Chapter 17: Radioactivity and Nuclear Chemistry

Transcription

1 Chapter 7: Radioactivity and Nuclear Chemistry Problems: -20, 24-30, 32-46, 49-70, 74-88, THE DISCOVERY OF RADIOACTIVITY In 896, a French physicist named Henri Becquerel discovered that uranium-containing crystals emitted rays that could expose and fog photographic plates. He found that these rays originated from changes within the atomic nuclei of the U atoms. He proposed that the uranium atoms were unstable. They emitted particles and/or energy to become more stable. These emissions he called uranic rays. One of the first women in France to do graduate work, Marie Sklodowska Curie carried out her doctoral work to determine if other substances emitted uranic rays. She discovered two elements, which she later named polonium (after her native Poland) and radium, which emitted high levels of radioactivity. Maric Curie changed the name of uranic rays to radioactivity (or radioactive decay). She shared the Nobel prize in physics with Becquerel and her husband, Pierre Curie, for discovering radioactivity. Except for H, every atom has protons and neutrons in its nucleus. Elements may exist as different isotopes. e.g. the three isotopes of hydrogen are H (hydrogen), 2 H (deuterium), and 3 H (tritium), which differ only in the number of neutrons (indicated by the mass number atomic number) Some isotopes are stable (e.g. H and 2 H ). Some isotopes are unstable and radioactive (e.g. 3 H). The unstable, radioactive isotopes are called radionuclides. Atomic Notation (or Nuclear Symbol): shorthand for keeping track of protons and neutrons in the nucleus # of protons + # of neutrons = mass number = A # of protons = atomic number = ZE=element symbol Atomic notation of common particles: electron: 0 e proton: p + neutron: 0 n CHEM 2 Chapter 7 page of 2

2 7.3 TYPES OF RADIOACTIVITY: ALPHA, BETA, AND GAMMA DECAY alpha (α) particle: a helium nucleus ( 24 α or 2 4 He) without electrons positively charged largest particle emitted by radioactive nuclei, has the highest charge beta (β) particle: a beta particle ( 0 β or 0 e ) is an electron emitted from an atomic nucleus positron: the antiparticle of an electron/beta particle, 0 β or 0 e The same size as an electron but with a positive charge gamma (γ) rays: high-energy rays (like X-rays) The Electromagnetic Spectrum shows the different forms of electromagnetic radiation, with cosmic and gamma rays having the highest frequency and the highest energy, making them potentially the most dangerous to humans Different Types of Radioactivity Alpha (α) emission: a helium nucleus, 24 α or 24 He, is emitted U 24 α + 90Th U 90 Th + 2α 238 CHEM 2 Chapter 7 page 2 of 2

3 Beta (β) emission: a beta particle (or electron), 0 β or 0 e, is emitted Th -0 β Pa Gamma (γ) emission: high-energy rays (like X-rays), 0 0 γ, are emitted 99m 0 43Tc γ Tc Positron emission: a positron (the antiparticle of an electron), 39 9 K e + 39Ar β or 0 e, is emitted WRITING NUCLEAR EQUATIONS nuclide: a specific atom with a given number of protons and neutrons We use the term parent-daughter nuclides to describe a parent nuclide decaying to produce a daughter nuclide. Balancing Nuclear Equations Differ from general chemical equations in that mass numbers (protons + neutrons) and atomic numbers are balanced, not the elements (or atoms) present. parent daughter 234 Th 238 For example: 92U 24 α + 90 where the mass numbers are equal to 238, and the atomic numbers are equal to 92. Ex. : Complete the following nuclear equations by identifying the unknown: 20 a. O 9 F b. 2Mg + p + 4 α + 2 c. 4 9 Be α 2 6C + Ex. 2: Write complete nuclear equations for the following processes: a. Iron-59 decays by beta emission. CHEM 2 Chapter 7 page 3 of 2

4 b. Ra-226 decays by alpha emission. c. Mn-50 decays by positron emission. d. Cs-8 is produced when a radionuclide decays by beta emission. 7.5 NATURAL RADIOACTIVITY AND HALF-LIFE When a radioactive sample decays, it emits particles and/or energy and continues to decay. When Becquerel studied radioactivity, he learned that unlike chemical reactions, the rate of the radioactive decay cannot be changed by changing temperature, pressure, or any other factors. Thus, a radioactive sample will always continue to decay. The amount of radiation given off is called the activity of the sample and can be measured using a Geiger counter. This instrument, developed by Hans Geiger, measures emissions from a radioactive sample as clicks. When more of the radioactive sample is present, the activity is higher. As the sample decays, less of the radioactive sample remains, so the activity decreases. CHEM 2 Chapter 7 page 4 of 2

5 Half-Life (t /2 ): the amount of time required for the amount or activity of a radioactive sample to decrease by half Note with each half-life passing, only half of the previous amount of sample is left. Example: Rn-222, a radioactive isotope, is one of the resulting daughter products in the U-238 decay series. The daughter product when Rn-222 decays is Po-28. a. If 96.0 mg of Rn-222 (t /2 =4 days) is inhaled but not exhaled or otherwise eliminated, what mass of it remains in the lungs after 2 days α Rn Po 84 b. Po-28 decays to form lead-24. If 84.0 mg of Po-28 (t /2 =3. minutes) formed in the lungs from the decay of Rn-222, what mass of it remains in the lungs after 9.3 minutes? α Po Pb 82 c. Lead-24 undergoes beta decay to form Bi-24. If 0.2 mg of lead-24 (t /2 =27 minutes) formed in the lungs from the decay of Po-28, what mass remains after 54 minutes? Pb e- + 24Bi CHEM 2 Chapter 7 page 5 of 2

6 7.0 THE EFFECTS OF RADIATION ON LIFE Penetrating Effects of Radiation Particles with the same energy but different masses can penetrate to different degrees. Alpha particles are massive with a high charge. They collide with other molecules and quickly lose energy. They cannot penetrate a few sheets of paper or outer cells of skin But exposure to alpha particles can cause severe burns. Beta particles penetrate more but can be stopped with a sheet of aluminum or plastic. Gamma and X-rays can penetrate even more but are stopped with a thick lead shield. can penetrate much deeper and are very dangerous to living organisms Neutrons can penetrate even more but can be stopped by a thick wall of concrete. ACUTE RADIATION DAMAGE AND GENETIC DEFECTS: Chemical Effects of Radiation: Why is radiation so dangerous? Acute radiation damage results from exposure to large amounts of radiation in a short period of time, which kills a large numbers of cells. Weakened immune systems. High-energy or ionizing radiation can take electrons from stable compounds in living organisms, leaving highly reactive unpaired electrons called radicals or free radicals. These free radicals are so reactive they cause reactions between otherwise stable materials in the cells of living organisms. If these reactions involve genetic materials (e.g. genes and chromosomes), the changes could lead to genetic mutations, cancer, or other devastating consequences. CHEM 2 Chapter 7 page 6 of 2

7 Health Hazards Associated with Radon Note: Because radon is a Noble gas with a long half-life, it is often inhaled and exhaled before it decays. However, two of its daughter nuclides, Po-28 and Po-24, are solids with much shorter half-lives (typically minutes). When radon decays in the lungs, these solid isotopes remain in the lungs tissue and continually emit α particles, damaging tissue and cells, and possibly even leading to cancer. 7. RADIOACTIVITY IN MEDICINE Radioactive nuclei (called "radiotracers") are used most commonly in medicine for diagnosis To minimize risk to patients, radiotracers should have the following characteristics:. They have short half-lives and a daughter product that is non-toxic and non-radioactive. They decay and emit radiation while the diagnosis is being done then decay so quickly that very little radiation is given off after the diagnosis 2. They emit penetrating gamma rays that can be detected by the detectors outside the body. 3. Their chemical properties should cause them to concentrate in diseased areas (make a hot spot ) or causing the diseased areas to reject it (make a cold spot ). Positron emission tomography (PET) is a technique used to study brain disorders. A patient is given glucose (C 6 H 2 O 6 ) containing carbon-, a positron emitter. The brain is then scanned to detect positron emission, and differences in glucose uptake and metabolism can be traced. Sodium-24 (a β-emitter) injected into the bloodstream can be monitored to trace the flow of blood and detect possible constrictions or obstructions. Isotopes of technetium are valuable as diagnostic tools. Patients drink or are injected with solution containing Tc-99m, which helps doctors imaging of organs like the heart, liver, or lungs. Some radioactive isotopes are used in cancer therapy Cobalt-60 is often used γ rays from this source are focused at small areas where cancer is suspected Thyroid cancer patients drink a solution of NaI containing radioactive iodide ions ( 3 I or 23 I). The iodine moves preferentially through the thyroid gland, where the radiation can (depending on the dosage) either image or destroy overactive thyroid cells. CHEM 2 Chapter 7 page 7 of 2

8 7.6 RADIOCARBON DATING: USING RADIOACTIVITY TO MEASURE THE AGE OF FOSSILS AND OTHER ARTIFACTS Radiocarbon Dating; Age of Organic Material Radioactive samples always decay at a CONSTANT rate (not affected by temperature, etc.) Knowing the half-life for a given radioactive isotope and knowing the current relative abundance for that isotope we can estimate the age of various objects. Carbon-4 is used to date organic materials (once living organisms) Carbon-4 is produced by neutron capture in the upper atmosphere: 4 4 7N + 0 n 6C + H Assuming the ratio of carbon-4 and carbon-2 in the atmosphere has been constant for about 50,000 years. Because living plants and animals have a constant intake of carbon compounds, they able maintain a ratio of carbon-4 to carbon-2 that is identical with the atmosphere. Once the organism dies, it no longer ingests carbon compounds, and the carbon-4 content decreases because of its radioactive decay: 4 6C 4 0 7N + β - t /2 = 5730 years Radiocarbon dating cannot be used to date objects older than 50,000 years because the radioactivity is too low to be measured accurately. Used to date charcoal found at Stonehenge, linen wrapped around the Dead Sea Scrolls, and other historical objects, etc. Uranium-Lead Dating; Age of Rocks The half-life for uranium-238 to decay to lead-206 is years: U Pb + 8 α e By analyzing rocks to determine the ratio of U-238 to Pb-206 present, one can determine the "age" of the rocks i.e. the amount of time since the rock solidified. For example, assuming the rock originally has no Pb-206 present, the Pb-206 could only have come from the U-238 decay, and equal amounts of both means half the U-238 has decayed to Pb-206. The rock is 4.5 x 0 9 years old (equal to U-238's half life). Used to estimate age of Earth and lunar rock samples brought back by the Apollo missions CHEM 2 Chapter 7 page 8 of 2

9 OTHER APPLICATIONS OF RADIOACTIVITY Agricultural Applications: Controlling insects without using pesticides Cobalt-60 emits gamma rays that sterilize male insects and reduce insect population. Gamma irradiation of processed food also destroys microorganisms. Cobalt-60 irradiation destroys parasites in pork (trichinosis) and chicken (salmonella). Gamma irradiation also increases shelf life without using preservatives. The photo below shows irradiated strawberries that do not become moldy after 25 days. INDUCED NUCLEAR REACTIONS Artificial Radioactivity Nuclei can also be induced to decay as a result of bombardment by high energy particles (e.g. neutrons, electrons, and other nuclei) These kinds of nuclear changes are called nuclear transmutation. This means we can cause nuclear reactions to occur! NUCLEAR ENERGY Nuclear Binding Energy energy required to break up a nucleus into its component nucleons The most stable nuclei have mass numbers of about Heavier nuclei (those with mass numbers > 80) can split into smaller, more stable nuclei: nuclear fission Smaller nuclei (those with mass numbers < 40) combine with other smaller nuclei to form more stable nuclei: nuclear fusion CHEM 2 Chapter 7 page 9 of 2

10 7.7 THE DISOVERY OF FISSION AND THE ATOMIC BOMB NUCLEAR FISSION: A heavy nucleus (mass number > 200) divides to form smaller, more stable nuclei, resulting in the release of large amounts of energy The Fission Process Many isotopes of heavy elements undergo fission if bombarded by high-energy neutrons Two naturally occuring isotopes, 92U and 94Pu, have practical applications. They undergo fission when hit with even low-energy, room temperature neutrons. Consider the following: U + 0 n Sr Xe n Note that additional neutrons are also produced The neutrons generated can induce fission in other nuclei, which turn produces more neutrons, and so on. nuclear chain reaction: self-sustaining sequence of nuclear reactions Nuclear chain reaction's also require a critical mass, the minimum amount of fissionable material needed to generate a self-sustaining chain reaction. 235 i.e., there must be enough 92U present, so the neutrons produced can collide with them to cause more fission reactions Fission Energy The energy released is kj/g of U, about equal to the explosion of 30 metric tons (3 0 4 kg) of TNT! CHEM 2 Chapter 7 page 0 of 2

11 7.8 NUCLEAR POWER: USING FISSION TO GENERATE ELECTRICITY Fission reactions are now used more commonly in nuclear reactors. Nuclear Reactors Entire reactor is contained in a concrete building to shield personnel and nearby residents from radiation. Fuel rods alternate with control rods in a containment chamber. fuel rods: UO 2 pellets in a zirconium alloy tube, where the uranium is "enriched" to contain 3% U-235, the fissionable material. Only about 0.7% of naturally occurring uranium is U-235 while 99.3% is U-238 sample must be "enriched" to increase the amount of U-235. control rods: cadmium or boron rods that capture neutrons and serve to control the number of neutrons, and thus, the nuclear fission reactions. Water is passed through the reactor to absorb the heat given off by fission and to moderate (or slow down) the neutrons produced. The heated water heats more water to steam, which drives a turbogenerator that produces electrical energy. Cooling water (usually a river) is also used to cool the steam for reuse. CHEM 2 Chapter 7 page of 2

12 Advantage of Nuclear Reactors: Produce energy without producing Greenhouse gases that contribute to global warming Disadvantage of Nuclear Reactors: Nuclear Waste Each year, on average, one-fourth of the intensely radioactive fuel rods used in nuclear reactors must be replaced. In 982, the Nuclear Waste Policy Act established a timetable for choosing and preparing sites for underground disposal of radioactive materials. Potential problems: It's estimated that 20 half-lives are required for the radioactivity to reach acceptable levels for biological exposure. Since Sr-90's half-life is 28.8 years, the wastes must be stored for at least 600 years. If Pu-239 is being stored, the storage must last much longer since Pu-239 's half-life is 24,000 years. There must be assurances that the containers do not crack, allowing radioactivity to find its way into underground water supplies. NUCLEAR FUSION: Lighter nuclei combine to form larger, more stable nuclei The energy available from such fusion reactions is considerably greater than that given off by fission reactions. This is the type of reaction that provides the Sun its energy. 2 H deuterium + 3 H tritium 4 2 He + 0 n Advantages of Nuclear Fusion: Produces more energy per gram of materials Light isotopes needed for fusion reactions are more abundant than heavy isotopes needed for fission reactions. Fusion products are not radioactive. Disadvantages of Nuclear Fusion: High energies are required to overcome repulsion between nuclei. For example, the fusion reaction show above requires temperatures around 40 million Kelvin. Such high temperatures have been achieved by using an atomic bomb to initiate the fusion process, but this approach can't be used for controlled power generation. In addition, no known structural material can withstand the enormous temperatures necessary for fusion on the scale needed for power generation. CHEM 2 Chapter 7 page 2 of 2