In this nuclear reaction, a neutron is used to catalyze the decay of U-235. Four products were produced: Ba-139, two neutrons, and an unknown element X.
To calculate this, you must know that what you start with should be what you end with. The reaction occurred with 236 atomic mass units (U-235 and one neutron), so the final products should add up to 236.
Taking away Barium and the two neutrons: 236-141 = 95. Now, we know that the element will have an overall mass of 95.
The number of protons should have stayed the same through the reaction as well, since the element didn’t undergo Beta or Positron decay. Starting with 92 protons (92 U), and subtracting the 56 contained in the Barium atom, the final number is 36 protons - the atomic number for Krypton.
