Chemistry Lecture No.4

Chemistry Lecture No.4______By : Asst. Lect. Tariq-H-Al-Mgheer
few isotopes are known to undergo fission reactions when bombarded by neutrons. Uranium-235 (U-235) is the only naturally occurring isotope that undergoes this reaction. Two other isotopes formed by nuclear bombardment, plutonium-239 and uranium-233, also undergo fission reactions. Second, a large amount of energy is released when a fission reaction occurs. Finally, fission reactions form neutrons and gamma radiation as well as other elements.
This last fact is particularly important because it allows a chain reaction to occur whenever enough uranium-235 atoms are present. A chain reaction is a series of repeated reactions in which one reaction produces a product that allows the series of reactions to continue. A chain reaction is shown in Figure 2. A slow neutron strikes an atom of uranium-235 and causes it to split into two nuclei of lighter mass with the emission of gamma radiation and two, three, or four neutrons. These neutrons can strike other nearby uranium-235 atoms and cause them to undergo fission reactions. Each fission reaction produces more neutrons, which cause more fission reactions, which produce more neutrons, and so forth. For a chain reaction to occur, enough fissionable nuclei must be present so that most of the neutrons will hit other fissionable nuclei. It is the emission of neutrons by a fission reaction that keeps the chain reaction going. The amount of material needed for a chain reaction to continue (to be self-sustaining) is called the critical mass.
The fission of uranium-235 can occur in more than 30 ways to produce about 200 different radioactive isotopes. These isotopes usually have atomic numbers between 30 and 64 and masses between 72 and 161. The following three equations are representative of the fission reactions of uranium-235:


Notice that more neutrons are formed than are consumed in each of these fission reactions.