Which description best defines nuclear fission?

• A. A neutron interaction in which a nucleus absorbs a neutron, splits into two new nuclei and releases a large amount of energy; the resulting fission fragments emit neutrons and gamma rays and then undergo radioactive decay
• B. Two light nuclei fuse together to form a heavier nucleus, releasing energy
• C. A neutron changing into a proton with the emission of an electron
• D. Nucleons rearranging without a change in mass number

Answer: (A)

Nuclear fission is when a heavy nucleus absorbs a neutron and splits into two lighter nuclei, releasing a large amount of energy. This splitting often releases prompt neutrons and gamma rays, and the resulting fission fragments are typically radioactive and decay over time. The energy comes from the mass defect—the difference between the mass of the original nucleus plus the absorbed neutron and the combined mass of the fission fragments and other emitted particles. The emitted neutrons can sustain a chain reaction if sufficient fissile material is present, which is the principle behind both reactors and certain types of bombs. The other descriptions refer to different processes: fusion combines light nuclei to form a heavier nucleus, beta decay involves a neutron transforming into a proton with emission of an electron, and nucleons rearranging without a change in mass number describes a non-fissional rearrangement. Hence, the description involving neutron-induced splitting of a heavy nucleus into two lighter nuclei with energy release (and accompanying neutrons and gamma rays) best defines fission.