Nuclear fission occurs when

uranium nuclei splits into heavier metals

hydrogen nuclei fuse together to make helium

uranium is split into two less heavier elements and a lot of energy is released

None of the above

Radioactive fission, where the center of a heavy element spontaneously emits a charged particle as it breaks down into a smaller nucleus, does notoccur often, and happens only with the heavier elements. Fission is different from the process of fusion, when two nuclei join together rather than split apart.