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 Pa, Np, and Pu as its parent isotopes. As it decays, it produces Th. 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.