In addition to the spontaneous decay of radioactive materials by emission of sub-atomic particles and energy, there is another way for unstable atoms to achieve nuclear structures which are stable.
A few elements undergo a process known as fission. One nucleus splits into two smaller nuclei and typically a few neutrons. Energy is also released based on the mass loss during the transformation.
U-235 is the only naturally occurring nuclide that fissions. However, both U-238 and Th-232 can be converted by neutron capture and subsequent beta decay into fissile isotopes: Pu-239 and U-233.
A great deal of data exists on the fission of U-235 due to research during World War II and also because of later nuclear power research. Perhaps the most impressive datum of all is the amount of energy that can be released when U-235 fissions.