An atomic bomb is composed of some heavy-duty explosives, capacitors, U-235 or P-239, and a neutron source.

The neutron source is in the center, surrounded by the Uranium or Plutonium, and then the explosives. Upon detonation, the capacitors all detonate the explosives simultaneously, crushing the nuclear material in on the neutron source. This causes the material to achieve critical mass and a massive chain reaction takes place.

Boom.