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NUCLEAR CHEMISTRY Atomic Structure Recall: Atoms – consist of a positively charged nucleus, which has protons and neutrons. Isotope – atoms of the same chemical element that have a different number of neutrons. Each isotope of a given element is designated by the total number of its protons plus its neutrons. Isotope Mo-101 H-2 C-14 U-238 Bi-210 He-4 Symbol 101 42 2 1 14 6 238 92 210 83 4 2 Mo H C U Bi He # protons # neutrons Atomic Mass 42 59 101 1 1 2 6 8 14 92 146 238 83 127 210 2 2 4 Nuclear Forces Mass Defect • Difference between the mass of an atom and the mass of its individual particles. 4.00260 amu 4.03298 amu Nuclear Binding Energy • Energy released when a nucleus is formed from nucleons. • High binding energy = stable nucleus. E = 2 mc E: energy (J) m: mass defect (kg) c: speed of light (3.00×108 m/s) Nuclear Binding Energy The seven most widely recognized magic numbers as of 2007 are 2, 8, 20, 28, 50, 82, 126 Unstable nuclides are radioactive and undergo radioactive decay. Types of Radiation • Alpha particle () – helium nucleus • Beta particle (-) – electron • Positron (+) – positron • Gamma () – high-energy photon 4 2 Charge Shielding He 2+ 0 -1 1- e 0 1 0 0 e paper lead 1+ 0 concrete Penetrating Ability of Radiation Nuclear Decay •Transmutation-One element becomes another. 238 92 U I 131 54 K 38 18 131 53 38 19 106 47 Th He 234 90 Ag 0 -1 4 2 Xe 0 -1 Ar 0 1 e 106 46 e e Pd More than 83 protons means that the nuclei is unstable (radioactive) Alpha Emission occurs when the nucleus has too many protons which cause excessive repulsion. 238 92 parent nuclide U Th He 234 90 daughter nuclide 4 2 alpha particle Numbers must balance!! Beta Emission occurs when the neutron to proton ratio is too great. 131 53 I 131 54 Xe e 0 -1 electron Neutron to Proton Beta Emission Ex. Polonium-210 undergoes beta decay to produce this daughter nuclide 210 84 A Z Po A Z X = 210 85 X At 0 + -1 e Positron Emission Occurs when the neutron to proton ratio is too small. 38 19 K Ar e 38 18 0 1 positron Positron Emission Ex. Polonium-210 undergoes positron emission to produce this daughter nuclide 210 84 A Z Po A Z X = 210 83 X Bi 0 + 1 e Electron Capture occurs when the neutron to proton ratio in the nucleus is too small. 106 47 Ag e 0 -1 106 46 Pd electron Electron Capture Ex. Polonium-210 captures an electron to produce this daughter nuclide 210 84 Po 0 + -1 A Z A Z e X = 210 83 Bi X Gamma Emission occurs when the nucleus is at too high an energy. – Emission of high energy electromagnetic wave. Gamma Emission Ex. Polonium-210 undergoes gamma decay to produce this daughter nuclide 210 84 A Z Po A Z X = 210 84 X Po + 0 0 Types of Radiation Nuclear Fission – splitting of heavier nuclei into lighter nuclei. 235 92 U 1 + 0 n 137 56 Ba 84 + 36 Xe 1 +15 0 + energy How much energy? E=mc2 _______= Energy _______ mass x (_____ speed of _ _____) light 2 n c=3.0x108 mass defect E=mc2 explains _____ _____ (total mass of nucleus is less than sum of individual particles) Nuclear Fusion - Energy released when two light nuclei combine or fuse •However, a large amount of energy is required to start a fusion reaction: repulsion forces o Need this energy to overcome ________ of protons. o Extremely high temperatures can provide start-up energy. More energy in fusing hydrogen that fission of uranium Nuclear Fusion A radioactive nucleus reaches a stable state by a series of steps A Decay Series Copyright© by Houghton Mifflin 27 Half-life Concept Decay Kinetics Decay occurs by first order kinetics (the rate of decay is proportional to the number of nuclides present) N0 = number of nuclides present initially N kt ln N0 N = number of nuclides remaining at time t k = rate constant t = elapsed time Calculating Half-life ln( 2) 0.693 t1/ 2 k k t1/2 = Half-life (units dependent on rate constant, k) Sample Half-Lives Half-life mf m ( ) 1 n i 2 mf: final mass mi: initial mass n: # of halflives