Some nuclei of the nuclide 256Fm undergo spontaneous fission.
Typical and well-known neutron-induced fission reactions are:
Since the mass excess of 235 or 238U is higher than nuclides with mass in the range of 80-160, fission reactions release a lot of energy. The annimation of neutron-induced fission is shown here:
This annimation is provided by Energy Matters - Fission theory.
All groups thought the reaction to be:
The Berlin group co-precipitated radioactive nuclides with barium. They have also precipitated radioactive materials using hydrogen sulfide. Half-life measurements indicated to them that not one but many elements were produced. They interpreted their results based on the fission reaction. The discovery was known to Frisch and Bohr, and eventually Fermi's group learned of the fission.
Further study revealed that only 235U undergoes fission when irradiated by slow neutrons.
Hungarian refugee scientists Leo Szilard, Eugene Wigner, and Edward Teller, who thought the time has come to unite political force with nuclear power.
composed a letter for Einstein, and Einstein signed the letter* as
the sender. They took the matter so serious that they convinced the
economist Alexander Sachs to personally deliver the letter to the
Einstein's letters to Roosevelt contains this letter plus others.