1. INTRODUCTORY CHEMISTRY INTRODUCTORY CHEMISTRY Concepts and Critical Thinking Sixth Edition by Charles H. Corwin 1 Chapter 18 © 2011 Pearson Education, Inc. Chapter 18 Nuclear Chemistry by Christopher Hamaker

2. 2 Chapter 18 © 2011 Pearson Education, Inc. Introduction As Earth’s supply of fossil fuels is diminishing, nations are increasingly turning to nuclear power. Currently, approximately 20% of the world’s electricity needs are met using nuclear power. However, disposal of radioactive waste and the threat of accidents are major concerns with the use of nuclear power. In this chapter, we will learn about nuclear chemistry.

3. 3 Chapter 18 © 2011 Pearson Education, Inc. Natural Radioactivity There are three types of radioactivity: 1.Alpha particles (  ) are identical to helium nuclei, containing two protons and two neutrons. 2.Beta particles (  ) are identical to electrons. 3.Gamma rays (  ) are high-energy photons.

4. 4 Chapter 18 © 2011 Pearson Education, Inc. Charges of Radiation Types Alpha particles have a +2 charge, and beta particles have a –1 charge. Both are deflected by an electric field. Gamma rays are electromagnetic radiation and have no charge, so they are not deflected.

5. 5 Chapter 18 © 2011 Pearson Education, Inc. Behavior of Radiation Since alpha particles have the largest mass, they are the slowest moving type of radiation. Gamma rays move at the speed of light since they are electromagnetic radiation.

6. 6 Chapter 18 © 2011 Pearson Education, Inc. Atomic Notation Recall that we learned atomic notation in Chapter 5. A nuclide is the nucleus of a specific atom. The radioactive nuclide strontium-90 has 90 protons and neutrons. The atomic number is 38. Sr 90 38 Sy symbol of the element mass number (p + and n 0 ) atomic number (p + ) A Z

7. 7 Chapter 18 © 2011 Pearson Education, Inc. Nuclear Reactions A nuclear reaction involves a high-energy change in an atomic nucleus. For example, a uranium-238 nucleus changes into a thorium-231 nucleus by releasing a helium-4 particle and a large amount of energy. U 235 92 He 4 2 Th 231 90 + In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products.

8. 8 Chapter 18 © 2011 Pearson Education, Inc. Balancing Nuclear Reactions 1.The total of the atomic numbers (subscripts) on the left side of the equation must equal the sum of the atomic numbers on the right side. 2.The total of the atomic masses (superscripts) on the left side of the equation must equal the sum of the atomic masses on the right side. 3.After completing the equation by writing all the nuclear particles in an atomic notation, a coefficient may be necessary to balance the reaction.

9. 9 Chapter 18 © 2011 Pearson Education, Inc. Common Nuclear Particles Below is a listing of the common nuclear particles used to balance nuclear reactions.

10. 10 Chapter 18 © 2011 Pearson Education, Inc. Alpha (  ) Emission Radioactive nuclides can decay by giving off an alpha particle. Radium-226 decays by alpha emission. Ra 226 88 He 4 2 X A Z + 1. First, balance the number of protons: 88 = Z + 2, so Z = 86 (Rn) 2. Balance the number of protons plus neutrons: 226 = A + 4, so A = 222 Ra 226 88 He 4 2 Rn 222 86 +

11. 11 Chapter 18 © 2011 Pearson Education, Inc. Beta (β) Emission Some radioactive nuclides decay by beta emission. Radium-228 loses a beta particle to yield actinium- 228. Beta decay is essentially the decay of a neutron into a proton and an electron. Ra 228 88 e 0 Ac 228 89 +

12. 12 Chapter 18 © 2011 Pearson Education, Inc. Gamma (γ) Emission Gamma rays often accompany other nuclear decay reactions. For example, uranium-233 decays by releasing both alpha particles and gamma rays. Note that a gamma ray has a mass and a charge of zero, so it has no net effect on the nuclear reaction. U 233 92 He 4 2 Th 229 90 +  0 0 +

13. 13 Chapter 18 © 2011 Pearson Education, Inc. Positron (β + ) Emission A positron (  + ) has the mass of an electron, but a +1 charge. During positron emission, a proton decays into a neutron and a positron. Sodium-22 decays by positron emission to neon- 22. Na 22 11 e 0 +1 + Ne 22 10

14. 14 Chapter 18 © 2011 Pearson Education, Inc. Electron Capture A few large, unstable nuclides decay by electron capture. A heavy, positively charged nucleus attracts an electron. The electron combines with a proton to produce a neutron. Lead-205 decays by electron capture. Pb 205 82 e 0 Tl 205 81 +

15. 15 Chapter 18 © 2011 Pearson Education, Inc. Decay Series Some heavy nuclides must go through a series of decay steps to reach a nuclide that is stable. This stepwise disintegration of a radioactive nuclide until a stable nucleus is reached is called a radioactive decay series. For example, uranium-235 requires 11 decay steps until it reaches the stable nuclide lead-207.

16. 16 Chapter 18 © 2011 Pearson Education, Inc. Uranium-238 Decay Series Uranium-238 undergoes 14 decay steps before it ends as stable lead-206. The decay series for uranium-238 is shown here. The series includes eight alpha decays and six beta decays.

17. 17 Chapter 18 © 2011 Pearson Education, Inc. Parent and Daughter Nuclides The term parent–daughter nuclides describes a parent nuclide decaying into a resulting daughter nucleus. For example, the first step in the decay series for U-238 is: U-238 is the parent nuclide and Th-234 is the daughter nuclide. U 238 92 He 4 2 Th 224 90 +

18. 18 Chapter 18 © 2011 Pearson Education, Inc. Activity The number of nuclei that disintegrate in a given period of time is called the activity of the sample. A Geiger counter is used to count the activity of radioactive samples. Radiation ionizes gas in a tube, which allows electrical conduction. This causes a clicking to be heard and the number of disintegrations to be counted.

19. 19 Chapter 18 © 2011 Pearson Education, Inc. Half-Life Concept The level of radioactivity for all radioactive samples decreases over time. Radioactive decay shows a systematic progression. If we start with a sample that has an activity of 1000 disintegrations per minute (dpm), the level will drop to 500 dpm after a given amount of time. After the same amount of time, the activity will drop to 250 dpm. The amount of time for the activity to decrease by half is called the half-life, t ½.

20. 20 Chapter 18 © 2011 Pearson Education, Inc. Half-Life Concept, Continued After each half-life, the activity of a radioactive sample drops to half its previous level. The decay curve on the right shows the activity of a radioactive sample over time.

21. 21 Chapter 18 © 2011 Pearson Education, Inc. Radioactive Waste A sample of plutonium-239 waste from a nuclear reactor has an activity of 20,000 dpm. How many years will it take for the activity to decrease to 625 dpm? The half-live for Pu-239 is 24,000 years. It takes five half-lives for the activity to drop to 625 dpm. 5 t ½ x = 120,000 y 1 t ½ 24,000 y

22. 22 Chapter 18 © 2011 Pearson Education, Inc. Half-Life Calculation Iodine-131 is used to measure the activity of the thyroid gland. If 88 mg of I-131 are ingested, how much remains after 24 days (t ½ = 8 days). First, find out how many half-lives have passed. 24 days x = 3t ½ 1 t ½ 8 days Next, calculate how much I-131 is left. 88 mg I-131 x = 11 mg I-131 1 2 1 2 1 2 x x

23. 23 Chapter 18 © 2011 Pearson Education, Inc. Radiocarbon Dating A nuclide that is unstable is called a radionuclide. Carbon-14 decays by beta emission with a half-life of 5730 years. C 14 6 e 0 + N 14 7 The amount of carbon-14 in living organisms stays constant with an activity of about 15.3 dpm. After the plant or animal dies, the amount of C-14 decreases.

24. 24 Chapter 18 © 2011 Pearson Education, Inc. Radiocarbon Dating, Continued The age of objects can therefore be determined by measuring the C-14 activity. This is called radiocarbon dating. The method is considered reliable for items up to 50,000 years old.

25. 25 Chapter 18 © 2011 Pearson Education, Inc. Uranium–Lead Dating Uranium-238 decays in 14 steps to lead-206. The half-life for the process is 4.5 billion years. The age of samples can be determined by measuring the U-238/Pb-206 ratio. A ratio of 1:1 corresponds to an age of about 4.5 billion years. e 0 + 6 U 238 92 He 4 2 Pb 206 82 + 8

26. 26 Chapter 18 © 2011 Pearson Education, Inc. Agricultural Use of Radionuclides Cobalt-60 emits gamma rays when it decays and is often used in agriculture. Gamma radiation is used to sterilize male insects instead of killing them with pesticides. Gamma-irradiation of food is used to kill microorganisms: –Irradiation of pork is used to kill the parasite that causes trichinosis. –Irradiation of fruits and vegetables is used to increase shelf life.

27. 27 Chapter 18 © 2011 Pearson Education, Inc. Critical Thinking: Nuclear Medicine The term nuclear medicine refers to the use of radionuclides for medical purposes. Iodine-131 is used to measure thyroid activity. The gas xenon-133 is used to diagnose respiratory problems. Iron-59 is used to diagnose anemia. Breast cancer can be treated using the isotope iridium-159.

28. 28 Chapter 18 © 2011 Pearson Education, Inc. Artificial Radioactivity A nuclide can be converted into another element by bombarding it with an atomic particle. This process is called transmutation. The elements beyond uranium on the periodic table do not occur naturally and have been made by transmutation. For example, rutherfordium can be prepared from californium. n 1 0 + 4 Cf 249 98 12 C 6 Rf 257 104 +

29. 29 Chapter 18 © 2011 Pearson Education, Inc. Nuclear Fission Nuclear fission is the process where a heavy nucleus splits into lighter nuclei. Some nuclides are so unstable, they undergo spontaneous nuclear fission. A few nuclides can be induced to undergo nuclear fission by a slow-moving neutron. n 1 0 + 4 Cf 252 98 Ba 142 56 + Mo 106 42 n 1 0 + 3 U 235 92 Ba 141 56 + Kr 92 36 + n 1 0 + energy

30. 30 Chapter 18 © 2011 Pearson Education, Inc. Nuclear Chain Reaction Notice that one neutron produces three neutrons. These neutrons can induce additional fission reactions and produce additional neutrons. If the process becomes self-sustaining, it is a chain reaction.

31. 31 Chapter 18 © 2011 Pearson Education, Inc. Nuclear Chain Reaction, Continued The mass of material required for a chain reaction is the critical mass.

32. 32 Chapter 18 © 2011 Pearson Education, Inc. Critical Thinking: Nuclear Power Plant Nuclear energy is an attractive source because of its potential. –1 gram of U-235 produces about 12 million times more energy than 1 gram of gasoline. A nuclear power plant produces energy from a nuclear chain reaction. Fuel rods are separated by control rods to regulate the rate of fission. A liquid coolant is circulated to absorb heat.

33. 33 Chapter 18 © 2011 Pearson Education, Inc. Nuclear Fusion Nuclear fusion is the combining of two lighter nuclei into a heavier nucleus. It is more difficult to start a fusion reaction than a fission reaction, but it releases more energy. Nuclear fusion is a cleaner process than fission because very little radioactive waste is produced. The Sun is a giant fusion reactor, operating at temperatures of millions of degrees Celsius.

34. 34 Chapter 18 © 2011 Pearson Education, Inc. Fusion in the Sun (and Other Stars) The Sun is about 73% hydrogen, 26% helium, and 1% all other elements. Three common fusion reactions that occur in the Sun are as follows: + H 1 1 + energy e 0 +1 H 1 1 H 2 1 + + H 2 1 + energy H 1 1 He 3 2 + 3 2 + energy e 0 +1 H 1 1 He 4 2 +

35. 35 Chapter 18 © 2011 Pearson Education, Inc. Fusion Energy There are two fusion reactions being investigated for use in commercial power generation. 1.The first reaction, as noted below, uses deuterium (H-2) as a fuel: 2. The second, as noted below, involves deuterium and tritium (H-3) as fuels: + H 2 1 + energy H 2 1 He 4 2 + H 3 1 + energy n 1 0 H 2 1 He 4 2 +

36. 36 Chapter 18 © 2011 Pearson Education, Inc. Chapter Summary There are three types of natural radiation: 1.Alpha particles 2.Beta particles 3.Gamma rays Gamma rays are electromagnetic radiation. Alpha particles are helium nuclei, and beta particles are electrons.

37. 37 Chapter 18 © 2011 Pearson Education, Inc. Chapter Summary, Continued Radioactive nuclides decay by four processes: 1.Alpha emission 2.Beta emission 3.Positron emission 4.Electron capture The parent nuclide decays to yield the daughter nuclide. If a nuclide decays through the emission of radiation in more than one step, the overall process is called a radioactive decay series.

38. 38 Chapter 18 © 2011 Pearson Education, Inc. Chapter Summary, Continued The time required for 50% of the radioactive nuclei in a sample to decay is constant and is called the half-life. After each half-life, only 50% of the radioactive nuclei remain. Artificial nuclides are produced by transmutation. The splitting of a heavy nucleus into two lighter nuclei is nuclear fission. The combining of two lighter nuclei into one nucleus is nuclear fusion.