Usually atomic bombs are composed of Uranium-235, Uranium-238, and Plutonium (if I am wrong, please correct me), all of which have an alpha decay. However, if alpha particles have the least penetrating power, and can be stopped through something a thin as a piece of paper, how is it so dangerous?
edit: Sorry for the confusion, I meant how is the radiation from it dangerous, not the initial explosion. However it seems my question has been answered on both accounts. Thank you to everyone who answered! I have a better understanding of it now.
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One way U-235 can decay to smaller bits is by ejecting an alpha particle. That is, as you said, not dangerous enough to make a weapon of mass destruction.
However, if you hit U-235 with a slow-moving neutron, it can split in a different way: basically shearing off into large clumps, and spitting off 2-3 new neutrons in the process: Eg, U-235 + n –> Barium-144 + Krypton-90 + n + n + heaps of energy; or U-235 + n —> Zircon-94 + Tellurium-139 + n + n + n + heaps of energy.
If those neutrons slow down to just the right speed (which can be arranged), then they in turn trigger more fissions, which trigger still more, so you get heaps of fission events, each releasing a lot of energy, packed into a very short period of time.
The thing that makes atomic bombs dangerous is
* the sudden release of truly enormous amounts of energy, and
* the production of a wide variety of radioactive isotopes that spread throughout the environment and ecosystem.
The energy vaporises everything in the immediate vicinity, sets fire to everything slightly further away, and causes a blast that smashes and burns everything further away still; there’s also a big burst of gamma radiation that causes its own set of damage to living things, and is very hard to shield oneself from.
The radioactive material consists of alpha, beta and gamma emitters (and their decay products), and these can each be dangerous in their own way. For example, even an alpha emitter can be dangerous if it becomes part of the food chain, and ends up irradiating people from the inside.
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