A weapon developed by the Americans during WWII and under research by other countries including Germany. The weapon is classified as either a Gun type or implosion type.
Gun type weapons are unreliable and do not provide a large blast, therefor rendering it obsolete. Though it is easier to make. The process includes accelerating a neutron(s) into a ball of Fissionable material and inducing a nuclear
Fission reaction.
Implosion type weapons are in current use, though, not much, they detonate explosives around a ball of Fissionable material and create immense pressure which induces nuclear Fission. They generally have a higher blast effect.
The Atom bomb was deployed twice at Nagasaki and Hiroshima to end WWII. Though, their use is still hotly debated today. Atom bombs have become very obsolete since the development of Hydogen bombs, Cobalt bombs, and then
Thermonuclear warheads.