Thorium-231 has 90 electrons and protons and 141 neutrons. It is the decay particle of Uranium-235. It is found in very small amounts on the earth and has a half life of 25.5 Hours.when It decays, emits an beta ray and forms Protactinium-231. It has a decay energy of 0.39 MeV. It has a mass of 231.0363043 grams/mole. Uranium-235 decays into this element.