Uranium is an extremely heavy metal, heavier than gold, and it has the largest atoms of any natural element. Moreover, the atoms have far more neutrons than protons, which does not make them easier to split, but does have an important bearing on the amount of energy they release during an explosion.

There are two ISOTOPES of uranium: an isotope is a form of an element distinguished by the number of neutrons in its atom. Natural uranium consists mostly of the isotope U-238, which has 92 protons and 146 neutrons (92+146=238). But mixed in with this is about 0.6 per cent of the other isotope, U-235, which has the same number of protons but only 143 neutrons. This isotope, unlike U-238, is fissionable (its atoms can be split), and so it is the one used for making bombs.