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1 CHAPTER 23 NUCLEAR CHEMISTRY 2 THE NATURE OF RADIOACTIVITY 3 Relative Penetration abilities 4 NUCLEAR REACTIONS • Nuclear reactions have a charge and mass balance, and produce one or more new elements by a nuclear change. • The charge balance is performed using the atomic numbers, Z, (subscripts) on the symbol of the element or nuclear species. • The mass balance is performed using the mass numbers, A, (superscripts) on the symbol of the element or nuclear species. 210 Po -----> 84 NUCLEAR REACTIONS x 206 X+ Pb Find the x's. x 82 Reactions Involving a and b Particles a decay, occurs when the nucleus is too massive. 239 Np -----> 256 Lr -----> 5 NUCLEAR REACTIONS 6 • b, emission, ( nuclear electron) occurs when the n/p ratio is too high; converts a neutron into a proton plus an electron. 1 0 218 84 n Po ---> ---> 1 1 p 218 85 0 + -1 At e + Try 214 83 Bi ---> 0 e -1 7 Uranium-238 Decay NUCLEAR REACTIONS Positron Emission and Electron Capture (K Capture) • These processes reduce the atomic number by one, by converting a proton into a neutron. • These processes occur when the p/n ratio is too large. • The particle produced is called a positron, and has the same mass as an electron, but has a positive charge. 8 NUCLEAR REACTIONS • Positron and electrons are anti matter and matter. When they meet, they are annihilated and a photon of light energy is emitted. 1 p ---> 1 1 n 0 0 + e +1 • In the electron capture process, a K (1s) electron is captured by a nuclear proton. 1 1 p + 0 -1 e -----> 1 0 n 9 NUCLEAR REACTIONS Try the following both ways: 38 19 39 20 K ---> Ca ---> 10 11 Beta emission 38 19 K b + 38 18 e 38 18 0 1 Ar Electron capture 38 19 K + 0 1 Ar 12 Beta emission 38 20 Ca 0 1 b + 38 19 K electron capture 38 20 Ca + e 0 1 38 19 K 13 STABILITY OF ATOMIC NUCLEI • Region of stability and modes of decay. • Notice that no elements above bismuth have stable isotopes (too massive). • Lower atomic numbers have equal numbers of protons and neutrons. • As the atomic number increases, so does the n/p ratio for stable isotopes. • Even numbers of protons and neutrons are more stable than odd numbers. 14 1 1 H 2 1 H 3 1 H hydrogen deutrium tritium STABILITY OF ATOMIC NUCLEI 15 Binding Energy • When protons and neutrons come together to form a nucleus, the mass decreases. • This mass decrease is changed into energy to hold the nucleus together. DE = (Dm)c2 • The binding energy per mole of nucleons • The fusion vs fission split occurs at Fe-56, the most stable nucleus. • Calculations of energy in J/mole require the mass in kilograms. 16 17 RATES OF DISINTEGRATION REACTIONS • The time required for one-half of a pure radioactive sample to decay is called the halflife, t1/2. • A short half-life means that the isotope decays quickly. Half-Life 18 HALF-LIFE is the time it takes for 1/2 a sample is disappear. For 1st order reactions, the concept of HALF-LIFE is especially useful. 19 Half-Life • Reaction is 1st order decomposition of H2O2. 20 Half-Life • Reaction after 654 min, 1 halflife. • 1/2 of the reactant remains. 21 Half-Life • Reaction after 3 half-lives, or 1962 min. • 1/8 of the reactant remains. 22 Half-Lives of Radioactive Elements Rate of decay of radioactive isotopes given in terms of 1/2-life. 238U --> 234Th + He 4.5 x 109 y 14C --> 14N + beta 5730 y 131I --> 131Xe + beta 8.05 d Element 106 - seaborgium 263Sg 0.9 s RATES OF DISINTEGRATION REACTIONS • As before in the kinetics chapter, t1/2 = (ln 2) / k. • Use -dN/dt = A = kN to find the rate at one point in time. • Use ln A/Ao = -kt, or ln N/No = -kt when the problem involves two times. 23 Half-Life Radioactive decay is a first order process. Tritium ---> electron + helium 3H 0 e 3He -1 If you have 1.50 mg of tritium, how much is left after 49.2 years? t1/2 = 12.3 years 24 Half-Life Start with 1.50 mg of tritium, how much is left after 49.2 years? t1/2 = 12.3 years Solution ln [A] / [A]0 = -kt [A] = ? [A]0 = 1.50 mg Need k, so we calc k from: t = 49.2 years k = 0.693 / t1/2 Obtain k = 0.0564 y-1, (from: t1/2 = (ln 2) / k) Now ln [A] / [A]0 = -kt = - (0.0564 y -1) • (49.2 y) ln [A] / [A]0 = - 2.77 Take antilog: [A] / [A]0 = e-2.77 = 0.0627 0.0627 is the fraction remaining ! 25 Half-Life Start with 1.50 mg of tritium, how much is left after 49.2 years? t1/2 = 12.3 years Solution [A] / [A]0 = 0.0627 0.0627 is the fraction remaining ! Because [A]0 = 1.50 mg, [A] = 0.094 mg But notice that 49.2 y = 4.00 half-lives 1.50 mg ---> 0.750 mg after 1 ---> 0.375 mg after 2 ---> 0.188 mg after 3 ---> 0.094 mg after 4 26 RATES OF DISINTEGRATION REACTIONS 27 Rate of Radioactive Decay • The rate of decay or activity, A, is directly proportional to the number of atoms present: -dN/dt = A = kN, where k is the rate constant and N is the number of atoms. • From this equation, we can see that the rate law is first order. Therefore, ln [A/Ao] = -kt, or ln [N/No] = -kt. RATES OF DISINTEGRATION REACTIONS Radiochemical Dating • C-14 dating is used to determine the carbon date of substances that were once living. • It is based on the assumption that the rate of C-14 in the atmosphere is and has been constant based on the conversion of N-14 to C-14 by cosmic neutron bombardment. • In the equation ln A/Ao = -kt, Ao = 14 d/min.g, the baseline specific activity. • Other methods involve Pb - U and K - Ar. 28 29 Carbon-14 changes 30 ARTIFICIAL TRANSMUTATIONS • The transuranium elements are made by bombarding target atoms of uranium with nuclei of other elements. • The process is also used to produce other isotopes of elements that do not naturally occur, O-17, I-123, etc. 31 NUCLEAR FISSION • Fission is the process of splitting heavy nuclei to produce lighter nuclei and energy. • This is the process used in nuclear reactors. • The most common element fissioned is U-235. • This isotope must first be converted to U-236 by a slow moving neutron 32 NUCLEAR FISSION Radioactive waste products and run-away reactions are a concern with this type of power production. 33 This figure demonstrates the chain reaction phenomenon of fission 34 Fission reactor NUCLEAR FUSION • Nuclear fusion is the joining together of two light nuclei to produce a heaver nucleus and energy. • The process occurs on the sun and in Hydrogen Bombs. • Attempts to use this process to produce electrical energy has been unsuccessful to date, but progress has been made. 35 36 RADIATION EFFECTS AND UNITS OF RADIATION • Rontgen, rad, rem, and curie are common units used. • Exposure comes from natural sources, about 65%; medical sources, about 32%; and artificial sources, about 3%. 37 APPLICATIONS OF RADIOACTIVITY • Food irradiation to reduce spoilage and kill bacteria, mold, and yeasts. • Radioactive tracers for following molecules. • Radioactive isotopes for biomedical purposed including cancer and medical imaging. 38 39 40