The release of nuclear energy is associated with changes from less stable to more stable nuclei and produces far more energy for a given mass of fuel than any other source of energy. In fission processes, a fissionable nucleus absorbs a neutron, becomes unstable, and splits into two nearly equal nuclei. In fusion processes, two nuclei combine to form a single, heavier nucleus. The most stable nuclei—those with the highest binding energies per nucleon holding their components together—are in the middle range of atomic weights, with the maximum stability at weights near 60. Thus, fission, which produces two lighter fragments, occurs for very heavy nuclei, while fusion occurs for the lightest nuclei.