Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei. There are two basic ways that nuclear energy can be released from an atom. In nuclear fission (pictured), scientists split the nucleus of an atom into two smaller fragments with a neutron. Nuclear fusion -- the process by which the sun produces energy -- involves bringing together two smaller atoms to form a larger one. In either process, fission or fusion, large amounts of heat energy and radiation are given off.
We can attribute the discovery of nuclear fission to the work of Italian physicist Enrico Fermi. In the 1930s, Fermi demonstrated that elements subjected to neutron bombardment could be transformed into new elements. This work resulted in the discovery of slow neutrons, as well as new elements not represented on the periodic table. Soon after Fermi's discovery, German scientists Otto Hahn and Fritz Strassman bombarded uranium with neutrons, which produced a radioactive barium isotope. They concluded that the low-speed neutrons caused the uranium nucleus to fission, or break apart, into two smaller pieces.
Their work sparked intense activity in research labs all over the world. At Princeton University, Niels Bohr worked with John Wheeler to develop a hypothetical model of the fission process. They speculated that it was the uranium isotope uranium-235, not uranium-238, undergoing fission. At about the same time, other scientists discovered that the fission process resulted in even more neutrons being produced. This led Bohr and Wheeler to ask a momentous question: Could the free neutrons created in fission start a chain reaction that would release an enormous amount of energy? If so, it might be possible to build a weapon of unimagined power.
And it was.