How does nuclear fission work with uranium?
Nuclear fission starts off with Uranium- 235, an isotope of uranium with 235 neutrons and 92 protons.
The Uranium- 235 is impacted deliberately by a slow neutron, adding it to its "collection" of neutrons, forming Uranium- 236.
Uranium- 236 is highly unstable, and begins to oscillate. Then, the Uranium- 236 "breaks" into Barium-141 and Krypton- 92, and 2 extra neutrons. This is one possible way that the Uranium-236 can split, as there are many other elements that it can form. It also releases gamma radiation (electromagnetic radiation with an extremely high frequency)
The reaction releases a ton of energy, and the 2 neutrons produced fly outwards and randomly strikes another Uranium-235 atom, repeating the process.
This is why the process is called a chain reaction. Each time a Uranium- 236 atom breaks apart, it "activates" 2 more atoms, speeding up the process every time.
An atomic bomb uses uncontrolled nuclear fission from either Uranium-233, Uranium-235, or Plutonium-239.