Fission is a process where an unstable nucleus splits into two daughter nuclei and in the process, energy and neutrons are released. The total mass number and total atomic number before and after fission are the same.
Fission can be spontaneous or induced. Spontaneous fission is very rare and does not require a neutron. Figure 1 shows induced fission, where a neutron is fired at a very large nucleus. This causes the nucleus to become unstable, inducing fission.
Nuclear reactors use fission to make energy. Uranium 235, uranium 233, and plutonium 239 are nuclides commonly used in nuclear reactors.
Figure 1: In fission, an unstable nucleus splits into two smaller nuclei. Neutrons and energy are released in the process.