Otto Hahn and Fritz Strassmann reported surprising results when they exposed uranium (element 92) to neutrons in 1939. They expected that the uranium nuclei would absorb one or more neutrons and undergo beta decay to produce elements with atomic numbers greater than 92. Instead, they found barium (element 56) among the reaction products. Lise Mietner and Otto Frisch explained the observation of Hahn and Strassmann by hypothesizing (correctly) that the uranium nucleus was split into two smaller nuclei. If a neutron splits the uranium nucleus into two other nuclei, one of which is a barium nucleus, the other fragment is a nucleus of what element?
