An atomic bomb uses a fissile material, which is a material that can undergo nuclear fission, a process in which the nucleus of an atom splits into two or more smaller nuclei, releasing a great amount of energy. The most common fissile materials used in atomic bombs are uranium-235 and plutonium-239. These materials are not found in nature in large enough quantities to be used in a bomb, so they must be enriched or produced through nuclear reactions.

In an atomic bomb, a small amount of fissile material is brought together quickly, creating a critical mass. This causes a chain reaction of nuclear fission to occur, releasing a large amount of energy in the form of heat, light, and radiation. The energy released by an atomic bomb is millions of times greater than that of conventional explosives.

Atomic bombs have been used twice in warfare, by the United States against the Japanese cities of Hiroshima and Nagasaki in August 1945. The bombs caused widespread death and destruction, leading to the end of World War II. Since then, atomic bombs have been developed and stockpiled by a number of countries as a deterrent against nuclear attack.