By Tim, John, Shane, Owen

By Tim, John, Shane, Owen

A few refreshers Atoms of the same element, which always have an identical number of protons, that have different numbers of neutrons, is an isotope. Protons and neutrons are nucleons. A ZX is an isotope, where A = mass number (protons + neutrons), Z = atomic number (# of protons), X = elemental symbol In a nuclear equation, the mass numbers of the reactants equal the mass numbers of the products, and the atomic numbers of the reactants equal the atomic number of the products. But the total mass for both sides will be different, due to the law of conservation of mass-energy. The mass of a neutron = 1.008665u The mass of a proton = 1.007276u

What is radioactivity? Radioactivity is a naturally occurring process in which atomic nuclei of heavy elements are spontaneously disintegrated, and emit radioactive particles. Artificial radioactivity is a man-made process in which even light elements can emit ionizing radioactivity or particles. Also called induced radioactivity, this can occur when an element is bombarded by an α-particle, x-rays, gamma rays, or other naturally occurring isotopes.

More about artificial radioactivity The phenomenon of artificial radioactivity was discovered by Irene Joliet-Curie and Frederic Joliot in an experiment that awarded them the Nobel Prize in 1935. The French couple conducted an experiment that saw them bombard aluminium with α-particles to make a radioactive isotope of phosphorus. The phosphorus isotope was the first to be synthesized artificially. 27 13Al + 4 2He 30 15P + 1 0n https://www.youtube.com/watch?v=3rn339v_q-w

Nuclear Fission Enrico Fermi discovered in the 1930 s that neutrons were the most effective at causing nuclear reactions because they do not repel positively charged protons. We call this nuclear fission because it resembles cell division. Here is a typical fission reaction. 1 0n + 235 92U 141 56Ba + 92 36Kr + 3 1 0n Note: ( 1 0n) represents one neutron being released. When there is a coefficient, that is how many neutrons are being released.

Nuclear Fission Note this equation again: 1 0 n + 235 92U 141 56Ba + 92 36Kr + 3 1 0n Uranium is a commonly used heavy element in nuclear fission reactions. Uranium has a much greater mass than the fission fragments (the elemental products), therefore a great amount of energy is released in the reaction. The products are bound tighter than the uranium nucleus, which means they re harder to break apart, and more stable. This occurs because their nuclei have less shielding of electrons.

Nuclear Fission So, how do we figure out how much energy was released, in MeV? 1 0 n + 235 92U 141 56Ba + 92 36Kr + 3 1 0n 1) Calculate the masses for both sides 2) Calculate the mass defect (m f m i )

Nuclear Fission 3) Find the energy released using E=mc 2

Nuclear Fission Example 2: How many atoms are released in 1.00kg of uranium-235? 1. Find the number of moles. 2. Find the number of particles from the number of moles.

Nuclear Fission In nuclear fission reactions, extra neutrons are produced, and one neutron is needed to start the reaction (note the 1 0 n & 3 1 0n in the reaction). The excess neutrons produced can go on to start other reactions, in a chain reaction process. FUN FACT: The first time nuclear fission was used was in the Hiroshima bombings on August 6, 1945. When a fission bomb explodes, radioactive fission fragments are released into the atmosphere; this is called radioactive fallout.

Nuclear Fission Radioactive fallout can be harmful to the environment and to organisms directly. The fission fragments can enter our food chain, and the results can be disastrous. α and β particles are too weak to affect us, but the stronger fission fragments can enter body through food, and affect our cells. Man-made fission reactions can be created and sustained by nuclear reactors.

Nuclear Reactors Nuclear reactors sound convenient, but there are problems with it. Neutrons emitted travel too fast for the reactor and they need to be slowed down to be absorbed by the uranium-235. A moderator does this usually deuterium ( 2 1H). Uranium-235 is hard to come by, and without enough of it, the neutrons will not be absorbed. Neutrons can escape the reaction before they go to start a new fission reaction (a minimum critical mass is needed).

Nuclear Fusion Nuclear fusion occurs when nuclei with smaller masses combine to make a nucleus with a larger mass. If the resultant mass is more tightly bound than the smaller masses, energy will be released. An example of a nuclear fusion reaction is: 1 1 H + 1 1H 2 1H + 0 1e + 0 0v 1 1 H + 2 1H 3 2He 3 2 He + 3 2He 4 2He + 2 1 1H This is the reaction that occurs in the sun

Nuclear Fusion Nuclear fusion has its advantages over nuclear fission, because: The total energy released is greater There is less radioactive waste to worry about The fuel is plentiful (ex: deuterium, can be found in oceans) Contrary to the benefits, nuclear fusion has its problems. Fusion reactions need very high temperatures to occur in (10 8 K). Fusion reactions are common in stars. This is why fusion reactions can also be called thermonuclear reactions. Fusion reactions are hard to control, and contain, after that high temperature is reached. This is why we do not yet have a practical nuclear fusion reactor.

Fission, Fusion, and Energy This chart demonstrates the relationship between the number of nucleons in the nucleus and the binding energy per nucleon. 1 H and 2 H have low binding energies for fusion, and 236 U and 238 U have low binding energies for fission. This is why they are the most commonly used elements for their respective reactions. 56 Fe is the most stable nucleus. It is the most tightly bound. This is why it has the highest binding energy out of all the elements.