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Nuclear Chemistry

Unit 15. Nuclear Chemistry. Overview. Nuclear Chemistry Isotopes Nuclear force Radioactive decay Alpha, beta, gamma decay Positron emission Electron capture Nuclear Stability. Radiometric Dating Half-life Nuclear fusion Nuclear fission Nuclear energy Mass Defect

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Nuclear Chemistry

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  1. Unit 15 Nuclear Chemistry

  2. Overview • Nuclear Chemistry • Isotopes • Nuclear force • Radioactive decay • Alpha, beta, gamma decay • Positron emission • Electron capture • Nuclear Stability • Radiometric Dating • Half-life • Nuclear fusion • Nuclear fission • Nuclear energy • Mass Defect • Nuclear binding energy

  3. Nuclear Chemistry • Involves the change in the nucleus of an atom • Nuclear reactions are everywhere • Produce sunlight • Create elements (synthetic and natural in stars) • Radiation therapy (cancer treatment) • Generate electricity • Nuclear weapons

  4. World Energy Use

  5. The Nucleus • Remember – the nucleus is comprised of the two nucleons (protons and neutrons) • Atomic Number – number of protons • Mass Number – number of protons and neutrons together • It is effectively the mass of the atom

  6. Nuclear Symbols Mass number (p+ + no) C Element symbol 12 6 Atomic number (number of p+)

  7. Isotopes • Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms • Example: There are three naturally occurring isotopes of uranium: • Uranium-234 • Uranium-235 • Uranium-238

  8. Nuclear Force • Strong nuclear force • Holds protons and neutrons in nucleus very close together • Strongest force known

  9. Nuclear Force • Nucleus is not stable when atoms experience certain ratios of protons to neutrons • Unstable atoms decay and emit radiation • Radioactive decay • Elements with more than 83 protons (bismuth) are naturally radioactive

  10. Radioactive Decay • Radionuclides: Radioactive elements • During radioactive decay • The makeup of the nucleus changes • The number of protons may change • Means that the element has changed

  11. Natural Radioactive Isotopes • Radon-222 • Comes from decomposition of Uranium rocks • 2nd leading cause of lung cancer • Comes up through cracks in basements • Radium-226 • Some radium salts glow in the dark • Early 1900s used to be used as paint for watches and clocks (workers licked paint brushes and got cancer – “radium girls”) • Uranium-238 • Rocks create radon gas • Used in radioactive dating • Potassium-40 • One of few light radioactive elements • Produces argon that is found in atmosphere

  12. Other Common Radioisotopes

  13. Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

  14. Radioactive Decay(3 Most Common Types) • Alpha (a, He) • 2 protons, 2 neutrons • Beta (b, e) • High energy electron • Gamma (g) • Electromagnetic radiation • High energy photons

  15. Alpha, Beta, Gamma Radiation

  16. 238 92 234 90 4 2 He or  U Th He 4 2 +  Alpha Decay: Loss of an -particle (a helium nucleus) 4 2

  17. 131 53 131 54 0 −1 e I Xe e  0 −1 0 −1 +  or Beta Emission: Loss of a -particle (a high energy electron)

  18. 0 0 Gamma Emission: • Loss of a -ray • High-energy radiation that almost always accompanies the loss of a nuclear particle • Not usually written in nuclear equation

  19. 11 6 11 5 0 1 C B e e  0 1 0 1 +  or Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge of an electron) Has a very short life because it is destroyed when it collides with an electron, producing gamma rays: e + e   0 1 0 0 0 -1

  20. 1 1 1 0 0 1 p n e +  Positron Emission • A positron can convert a proton to a neutron

  21. 0 −1 1 1 1 0 p e n +  Electron Capture • Capture by the nucleus of an electron from the electron cloud surrounding the nucleus • Addition of an electron to a proton in the nucleus • As a result, a proton is transformed into a neutron

  22. Nuclear Stability • Several factors predict whether a particular nucleus is radioactive • Neutron-to-proton ratio • Radioactive series • Magic Numbers • Evens and Odds

  23. Neutron-Proton Ratios • The strong nuclear force helps keep the nucleus from flying apart • Protons repel each other • Neutrons help the strength of the nuclear force • As protons increase, neutrons have to counter-act increasing proton-proton repulsions • In low atomic number elements (1-20) protons and neutrons are approximately equal • In high atomic number elements number of neutrons much larger than protons • Neutron-proton ratio helps stabilize nucleus

  24. Neutron-Proton Ratios For smaller nuclei (Atomic Number  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

  25. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

  26. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

  27. Stable Nuclei • Nuclei above this belt have too many neutrons. • They tend to decay by emitting beta particles. • (If an isotopes mass number is greater than its atomic weight, the same trend will happen example C) 16 6

  28. Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture. • (If an isotopes mass number is less than its atomic weight, the same trend will happen example C) 11 6

  29. Stable Nuclei • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission. • Decreases both protons and neutrons

  30. Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). • Often occur in nature

  31. Magic Numbers • Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons. • These are called the “Magic Numbers”

  32. Evens and Odds • Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

  33. 0.693 k = t1/2 Kinetics of Radioactive Decay • Radioactive decay is a 1st order process • Remember this equation:

  34. Radiometric Dating • Half life can help determine the age of different objects • Carbon-14 • Half life of 5,715 years • Can determine age of organic materials up to about 50,000 years old

  35. Radiometric Dating • Uranium-238 • Half life of 4.5×109 years • Used to determine age of Earth (measured rocks) • Oldest rock found is almost 4.5 billion years old

  36. Nuclear Fusion • Elements can be man-made by bombarding nuclei with particles • Alpha particles accelerated and collided with nucleus • Neutrons bombard nucleus • Bombard nuclei to create transuranium elements • Heavy elements beyond uranium on periodic table

  37. Particle Accelerators • Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide • These particle accelerators are enormous, having circular tracks with radii that are miles long

  38. Nuclear Fission • The splitting of heavy nuclei • (Fusion is the combination of light nuclei) • Process begins by bombarding heavy nucleus with a neutron • 2 main commercial uses • Nuclear Weaponry • Nuclear Energy

  39. Nuclear Fission • About 2 neutrons are produced for each fission • These 2 neutrons cause 2 additional fissions • Which cause 2 more fissions each • Which cause 2 more fissions each… • This is called a chain reaction

  40. Nuclear Fission • Chain reactions can escalate quickly • If the reaction is not controlled, it results in a violent explosion because of the release of too much energy too quickly

  41. Nuclear Energy • We can control fission reactions and use it to create energy

  42. Nuclear Energy • Fission reactions are carried out in nuclear reactors • The reaction is kept in check by the use of control rods • These block the paths of some neutrons, keeping the system from escalating out of control • The heat generated by the reaction is used to produce steam that turns a turbine connected to a generator Video: http://www.youtube.com/watch?v=VJfIbBDR3e8

  43. Debates on Nuclear Energy • Pros… • Cleaner energy than coal and fossil fuel plants • Doesn’t add to global warming • High amount of electricity can be generated in one plant • Cheaper to run a nuclear facility than a fossil fuel plant • Cons… • Nonrenewable source of energy • Produces nuclear waste that must be stored for thousands of years • Accidents (Chernobyl, Three Mile Island, Fukushima) • http://www.youtube.com/watch?v=eGI7VymjSho • Very expensive to build a nuclear facility (about $10 billion per reactor)

  44. Nuclear Energy • We can measure the energy associated with nuclear reactions E = mc2 E = energy (J) m = change in mass (kg) during reaction (mass of products-mass of reactants) c = speed of light (3.0×108 m/s) When a system loses mass, it is exothermic (-E) When a system gains mass, it is endothermic (+E)

  45. Nuclear Energy • The mass change in chemical reactions is so small that we treat them as though mass is conserved • Ex: Mass change for exothermic process of combustion of 1 mol of CH4 is -9.9×10-9 grams • Mass change in nuclear reactions is measureable • Ex: Mass change accompanying decay of 1 mol of uranium-238 is 50,000 times greater than combustion of CH4

  46. Nuclear Energy (example) For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g. The change in energy, E, is then E = (m) c2 E = (−4.6  10−6 kg)(3.00  108 m/s)2 E = −4.1  1011 J

  47. Mass Defect • When protons and neutrons form a nucleus, the mass of the nucleus is less than the sum of the masses of its constituent protons and neutrons Example: Helium (He) – 2 protons, 2 neutrons Protons and NeutronsMass of Nucleus Mass of 2 protons (2×1.0073 = 2.0146) 4.0015 amu Mass of 2 neutrons (2×1.0087 = 2.0174) Total mass = 4.0320 amu Difference = 4.0320 – 4.0015 = 0.0305 amu (mass defect)

  48. Mass Defect • To measure the energy associated with the mass defect use E = mc2 Example: Helium (He) – 2 protons, 2 neutrons E = (5.1×10-29 kg)(3.0×108 m/s)2 E = 4.6×10-12 J NOTE: 1 gram = 6.022×1023amu

  49. Nuclear Binding Energy • Energy required to separate a nucleus into its individual nucleons (protons and neutrons) • Also use E = mc2 • The larger the binding energy, the more stable the nucleus toward decomposition

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