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.