Radioactive element (it doesn't have stable isotopes). Uranium was the first element that was found to be fissionable, i.e. upon bombardment with slow neutrons, its U235 isotope becomes the very short lived U236, that immediately divides into two smaller nuclei, liberating energy and more neutrons. If these neutron are absorbed by other U235 nuclei, a chain reaction occurs, and if there isn't anything to absorb some neutrons and slow the reaction, it is explosive. The first atomic bomb worked with by this principle (fission). A more accurate name for both this and the Hydrogen bomb (fusion) would be "nuclear bomb", because only the nuclei participate.