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. what is the atomic difference between uranium-235 and uranium-238? uranium-235 has three fewer protons than uranium-238. uranium-235 has three fewer neutrons than uranium-238. uranium-235 has three fewer electrons than uranium-238. uranium-235 has three fewer atoms than uranium-238.