The nucleus is like a ball of glue (neutrons) and magnets that are all repelling each other (protons). But the balance is delicate and just right to keep it together. And it was forced together with quite a bit of energy when it was made in a star.
When a stray bit of glue comes along and hits the ball, it is too much to handle and the ball falls apart. This splits into a few large pieces, some single bits of glue (neutrons) and the energy from when it was made. If your bomb is crafted properly, at least one of these neutrons will hit the neighbouring atoms and cause another fission. This chain reaction will affect the whole material and release a huge amount of energy.
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