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.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 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.The mass balance is performed using the mass numbers, A, (superscripts) on the symbol of the element or nuclear species.

5 NUCLEAR REACTIONS 210 Po -----> x X Pb Find the x's. 210 Po -----> x X Pb Find the x's. 84 x x 82 Reactions Involving  and  Particles  decay, occurs when the nucleus is too massive. 239 Np -----> 239 Np -----> 256 Lr -----> 256 Lr ----->

6 NUCLEAR REACTIONS , emission, ( nuclear electron) occurs when the n/p ratio is too high; converts a neutron into a proton plus an electron. , emission, ( nuclear electron) occurs when the n/p ratio is too high; converts a neutron into a proton plus an electron. 1 n ---> 1 p + 0 e n ---> 1 p + 0 e Po ---> 218 At + 0 e Try 214 Bi ---> 83

7 Uranium-238 Decay

8 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 reduce the atomic number by one, by converting a proton into a neutron. These processes occur when the p/n ratio is too large.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.The particle produced is called a positron, and has the same mass as an electron, but has a positive charge.

9 NUCLEAR REACTIONS Positron and electrons are anti matter and matter. When they meet, they are annihilated and a photon of light energy is emitted.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 n + 0 e In the electron capture process, a K (1s) electron is captured by a nuclear proton.In the electron capture process, a K (1s) electron is captured by a nuclear proton. 1 p + 0 e -----> 1 n

10 NUCLEAR REACTIONS Try the following both ways: 38 K ---> Ca ---> 20

11 Beta emission Electron capture

12 Beta emission electron capture

13 STABILITY OF ATOMIC NUCLEI Region of stability and modes of decay.Region of stability and modes of decay. Notice that no elements above bismuth have stable isotopes (too massive).Notice that no elements above bismuth have stable isotopes (too massive). Lower atomic numbers have equal numbers of protons and neutrons.Lower atomic numbers have equal numbers of protons and neutrons. As the atomic number increases, so does the n/p ratio for stable isotopes.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.Even numbers of protons and neutrons are more stable than odd numbers.

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15 STABILITY OF ATOMIC NUCLEI Binding Energy When protons and neutrons come together to form a nucleus, the mass decreases.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.This mass decrease is changed into energy to hold the nucleus together.  E = (  m)c 2 The binding energy per mole of nucleonsThe binding energy per mole of nucleons The fusion vs fission split occurs at Fe-56, the most stable nucleus.The fusion vs fission split occurs at Fe-56, the most stable nucleus. Calculations of energy in J/mole require the mass in kilograms.Calculations of energy in J/mole require the mass in kilograms.

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17 RATES OF DISINTEGRATION REACTIONS The time required for one-half of a pure radioactive sample to decay is called the half- life, t 1/2.The time required for one-half of a pure radioactive sample to decay is called the half- life, t 1/2. A short half-life means that the isotope decays quickly.A short half-life means that the isotope decays quickly.

18 Half-LifeHalf-Life 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 Reaction is 1st order decomposition of H 2 O 2.Reaction is 1st order decomposition of H 2 O 2. Half-LifeHalf-Life

20 Half-LifeHalf-Life Reaction after 654 min, 1 half- life.Reaction after 654 min, 1 half- life. 1/2 of the reactant remains.1/2 of the reactant remains.

21 Half-LifeHalf-Life Reaction after 3 half-lives, or 1962 min.Reaction after 3 half-lives, or 1962 min. 1/8 of the reactant remains.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. Rate of decay of radioactive isotopes given in terms of 1/2-life. 238 U --> 234 Th + He4.5 x 10 9 y 14 C --> 14 N + beta5730 y 131 I --> 131 Xe + beta8.05 d Element seaborgium 263 Sg 0.9 s

23 As before in the kinetics chapter,As before in the kinetics chapter, t 1/2 = (ln 2) / k. Use -dN/dt = A = kN to find the rate at one point in time.Use -dN/dt = A = kN to find the rate at one point in time. Use ln A/A o = -kt, or ln N/N o = -kt when the problem involves two times.Use ln A/A o = -kt, or ln N/N o = -kt when the problem involves two times. RATES OF DISINTEGRATION REACTIONS

24 Half-LifeHalf-Life Radioactive decay is a first order process. Tritium ---> electron + helium Tritium ---> electron + helium 3 H 0 -1 e 3 He 3 H 0 -1 e 3 He If you have 1.50 mg of tritium, how much is left after 49.2 years? t 1/2 = 12.3 years

25 Solution ln [A] / [A] 0 = -kt [A] = ?[A] 0 = 1.50 mgt = 49.2 years Need k, so we calc k from: k = / t 1/2 Obtain k = y -1, (from: t 1/2 = (ln 2) / k) Now ln [A] / [A] 0 = -kt = - ( y -1 ) (49.2 y) ln [A] / [A] 0 = ln [A] / [A] 0 = Take antilog: [A] / [A] 0 = e = is the fraction remaining ! Start with 1.50 mg of tritium, how much is left after 49.2 years? t 1/2 = 12.3 years Half-LifeHalf-Life

26 Solution [A] / [A] 0 = is the fraction remaining ! Because [A] 0 = 1.50 mg, [A] = mg But notice that 49.2 y = 4.00 half-lives 1.50 mg ---> mg after mg ---> mg after 1 ---> mg after 2 ---> mg after 2 ---> mg after 3 ---> mg after 3 ---> mg after 4 ---> mg after 4 Half-LifeHalf-Life Start with 1.50 mg of tritium, how much is left after 49.2 years? t 1/2 = 12.3 years

27 Rate of Radioactive Decay The rate of decay or activity, A, is directly proportional to the number of atoms present: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. 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.From this equation, we can see that the rate law is first order. Therefore, ln [A/A o ] = -kt, or ln [N/N o ] = -kt. RATES OF DISINTEGRATION REACTIONS

28 Radiochemical Dating C-14 dating is used to determine the carbon date of substances that were once living.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.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/A o = -kt, A o = 14 d/min. g, the baseline specific activity.In the equation ln A/A o = -kt, A o = 14 d/min. g, the baseline specific activity. Other methods involve Pb - U and K - Ar.Other methods involve Pb - U and K - Ar. RATES OF DISINTEGRATION REACTIONS

29 Carbon-14 changes

30 ARTIFICIAL TRANSMUTATIONS The transuranium elements are made by bombarding target atoms of uranium with nuclei of other elements.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.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.Fission is the process of splitting heavy nuclei to produce lighter nuclei and energy. This is the process used in nuclear reactors.This is the process used in nuclear reactors. The most common element fissioned is U-235.The most common element fissioned is U-235. This isotope must first be converted to U-236 by a slow moving neutronThis 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

35 NUCLEAR FUSION Nuclear fusion is the joining together of two light nuclei to produce a heaver nucleus and energy.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.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.Attempts to use this process to produce electrical energy has been unsuccessful to date, but progress has been made.

36 RADIATION EFFECTS AND UNITS OF RADIATION Rontgen, rad, rem, and curie are common units used.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%.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.Food irradiation to reduce spoilage and kill bacteria, mold, and yeasts. Radioactive tracers for following molecules.Radioactive tracers for following molecules. Radioactive isotopes for biomedical purposed including cancer and medical imaging.Radioactive isotopes for biomedical purposed including cancer and medical imaging.

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