The immense destructive power of atomic weapons derives from a sudden release of energy produced by splitting the nuclei of the fissile elements making up the bombs' core. The U.S. developed two types of atomic bombs during the Second World War. The first, Little Boy, was a gun-type weapon with a uranium core. Little Boy was dropped on Hiroshima. The second weapon, dropped on Nagasaki, was called Fat Man and was an implosion-type device with a plutonium core.
The isotopes uranium-235 and plutonium-239 were selected by the atomic scientists because they readily undergo fission. Fission occurs when a neutron strikes the nucleus of either isotope, splitting the nucleus into fragments and releasing a tremendous amount of energy. The fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. This is known as a chain reaction and is what causes an atomic explosion.
When a uranium-235 atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium-238 atom. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, and so on. This causes a nuclear chain reaction. For more on this topic, see Nuclear Fission.
In order to detonate an atomic weapon, you need a critical mass of fissionable material. This means you need enough U-235 or Pu-239 to ensure that neutrons released by fission will strike another nucleus, thus producing a chain reaction. The more fissionable material you have, the greater the odds that such an event will occur. Critical mass is defined as the amount of material at which a neutron produced by a fission process will, on average, create another fission event.
The Difference Between the Bombs
Little Boy and Fat Man utilized different elements and completely separate methods of construction in order to function as nuclear weapons. Little Boy detonated due to a fission chain reaction involving the isotope U-235 of uranium, while Fat Man used plutonium’s Pu-239 form.