Nuclear fission is the process of splitting atoms. Fission usually takes place when neutrons interact with large nuclei. Fissionable materials are those radioisotopes that will undergo nuclear fission. The number of fissionable radioisotopes is very small. The common fissionable radioisiotopes include: uranium-235, uranium-238 and plutonium-239.
In a chain reaction, there is a net >= of one neutron produced by fission that goes on to be involved in a subsequent fission. To have a chain reaction, one needs a minimum amount of fissile material. The amount of fissile material required to sustain a chain reaction is called the critical mass. In making atomic bombs [local], the critical mass is achieved by surrounding the fissile material with explosive material intended to compress the nuclear fuel.
Very large nuclei may experience spontaneous fission. Most fission events are the result of a nucleus absorbing a neutron, and the resulting nucleus being produced in an excited state from which fission is essentially instantaneous. The distribution of the nucleons among the daughter nuclides is discussed in probabilistic terms.
