1 / 49

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

jasper
Download Presentation

Nuclear Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related