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NUCLEAR CHEMISTRY Radioactivity • Radioisotopes – A isotope that has an unstable nucleus and undergoes radioactive decay • Always accompanied by the emission of large amounts of energy • Nuclear reactions NOT Affected by: – Changes in temperature, pressure or the presence of catalyst – Unaffected by compounds in which the unstable isotopes are composed – Can not be slowed down, speed up or turned off Radioactivity Antoine Henri Becquerel (1852-1908): 1896- while studying uranium salts, observed that they fogged up photographic film plates Marie Curie (1867-1934) & Pierre Curie (18591906): Demonstrated that fogging was caused by rays emitted by uranium atoms in the ore Radioactivity-process by which materials give off such rays Radiation-penetrating rays and particles emitted by a radioactive source Radioactivity •Radioisotopes have unstable nuclei •Stability dependent on proportion of neutrons to protons and overall size •Radioactive Decay: The spontaneous disintegration of a nucleus into a slightly lighter and more stable nucleus, accompanied by emission of particles, electromagnetic radiation, or both Types of Radioactive Decay Alpha Radiation Helium nuclei that have been emitted from a radioactive source 1. Alpha particle (α) is a helium nucleus, contains 2 proton and 2 neutron so it has a 2+ charge. 2. Loss of an alpha particle reduces atomic # by 2 and atomic mass by 4 Beta Radiation fast moving electrons formed by the decomposition of a neutron in an atom 1. Beta particle (β) is an electron emitted from the nucleus during nuclear decay 2. Beta particles are emitted when a neutron is converted into a proton and an electron, atomic # increases by 1 Gamma Emission 1. Gamma rays () are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state 2. Gamma emission usually follows other types of decay that leave the nucleus in an excited state Nuclear Radiation A. Penetrating Ability 1. Alpha Particles a. Least penetrating ability due to large mass and charge b. Travel only a few centimeters through air c. Cannot penetrate skin d. Can cause harm through ingestion or inhalation 2. Beta Particles a. Travel at speeds close to the speed of light b. Penetrating ability about 100 times greater than that of alpha particles. c. They have a range of a few meters in air. 3. Gamma rays a. Greatest penetrating ability b. Protection requires shielding with thick layers of lead, cement, or both B. Penetrating ability of radiation C. Radioactive Elements 1. All isotopes of all man-made elements are radioactive 2. Some naturally isotopes are radioactive a. All isotopes of all elements beyond bismuth (atomic #83) are radioactive Half-Life Half-Life (t1/2) The time required for half the atoms of a radioactive nuclei to decay a. More stable nuclei decay slowly b. Less stable nuclei decay rapidly Half-Life Equations Total mass of decay = number of half-lives x number of years Half - life Fraction of sample remaining = final mass of sample Initial mass of sample 1. If 100.0g of carbon-14 decays until only 25.0 g of carbon is left after 11460 yrs, what is the half-life of carbon-14? 2. Thallim-208 has a half-life of 3.053 min. How long will it take for 120.0g to decay to 7.50 g? 3. Gold-198 has a half-life of 2.7 days. How much of a 96 g sample of gold -198 will be left after 8.1 days? Radioactive Decay Modeling Model the hypothetical nuclei of Mysterium (My) and it’s decay 1. Using toothpicks connect 1 large marshmallow + 1 small marshmallow (large = proton ; small = electron) (proton + electron = Neutron) 2. Form 6 Neutrons in total 3. Combine 6 Neutron + 4 protons in one pile: Illustrate on poster and write initial state AZ My 4. Illustrate alpha emission with equation and marshmallows (reduce by 2 protons + 2 neutrons) AZ My AZ X + 4 He 5. Illustrate beta emission using new isotope in step 4 with equation and marshmallows AZ Y + 0-1 β 6. Label the following decay sequence as α or β emission: 238 U 234 Th 234 Pa 230 Th 226 Ra 222 Rn 92 90 91 92 88 86 Neutron-Proton Ratios •Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. •A strong nuclear force helps keep the nucleus from flying apart. •Neutrons play a key role stabilizing the nucleus. •Therefore, the ratio of neutrons to protons is an important factor. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the socalled band of stability. Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron (mass of an electron but a positive charge) emission or electron capture. Transmutation Reactions •Transmutation: the conversion of an atom of one element to an atom of another element oHigh energy particles bombard the nucleus of an atom •Naturally Occurring oNitrogen-14 Carbon-14 (upper atmosphere) oFlourine-18 Oxygen-17 and one proton •Transuranium elements: elements in the periodic table with an atomic # > 92, all undergo transmutation •None occure in nature •All radioactive Nuclear Fission A very heavy nucleus splits into more stable nuclei of intermediate mass – Occurs by chain reaction The mass of the products is less than the mass of the reactants. Missing mass is converted to energy Nuclear Chain Reaction In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons, which react with still more fissionable atoms… Nuclear Reactors Use controlled fission to produce useful energy Energy genreated in form of heat Coolant fluid, removes heat from the core Heat generates steam which drives a turbine that generates electricity Two step process: 1. Neutron Moderation: reduces speed of neutrons to continue chain reaction (water & carbon) 2. Neutron absorption: decreases the # of slow moving neutrons, prevents chain reaction from going too fast Nuclear Waste 300 fuel rods = assembly 100 assemblies = reactor core Spent fuel rods are high-level nuclear waste (highly radioactive) All nuclear power plants have holding tanks for spent rods 12 meter deep and filled with water – cools rods and acts as a radiation shield Spend a decade or more in tank Limited time for plant operation due to contamination Nuclear Fusion Nuclear Fusion Light-mass nuclei combine to form a heavier, more stable nucleus Fusion Reactions 1. More energetic than fission rxns 2. Occur only at high temperature – in excess of 40 000 000 °C 3. Source of energy of the hydrogen bomb 4. Potential energy source that is inexpensive and readily available and not radioactive Problems: achieving high temperatures, uncontrolled, plasma state of products Detecting Radiation Ionizing Radiation: radiation with enough energy to knock electrons off atoms of bombarded substances to form ions Monitoring Devices: Geiger Counter: a gas-filled metal tube used to detect radiation ionizing radiation penetrates the thin window at one end ionizing the gas which conducts electricity making audible clicks Scintillation counter: A device that uses a specially coated phosphor surface to detect radiation; ionizing radiation striking the phosphor plate produces bright flashes of light Uses for Radiation Nuclear Medicine Thyroid imaging using Tc-99m Food Irradiation •Food can be irradiated with g rays from 60Co or 137Cs. •Irradiated milk has a shelf life of 3 mo. without refrigeration. •USDA has approved irradiation of meats and eggs. Chernobyl Disaster April 26, 1986 Reactor # 4 in a Nuclear Power plant in Ukraine exploded. Further explosions and resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical location http://www.youtube.com/watch?v=uoEgkGNO-sQ&feature=related http://ngm.nationalgeographic.com/2006/04/inside-chernobyl/audio-interactive