Introduction

2. Introduction • As the Earth’s supply of fossil fuels is used up, 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. Chapter 18

3. Natural Radioactivity • There are three types of radioactivity: • alpha particles, beta particles, and gamma rays • Alpha particles (a) are identical to helium nuclei, containing 2 protons and 2 neutrons. • Beta particles (b) are identical to electrons. • Gamma rays (g) are high-energy photons. Chapter 18

4. Charge 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. Chapter 18

5. 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. Chapter 18

6. Atomic Notation Sr 90 38 • Recall, 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. Chapter 18

7. Nuclear Reactions U Th + He 235 4 231 92 2 90 • A nuclear reactioninvolves 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. • In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products. Chapter 18

8. Balancing Nuclear Reactions • 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. • 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. • After completing the equation by writing all the nuclear particles in an atomic notation, a coefficient may be necessary to balance the reaction. Chapter 18

9. Common Nuclear Particles • Here is a listing of the common nuclear particles used to balance nuclear reactions. Chapter 18

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

11. Beta (β ) Emission Ra + e Ac 228 0 228 88 -1 89 • 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. Chapter 18

12. Gamma (γ ) Emission g + + U He Th 4 0 233 229 2 92 90 0 • 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. Chapter 18

13. Positron (β +) Emission Na Ne e + 22 22 0 11 10 +1 • A positron (b+) 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. Chapter 18

14. Electron Capture Pb + e Tl 205 82 0 205 -1 81 • 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. Chapter 18

15. 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. Chapter 18

16. 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 8 alpha-decays and 6 beta-decays. Chapter 18

17. Parent and Daughter Nuclides U He Th + 238 4 224 92 2 90 • 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. Chapter 18

18. 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. Chapter 18

19. 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 the half-life, t½. Chapter 18

20. Half-Life • After each half-life, the activity of a radioactive sample drops to half its previous level. • A decay curve shows the activity of a radioactive sample over time. Chapter 18

21. Radioactive Waste 24,000 y 5 t½ × = 120,000 y 1 t½ • 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 5 half-lives for the activity to drop to 625 dpm. Chapter 18

22. Half-Life Calculation 1 t½ 24 days × = 3t½ 8 days 1 1 1 88 mg I-131 × × × = 11 mg I-131 2 2 2 • 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 (t½ =8 days). • First, find out how many half-lives have passed: • Next, calculate how much I-131 is left: Chapter 18

23. Radiocarbon Dating + e 0 -1 C N 14 14 6 7 • A nuclide that is unstable is called a radionuclide. • Carbon-14 decays by beta emission with a half-life of 5730 years. • 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. Chapter 18

24. 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. Chapter 18

25. Uranium-Lead Dating + 8 + 6 U He Pb e 0 -1 206 4 238 2 82 92 • 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. Chapter 18

26. 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 to kill the parasite that causes trichinosis • irradiation of fruits and vegetables to increase shelf life Chapter 18

27. 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. Chapter 18

28. Artificial Radioactivity Cf Rf n 1 0 + + 4 C 12 6 249 257 104 98 • 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: Chapter 18

29. Nuclear Fission + 4 + Cf Mo Ba n 1 0 + energy + + + 3 n Ba U Kr n 0 1 1 0 235 252 142 92 106 141 56 98 42 92 36 56 • 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. Chapter 18

30. 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. Chapter 18

31. Nuclear Chain Reaction • The mass of material required for a chain reaction is the critical mass. Chapter 18

32. 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. Chapter 18

33. 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. Chapter 18

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

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

36. Chapter Summary • There are three types of natural radiation: • alpha particles • beta particles • gamma rays • Gamma rays are electromagnetic radiation. • Alpha particles are helium nuclei, and beta particles are electrons. Chapter 18

37. Chapter Summary, continued • Radioactive nuclides decay by 4 processes: • alpha emission • beta emission • positron emission • 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. Chapter 18

38. 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. Chapter 18