also called ATOM BOMB, weapon with great explosive power that results from the sudden release of energy upon the splitting, or fission, of the nuclei of such heavy elements as plutonium or uranium. When a neutron strikes the nucleus of an atom of the isotopes uranium 235 or plutonium-239, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In …showed first 75 words of 1041 total…

…showed last 75 words of 1041 total…initiated surrender negotiations the next day. After the war, the United States conducted dozens of test explosions of atomic bombs in the Pacific at Enewetak (Eniwetok) atoll and in Nevada. In subsequent years, the Soviet Union (1949), Great Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998) tested fission weapons of their own. The great temperatures and pressures created by a fission explosion are also used to initiate fusion and thus detonate a thermonuclear bomb. See also nuclear weapon.