Nuclear fission is when you can split an atom's nucleus into two fragments with a neutron. It involves the isotopes of uranium (235, and 233) or plutonium 239. Nuclear fusion is when you take two atoms combining them to form another substance. In processes, fission and fusion, large amounts of heat energy and radiation are released. A source of fission or fusion fuel, trigger device, and a fuse to make an atomic bomb.
A fusion bomb, which is called a "Little Boy", was used in Hiroshima, Japan in 1945, and an implosion-triggered fission bomb, which is called a "Fat Man", was used in Nagasaki, Japan also in 1945.
Fission bombs need uranium (235) to make a nuclear explosion because ranium has an extra property. Uranium (235) is one of the few materials that can undergo induced fission. When a free neutron crosses uranium (235), its nucleus takes the neutron and becomes unstable and splits.
The fuel of a fission bomb must be kept in a separate mass called sub critical. The minimum mass of a fissionable material required to sustain a nuclear fission reaction is critical mass. A tamper is the dense material that confines with the fission reaction.
Another way is a Gun-triggered fission bomb, which is the simplest. This bomb has a gun that fires one mass into the other. Then the fission reaction begins, and of course the bomb explodes. The Little Boy is a gun-triggered bomb, which has about 14.5 kiloton of explosives.
I think that an atomic bomb is a powerful bomb, because it can destroy anything in the blast radius. I wonder where they discovered uranium and plutonium. I never knew that they could split atoms and use it for explosives. The smallest things can be used for big experiments and even blow up a big city.