Introductory Chemistry: Concepts & Connections Introductory Chemistry: Concepts & Connections 4 th Edition by Charles H. Corwin Nuclear Chemistry Christopher G. Hamaker, Illinois State University, Normal IL © 2005, Prentice Hall Chapter 18

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 183 Natural Radioactivity There are three types of radioactivity: –Alpha particles, beta particles, and gamma rays Alpha particles (  ) are identical to helium nuclei, containing 2 protons and 2 neutrons. Beta particles (  ) are identical to electrons. Gamma rays (  ) are high energy photons.

Chapter 184 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 185 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 186 Atomic Notation 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. Sr 90 38

Chapter 187 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 He 4 2 Th In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products.

Chapter 188 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 atomic notation, a coefficient may be necessary to balance the reaction.

Chapter 189 Common Nuclear Particles Here is a listing of the common nuclear particles used to balance nuclear reactions.

Chapter 1810 Alpha Emission Radioactive nuclides can decay by giving off an alpha particle. Radium-226 decays by alpha emission. Ra He 4 2 X A Z + 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. Ra He 4 2 Rn

Chapter 1811 Beta 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 e 0 Ac

Chapter 1812 Gamma 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 He 4 2 Th  0 0 +

Chapter 1813 Positron 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 e Ne 22 10

Chapter 1814 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 e 0 Tl

Chapter 1815 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 1816 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 1817 Parent & 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 He 4 2 Th

Chapter 1818 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 1819 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, t ½.

Chapter 1820 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 1821 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 5 half-lives for the activity to drop to 625 dpm. 5 t ½ ×= 120,000 y 1 t ½ 24,000 y

Chapter 1822 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 ×= 3t ½ 1 t ½ 8 days Next, calculate how much I-131 is left: 88 mg I-131 ×= 11 mg I ××

Chapter 1823 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.

Chapter 1824 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 1825 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 U He 4 2 Pb

Chapter 1826 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 Cf C 6 Rf

Chapter 1827 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 Cf Ba Mo n U Ba Kr n energy

Chapter 1828 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 1829 Nuclear Chain Reaction The mass of material required for a chain reaction is the critical mass.

Chapter 1830 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 1831 Fusion in the Sun The Sun is about 73% hydrogen, 26% helium, and 1% all other elements. Three common fusion reactions that occur in the sun are: + H energy e 0 +1 H 1 1 H H energy H 1 1 He energy e 0 +1 H 1 1 He 4 2 +

Chapter 1832 Fusion 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: + H energy H 2 1 He H energy n 1 0 H 2 1 He 4 2 +

Chapter 1833 Conclusions 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 1834 Conclusions 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 1835 Conclusions 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.