Uranium is a metallic element that is used in nuclear reactors and nuclear weapons. the vast majority of uranium found on earth is in the form of uranium-238, an isotope with a mass number of 238, while the uranium that is used for nuclear reactors and weapons is uranium-235, an isotope with a mass number of 235. if the atomic number of uranium is 92, how many neutrons does an atom of uranium-235 have in its nucleus?