A stable atom of Uranium-235 becomes an
unstable (radioactive) atom of Uranium-236
when it absorbs a neutron. The nucleus of
an unstable U-236 atom breaks apart spontaneously,
releasing a large amount of energy, nuclei
of two lighter elements, and three neutrons.
Other U-235 atoms can absorb the neutrons
that are released in the reaction, forming
more unstable atoms. As increasing numbers
of unstable atoms experience fission, more
energy and neutrons are released, resulting
in a chain reaction of fission.