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If you've ever watched a World War II documentary, chances are you've seen footage of an atom bomb exploding over Hiroshima. The image of the expanding mushroom cloud is both terrifying and fascinating, leaving one to wonder: how did atoms, some of the smallest particles known to exist, give rise to such a gigantic explosion?

The answer involves something even smaller than an atom--the atomic nucleus. Basically, the nuclear explosion over Hiroshima occurred when the atomic nuclei inside the bomb were caused to split--a process called nuclear fission. When fission occurs on a large enough scale, a tremendous amount of energy is released.

To create a bomb based on nuclear fission, the scientists working on the top secret Manhattan Project had to find fissionable fuel. They settled on enriched uranium. When this substance was exposed to stray neutrons given off by other atoms, the nuclei of the uranium atoms would absorb the neutrons, immediately become unstable, and split.

When these nuclei split they would give off more neutrons, which would then be absorbed by other atoms and cause them to split, and so on. Once triggered, this chain reaction happened almost instantly, releasing a huge amount of energy in the form of heat and gamma radiation.

To get a sense of the power of that explosion, consider that a few pounds of fissioning uranium completely destroyed everything within a one mile radius. What's worse, nuclear weapons built today are many times more powerful, their destructive capabilities almost beyond imagination. So although nuclear weapons helped America win World War II, most people hope we never have to use them again.

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