The atomic bomb dropped on Nagasaki before Japan surrendered was code-named Fat Man.
Fat Man was an implosion-type plutonium nuclear weapon developed by the Manhattan Project. The United States dropped it on Nagasaki on 9 August 1945, three days after Little Boy was used against Hiroshima. The B-29 bomber Bockscar carried the weapon, and the attack intensified the Japanese leadership’s debate over ending the war.
Fat Man was not the same design as Little Boy. Little Boy used a gun assembly and uranium-235, while Fat Man used conventional explosives arranged to compress a plutonium core. The first device of this general implosion design, called the Gadget, had been tested at Trinity in New Mexico in July 1945.
Japan’s surrender followed several simultaneous developments: the two atomic bombings, the Soviet Union’s entry into the war against Japan, and the Emperor’s intervention in the surrender decision. The formal surrender ceremony came later, on 2 September 1945.