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Uranium-235 is a radioactive isotope of uranium. Uranium-235 has 143 neutrons, 92 protons, and an isotope mass of 235.0439299. Uranium-235 has 235Pa, 235Np, and 239Pu as its parent isotopes. As it decays, it produces 231Th. Uranium-235 has a half-life of 703.8 million years. Uranium-235 makes up about 0.72% of natural uranium. When it is separated from the other uranium isotopes, it is called enriched uranium. The main use for enriched uranium is in Nuclear reactors.

Enriched uranium is best known for its use in the bombing of Hiroshima on August 6, 1945. The bomb called Little Boy used Uranium-235. Uranium-235 can be used for nuclear fission. This fission is about 504.81 barns for slow thermal neutrons and 1 barn for fast thermal neutrons. Uranium-235 was discovered in 1935 by Arthur Jeffrey Dempster.