(3) Lead Explanation: Natural uranium consists of three major isotopes: U238 (99.28% natural abundance), U235 (0.71%), and U234 (0.0054%). All three are radioactive, emitting alpha particles, with the exception that all three of these isotopes have small probabilities of undergoing spontaneous fission, rather than alpha emission. U23° is usually an alpha emitter (occasionally, it undergoes spontaneous fission), decaying through the "Uranium Series" of nuclear decay, which has 18 members, all of which eventually decay into Pb206, by a variety of different decay paths.