Uranium is not the only material used for making A-bombs. Another material is the element plutonium, in its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from uranium. This can be done by putting U-238 in a nuclear reactor. After a while, the intense radioactivity causes it to pick up the extra particles, so that more and more of its atoms turn into plutonium.

Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the plutonium itself.

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.

The A-bomb, or atomic bomb, has been used only twice in war, at Hiroshima and Nagasaki, and both times it has totally destroyed a large city in a single explosion.

Strictly speaking, the term atomic bomb includes the H-bomb, which also uses atomic power. But in general usage, atomic bomb is reserved for earlier weapons that work by nuclear fission, that is, splitting atoms. The hydrogen bomb works by nuclear fusion - joining small atoms together to make larger ones. Both fission and fusion release huge amounts of energy and radioactivity.

Nuclear fission