When a neutron collides with a heavy atomic nucleus, such as uranium-235 or plutonium-239, it can be absorbed by the nucleus.
- The absorption of the neutron destabilizes the atomic nucleus, making it highly unstable and prone to splitting apart.
- The nucleus undergoes a process called nuclear fission, breaking into two smaller nuclei, known as fission fragments.
- Along with the fission fragments, several neutrons are also released as a result of the fission process.

3. Chain Reaction:
- The release of additional neutrons from the fission process can initiate a chain reaction.
- These released neutrons can go on to collide with other heavy atomic nuclei, causing them to undergo fission as well.
- As each nucleus splits, it releases more neutrons, which can then continue the chain reaction by causing further fission reactions.
- This chain reaction can lead to an exponential increase in the number of fission reactions, resulting in a large release of energy.

4. Energy Release:
- The energy released during nuclear fission is a result of the conversion of mass into energy, as described by Einstein's famous equation, E=mc².
- The total mass of the fission products is slightly less than the mass of the original nucleus, and this mass difference is converted into a tremendous amount of energy according to the equation.

In summary, nuclear fission is caused by the impact of a neutron on a heavy atomic nucleus. The absorption of the neutron by the nucleus destabilizes it, leading to the splitting apart of the nucleus into two smaller fragments and the release of a large amount of energy. The released neutrons can then go on to initiate a chain reaction, resulting in a self-sustaining process that releases even more energy.

Neutrons