According to the Encyclopedia on yahoo.com, the atom bomb, or A-bomb, is a weapon that gets its explosive force from the release of atomic energy through the fission of heavy nuclei, resulting in nuclear energy. The first atomic bomb was made in a lab in Los Alamos, New Mexico, and was first tested successfully on July 16, 1945. This was the goal of the Manhattan project led by Dr. Robert Oppenheimer. It began two years after 1940 when two German scientists, Otto Hahn and Fritz Strassman,  discovered nuclear fission. An A-bomb was first dropped in Aug 6, 1945, on Hiroshima. The explosive force was estimated at about 12,500 tons of TNT, and it was followed three days later in Nagasaki by a more powerful bomb. Both bombs caused much death and destruction, and it is still debated to this day about whether or not it was neccassary to use these weapons of mass destruction.

The "little boy" bomb that was dropped on Hiroshima on Aug 6,1945, was a Gun-type fission bomb. After being dropped from a B-29 bomber it was detonated at 1,850 feet from the ground. The non-nuclear explosive was set of by a radar echo device. The explosion wedged a piece of uranium, U-235, into a larger piece of the same uranium, thus settin off the blast.

The "fat man" bomb dropped on Nagasaki on Aug 9, 1945, was an Implosion-type fission bomb. The explosion was also set of by a radar echo device. This explosion crushed a sphere of plutonium into a core made up of the chemical elements beryllium and polonium. The core released neutrons, triggering a fission chain reaction in the plutonium.