Chapter 20 Nuclear Chemistry - UMass D ?· Chapter 20 Nuclear Chemistry In chemical reaction, ... The…

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<ul><li><p>Chapter 20 </p><p>Nuclear Chemistry </p><p>In chemical reaction, only the outer electrons of the atoms are disturbed and the </p><p>nuclei of the atoms are not affected. </p><p>Two types of nuclear reactions: </p><p>1. Radioactive decay - the process in which a nucleus spontaneously disintegrates, </p><p>giving off radiation. </p><p>2. Nuclear Bombardment reaction - a nuclear reaction in which a nucleus is </p><p>bombarded, or struck, by another nucleus or by a nuclear particle. </p><p>Radioactivity </p><p>Writing an equation for the nuclear reaction corresponding to the decay of </p><p>uranium-238 just as you would an equation for a chemical reaction. </p></li><li><p>Alpha rays consist of a helium -4 nuclei as seen in the uranium example. </p><p>Beta rays consist of high speed electrons. </p><p>Gamma rays are similar to x rays except they are higher in energy. </p><p>Nucleon is a collective name for two particles: a proton and a neutron </p><p>The nuclear force is a strong force of attraction between nucleons that act only at a </p><p>very short distances ( about 10-15</p><p> m). </p><p>The shell model of the nucleus is a model in which protons and neutrons exist in </p><p>levels, or shells, analogous to the shell structure that exists for electrons in an atom. </p><p>Certain numbers of protons or neutrons appear to be more stable than others, the </p><p>numbers of interest are called magic numbers - the number of nuclear particles in </p><p>a completed shell of protons and neutrons </p><p>For protons the magic numbers are: 2, 8, 20, 28, 50 and 82. </p><p>For neutrons they are: 2, 8, 20, 28, 50, 82 and 126. </p><p>Types of Radioactive Emission </p><p>1. Alpha emission </p><p>photon Gamma</p><p>or ePositron</p><p>or eElectron</p><p> nNeutron</p><p> Por HProton</p><p>0</p><p>0</p><p>0</p><p>1</p><p>0</p><p>1</p><p>0</p><p>1-</p><p>0</p><p>1-</p><p>1</p><p>0</p><p>11</p><p>11</p></li><li><p>2. Beta emission </p><p>3. Positron emission </p><p>4. Electron capture </p><p>5. Gamma emission </p></li><li><p>6. Spontaneous Fission </p><p>All nuclei with an atomic number greater than Z= 83 are radioactive. Many decay </p><p>by alpha emission. Alpha particles are, or </p><p> He nuclei are especially stable. </p><p>Naturally radioactive elements, such as uranium-238, give a radioactive decay </p><p>series until it reaches a stable nucleus, which is an isotope of lead. </p><p>Transmutation is the change of one element to another by bombarding the </p><p>nucleus of the element with nuclear particles or nuclei. </p><p>Transuranium elements are elements with atomic numbers greater than that of </p><p>uranium. (Z = 92), the naturally occurring element of greatest Z. </p><p>Transuranium elements have a number of commercial uses. </p><p>Example: Plutonium-238 and Americium-241 </p></li><li><p>Biological Effects and Radiation Dosage </p><p>To monitor the effect of nuclear radiations on biological tissue, it is necessary to </p><p>have a measure of radiation dosage. </p><p>The rad - radiation absorbed dose, is the dosage of radiation that deposits 1 10-2 </p><p>J </p><p>of energy per kilogram of tissue. </p><p>The biological effect of radiation depends on not only the energy deposited, but the </p><p>type of radiation. </p><p>Neutrons are more destructive than gamma rays of the same dosage measured in </p><p>rads. </p><p>Rem - unit of radiation dosage used to regulate various kinds of radiation in terms </p><p>of biological destruction. </p><p>Rem = rad RBE ( relative biological effectiveness) </p><p>Rate of radioactive decay is the number of nuclei disintegrating per unit time </p><p> Rate = k Nt </p><p>Where Nt = number of radioactive nuclei at time t, and k = the radioactive decay </p><p>constant. </p><p>Example: A 1.0 mg sample of technetium-99 has an activity of 1.7 10-5</p><p> Ci, </p><p>decaying by beta emission. What is the decay constant for </p><p> Tc? </p></li><li><p>The half life of a radioactive nucleus is the time it takes for one half of the nuclei </p><p>in a sample to decay. The half life is independent of the amount of the sample. </p><p> = .</p><p>Example: The decay constant, k, for the beta decay of </p><p>Tc was found to be 1.0 </p><p> 10-13</p><p> /s. What is the half life of this isotope in years? </p><p>Radioactive Dating - because the rate of radioactive decay of a nuclide is constant, </p><p>this can serve as a clock for dating very old rocks and human implements. Once </p><p>you know the decay constant for a radioactive isotope, you can calculate the </p><p>fraction of the radioactive nuclei that remains after a given period of time by the </p><p>following equation </p><p> ln </p><p> = -kt </p><p>Carbon -14 Dating Method Example </p><p> A piece of charcoal from a tree killed by the volcanic eruption that formed the crater in Crater </p><p>Lake, Oregon, gave 7.0 disintegrations of carbon-14 nuclei per minute per gram of total carbon. </p><p>Present day carbon gives 15.3 disintegrations per minute per gram carbon. Determine the date of </p><p>the volcanic eruption. Recall that the half life of carbon-14 is 5730 y. </p></li><li><p>Nuclear Power Plants and Nuclear Fission and Fusion </p><p>Nuclear reactions, like chemical reactions, involve changes in energy. The changes </p><p>in nuclear reactions are enormous by comparison with those in chemical reactions. </p><p>Nuclear fission and fusion are reactions in which nuclei attain sizes closer to an </p><p>intermediate range (approx mass number s around 50). Nuclear fission of uranium-</p><p>235 is employed in nuclear power plants to generate electricity. </p><p>Critical mass is the smallest mass of fissionable material in which a chain reaction </p><p>can be sustained. </p><p>A Nuclear fission reactor is a device that permits a controlled chain reaction of </p><p>nuclear fissions. A power plant with a nuclear reactor is used to produce heat, </p><p>which is then used to produce steam to drive an electric generator. </p><p>Fuel rods are the cylinders that contain fissionable material. </p><p>Control rods are cylinders composed of substances that absorb neutrons, such as </p><p>boron and cadmium, and can therefore slow the chain reaction. </p><p>Fission of uranium nuclei produce </p><p>approximately 30 different elements </p><p>of intermediate mass. </p><p>When the uranium-235 nuclei splits, </p><p>approx 2 or 3 neutrons are released. </p><p>If the neutrons from each nuclear </p><p>fission are absorbed by other </p><p>uranium-235 nuclei, these nuclei split </p><p>and release even more neutrons. In </p><p>this way, a chain reaction can occur. </p></li><li><p>Light water nuclear reactor consists of fuel rods with interspersed control rods. </p></li></ul>