Nuclear Chemistry

Nuclear Chemistry Chapter 22 Nuclear Chemistry Nuclear chemistry involves changes in the nucleus (protons and neutrons) of radioactive atoms.  Applications of nuclear chemistry:  medical diagnosis and treatment  C-14 dating  nuclear power plants  create new elements  Nuclei and Nuclear Reactions Spontaneous emission of particles or electromagnetic radiation is known as radioactivity. Atomic notation for Isotopes Recall atomic notation: All elements with Z > 83 are radioactive. A Z X A = mass # = # neutrons + # protons Z = atomic # = # protons  #neutrons for element may vary – referred to as an isotope or nuclide of element   Example: C has 3 different nuclides: 12 6  There arere, others are radioactive. C 14 6 C Radioactive decay/emission: an unstable atom emits a particle or energy. parent nuclide  daughter nuclide + radiation Parent nuclide undergoes decay. Radionuclide -unstable isotope that undergoes nuclear decay. Daughter nuclide is formed by the decay.  All elements with Z > 83 protons are radioactive; specific nuclides of lighter elements are also radioactive. 13 6 Two Types of Nuclear Processes Nuclide - atom with a specific number of protons and neutrons in its nucleus C Transmutation: atoms are bombarded by high energy particles to create elements/isotopes. 14 7 N + 24 He  238 92 U 17 8  42 He  O + 11H 239 94 Pu  3 01n 1 1 Nuclear Particles Alpha Decay: 42He is product 4 Alpha particle: a = 24 He2+; written as a or 2 He 0  Beta particle (an e-): b or -1 e 0  Gamma ray: g = high energy wave = 0 g 0 +  Positron particle (positive e ): b or +1 e 1  Neutron: n0 = 0 n   1 Proton: p+ = 1 p  a = 42He or 42a  most massive particle but least harmful since its least penetrating – stopped by paper, skin Heavy radioactive isotopes tend to emit a particles:  238 92  232 88Ra  90Th  4 2 Ηe 4 2 Ηe 4 2 Ηe   222 86 Rn can be writtenas 42 α = positron = positive electron: 0 1 b+ or 0 0 1e  Electron capture is the opposite of b emission. A nucleus captures an electron, converting a p+ to a n. 197 0 80 Hg  -1e  daughter: mass # is same, atomic #  by 1 or -1e 0 0g is product  g = high energy radiation with no mass or charge  Most penetrating radiation - stopped by 5 - 11 cm of aluminum or thick layer of concrete or lead  Often accompanies alpha or beta decay reactions  The release of gamma particles does not change the mass # or atomic #. 40 18 Ar Electron Capture   more penetrating - stopped by 0.05 - 0.1 cm of Al b emission: A high energy 0-1e is ejected from the nucleus & a neutron is converted into a proton. 0 1e is ejected from the nucleus & a p+ is converted to a n. Thus, mass # is same, atomic #  by 1  b = electron:  Gamma Radiation:  01e 40 19 K is product  0 -1b 0 b -1  E.g. 23490Th  23491Pa + 0-1e Predict products for this Beta decay rxn: 239U   01e  239 92 93 Np 228 88 Ra Positron Emission  b+ Th + 24 He Beta Emission: Daughter: mass #  by 4, atomic #  by 2 Write alpha decay rxns for the following: 226 234 90 Note: mass # = # n + # p and atomic # = # p are the same on both sides Alpha Decay Reactions  U   197 79 Au 2 2 Nuclear Reactions  Answers: Nuclear rxns Write balanced equations for: 1.Alpha emission from curium-242 242 4 238 96 Cm  2 He  94 Pu 2.Beta emission from magnesium-28     28 0 28 12 Mg -1e  13 Al What particle is produced by decay of thorium-214 to radium-210? 214 4 210 90Th 2 He  88 Ra A radioisotope decays to give an alpha particle and Rn-222. Identify the radioisotope. 226 4 222 88 Ra  2 He  86 Rn Particle = a Parent = Ra-226 Nuclear decay is 1st order process 1st order Decay of a Radionuclide A  ln t   kt  A0  A = number of radioactive nuclei present (symbol N used for A for nuclear problems)  A = 1/2 Ao at t = t1/2 : k  ln2 0.693  t1 t1 2  A   t ln  t    0.693  t1  A0   2 half-life, t1/2, is the time required for the number of radioactive nuclei in a sample to drop to half its initial value.  t1/2 is constant for a given radioisotope 2       Half-lives can vary from milliseconds to millions of years! Nuclear Kinetics Calcs Nuclear Fission and Fusion Use 1st order IRL and half-life relationships! Nuclear fission: nuclei of heavier isotopes are split to form lighter, more stable ones after bombardment by a neutron.  This often results in a chain reaction.  Nuclear fusion: nuclei of atoms are joined together or fused. This requires very hot conditions (i.e., the sun).  The hydrogen bomb works by fusion. An atomic bomb is used to provide the heat needed.   If you ingest a sample containing I-131, how much time is required for a 75.0 mg sample to decay to 12.5 mg? The half-life for I-131 is 8.05 days.  The half-life of Au-98 is 2.7 days. If you begin with 5.6 mg of this gold isotope, what mass remains after 9.5 days? 3 3 Chain Reaction   Nuclear Reactor uses Fission One step in a chain reaction produces more neutrons than it consumes. Each step gets faster. Critical mass is the mass required for the chain reaction to become self-sustaining. Nuclear Energy        17% of world’s energy comes from nuclear plants. Currently there are more than 110 nuclear power plants in the U.S.; about 435 worldwide. B or Cd control rods are lowered and raised to “control” the rate of the fission reaction. Energy produced is used to heat water and drive steam turbines. Storage of used rods is a major challenge. Natural disasters, terrorist acts, operator errors, etc. can lead to huge release of radioactive energy. Chernobyl (1986) had no containment, so the radiation leak was worse than Three Mile Island (1979). Nuclear Fusion       Nuclear Fusion is the formation of heavier nuclei by the joining of lighter ones. Fusion products are generally not radioactive. Smaller atoms are fused together into one larger atom: 11H + 21H  32He (i.e., in the sun) Called thermonuclear because they require extremely high temperatures. Positive side: H is cheap, plentiful; products are not radioactive or toxic Negative side: Still have trouble controlling the reactions; require temps ~ 40 million Kelvin!. Atomic vs Hydrogen Bombs    Atomic weapons utilize the splitting of atoms (235U) and are detonated through fission and its resulting chain reaction. Hiroshima = 13 ktons Hydrogen bombs are detonated through fusion. They are 1000 times more explosive than atomic bombs. Energy is released because of overall loss of mass. Typical = 10 megatons! Great site - summary of nuclear weapons: http://library.thinkquest.org/3471/abomb.html 4 4