Both isotopes of uranium are naturally radioactive; their bulky atoms disintegrating over time. Given enough time (hundreds of thousands of years) uranium will eventually lose so many particles that it will turn into lead. This process of decay can be greatly accelerated in what is known as a chain reaction. Instead of disintegrating naturally and slowly, the atoms are forcibly split by bombardment with neutrons.

A blow from a single neutron is enough to split the less-stable U-235 atom, creating atoms of smaller elements (often barium and krypton) and releasing heat and gamma radiation (the most powerful and lethal form of radioactivity). The chain reaction occurs when "spare" neutrons from this atom fly out with sufficient force to split other U-235 atoms they come in contact with. In theory, it is necessary to split only one U-235 atom, which will release neutrons which will split other atoms, which will release neutrons ... and so on. This progression is not arithmetic; it is geometric and takes place within a millionth of a second.

The minimum amount to start a chain reaction as described above is known as super critical mass. For pure U-235, it is 110 pounds (50 kilograms). No uranium is ever quite pure, however, so in reality more will be needed. U-235, U-238 and Plutonium

Uranium is not the only material used for making atomic bombs. Another material is the Pu-239 isotope of the man-made element plutonium. Plutonium is only found naturally in minute traces, so useable amounts must be produced from uranium. In a nuclear reactor, uranium's heavier U-238 isotope can be forced to acquire extra particles, eventually becoming the 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. In certain types of bombs, a mixture of the elements Beryllium and Polonium is used to bring about this reaction. Only a small piece is needed (super critical mass is about 32 pounds, though as little as 22 can be used). The material is not fissionable in and of itself, but merely acts as a catalyst to the greater reaction.

Related Links
Nuclear Innovations
More detailed information on different nuclear inventions and the history of nuclear physics. Timeline of nuclear technology and major figures in radiation history.
Albert Einstein
German-American physicist who developed the special and general theories of relativity - the irony for this idealistic man was that his famous postulation of an energy-mass equation, which states that a particle of matter can be converted into an enormous quantity of energy, had its spectacular proof in the creation of the atomic and hydrogen bombs, the most destructive weapons ever known.
Plutonium on the Internet

Cheers Theis

That's all nice and well, but I have yet to find out why they pretty little mushroom cloud is made.

Fission (A-Bomb) & Fusion (H-Bomb)
AH = Hitler's initials; he did kind of fall apart and left everything in 'con'fusion.

Thanks Theis, but I've kinda come to the conclusion long ago I need to make friends with someone at NASA or something, becuase no one seems to know (Wikipedia sure doesn't).  What I meant was I'd like to know why nuclear (or any large explosion) produces a mushroom.  All I can theorize is weight and density.

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Imagine how screwed your day will be if this is one of the first things you read after having got up.

Never mind about those coincidence dates. I was born on April 24th, so I share my birthday with Phillipe Pétain, the french general who won Verdun and collaborated with the Nazis from 1940 till 1944.

If someone asks me for my birthday, how ever, I always say either "Right between Lenin and Tchernobyl" or "Between Adolf and Tchernobyl".

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Sorry, I don't know that much about british history.

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I've just overheard the "ruthless dictator" thing.

- Edit: Depends on who you were referring to. Might agree concerning Hitler, will disagree concerning Lenin 'though.