Paraphrase the following selection. Nuclear energy becomes available when radioactive (unstable) atoms split apart. The natural or artificial breaking up of atoms into smaller pieces is called fission. The newly discovered ability to "split" the atom is what makes atoms into a source of energy--nuclear energy. Scientists discovered that uranium 235 can be caused to split apart when hit by a neutron traveling at a medium speed. The uranium atom divides and subsequently becomes two new elements, barium and krypton.