The phenomenon that occurs when an atomic nucleus splits is called nuclear fission. This process releases a significant amount of energy, utilized in nuclear power generation and atomic weapons. During fission, heavy nuclei, such as uranium-235 or plutonium-239, absorb a neutron, become unstable, and subsequently split into smaller nuclei, releasing additional neutrons and energy in the form of thermal energy. Understanding nuclear fission is critical for both the development of energy sources and the implications for nuclear safety and non-proliferation measures globally.