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 this principle (fission). A more accurate
name for both this and the
Hydrogen bomb (fusion) would be "nuclear
bomb", because only the nuclei participate.