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Fission and Fusion

Atoms that are unstable can become more stable by rearranging the nucleus.

Large unstable atoms can become more stable by splitting into smaller, more stable atoms. This is called fission.

Small atoms can become more stable by joining together into larger, more stable atoms. This is called fusion.


Large, unstable atoms can turn into stable ones by rearranging the nucleons. There are usually some particles left over after this rearranging, and these particles are ejected from the nucleus. This process of rearranging a large unstable nucleus into smaller nuclei is called fission.

The process of fission was first described by Otto Hahn and Lise Meitner.

There are several kinds of fission processes.

A small change can be achieved by the nucleus ejecting an electron. This is called Beta (β) radiation. It results in one of the neutrons in the nucleus changing into a proton. (Sometimes the reverse can happen, and the nucleus captures an electron, changing a proton into a neutron.)

A bigger change can happen by the nucleus spitting out four particles—two protons and two neutrons. (This combination of particles is a very stable arrangement. It is the nucleus of a helium atom.) This is called Alpha (α) radiation.

A really big change can involve the nucleus completely breaking up into two or more smaller nuclei. This kind of splitting is often accompanied by α and/or β radiation as well.

When matter is ejected, usually electromagnetic radiation is ejected as well in the form of X-rays, or gamma rays. This electromagnetic radiation can have a lot of energy.


Small atoms can become more stable by joining together to form a larger nucleus. However in order to fuse together, the two nuclei have to come very close together. It is only at very short ranges that the strong nuclear force can overcome the electric force of repulsion between the atoms.

In order for the nuclei to come close enough together to fuse, the nuclei have to be travelling very fast. This means that the atoms have to be very hot. The only place hot enough for fusion to happen naturally is at the centre of stars, like our Sun. Solar water heaters tap directly into fusion power. Fusion reactions in the Sun are the basis of the sunlight energy that sustains life on Earth.

On Earth, scientists have tried to produce fusion reactions in the hope of generating power in a similar way. So far only short fusion reactions have been achieved. These reactions have taken more energy to start than they have released, so they have not been useful for generating power.

Lise Meitner
magnifyLisa Meitner

The Sun
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