The main elements used in the atomic bombs dropped on Hiroshima and Nagasaki were uranium-235 and plutonium-239. Uranium-235 is a naturally occurring isotope of uranium, while plutonium-239 is a synthetic element that is produced by irradiating uranium-238 with neutrons. In the atomic bombs, these elements were used to create a chain reaction, in which the nuclei of the atoms were split, releasing a tremendous amount of energy.