Chapter 11


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Chapter 11

  1. 1. Chapter 11 Nuclear Chemistry
  2. 2. Sec 11.1 Stable and Unstable Nuclides Nuclear chemistry deals with the concept of radioactivity and particles that are given off by radioactive substances The nucleus of an atom can be stable, which means that it does not undergo change If the nucleus of an atom is unstable, it will spontaneously undergo change
  3. 3. Some isotopes for an element are stable, while others are radioactive Radioactive simply means that the substance emits radiation (such as alpha, beta or gamma radiation) Radiation can occur when an isotope is imbalanced and emits a particle to become more stable Sec 11.1 Stable and Unstable Nuclides
  4. 4. Sec 11.2 The Nature of Radioactivity The field of study was pioneered by people such as Marie Curie in early 1900s The three main types of radiation are:  Alpha particles (positively charged particles)  Beta particles (stream of electrons, neg charged)  Gamma Rays (no particles, high energy)
  5. 5. Sec 11.2 The Nature of Radioactivity The electromagnetic spectrum Note how small the spectrum of visible light truly is Page 65
  6. 6. Sec 11.3 Radioactive Decay Radioactive decay is the process by which an unstable nucleus emits radiation and undergoes a change  One element can change into a different element through the process of radioactive decay
  7. 7. Sec 11.3 Radioactive Decay Alpha:  Normally given off by heavy elements  We write this as the following 238 4 234 92U 2He (α)+ 90Th  Alpha Particles: Basically a helium nucleus with mass 4 and charge +2
  8. 8. Sec 11.3 Radioactive Decay Alpha Emission  If a heavy element is unstable it may emit an alpha particle, which can be though of as a Helium Nucleus (He 4/2)  Note that Alpha Particles have a positive charge
  9. 9. Sec 11.3 Radioactive Decay Alpha particles transform the nucleus into another element with a change of mass number by 4 and a change of atomic number by 2 Rule of thumb, alpha radiation converts an element two places to the left
  10. 10. Sec 11.3 Radioactive Decay Beta Emission:  If a nucleus has too many neutrons, it can convert a neutron to a proton and an electron  We write this as the following 1 1 0 0n 1H + -1e  Beta particles: e (-1) are basically electrons that are emitted from the nucleus
  11. 11. Sec 11.3 Radioactive Decay Beta Particles transform the nucleus into another element with the same mass number but with an atomic number of +1 Example: P S + e- (Page 67) Rule of thumb, beta radiation converts an element one place to the right
  12. 12. Sec 11.3 Radioactive Decay Gamma:  Gamma radiation doesn’t change the identity of the element  We write gamma as the following 11 0 11 5B* 0 γ + 5B  Gamma emission: Basically gamma rays are energy from higher state atoms to ground state atoms. Gamma has no mass or charge
  13. 13. Sec 11.3 Radioactive Decay Summary of Types of Radiation
  14. 14. Sec 11.4 Rate of Radioactive Decay Not all radioactive nuclei decay at the same rate, there is a large variation Half-life is the time it takes for one half of any sample to decay Logically, the faster the half life means that the nucleus is less stable
  15. 15. Sec 11.4 Rate of Radioactive Decay It is important to realize that considering half-lives, a radioactive sample will never decay completely Also: **We do not currently know of any method to speed up or slow down radioactive decay Half-Lives can be seconds, days, or years
  16. 16. Sec 11.4 Rate of Radioactive Decay Figure 11.3 Page 271. An example of a half life of 8 days
  17. 17. Sec 11.4 Rate of Radioactive Decay
  18. 18. Sec 11.5 Bombardment Reactions There are two main ways that a radioactive decay reaction takes place  Transmutation reactions, the type discussed in the previous section, describes a natural and spontaneous radioactive decay  Bombardment reactions are brought about by bombarding a stable nucleus with small particles, which then leads to radioactivity
  19. 19. Sec 11.5 Bombardment Reactions Bombardment reactions were and still are used to discover the “synthetic” elements on the periodic table  All the elements beyond uranium are radioactive and were produced through this type of experiment  Many of the elements have a short half-life time, which makes them difficult to characterize, much less use
  20. 20. Sec 11.5 Bombardment Reactions Table 11.2 Page 274
  21. 21. Sec 11.6 Radioactive Decay Series In many cases, a radioactive substance with a high atomic number (the elements starting with uranium and beyond) undergo a series of radioactive decay steps to ultimately end with a stable form  Uraniun-238 for example, undergoes 14 steps including both alpha and beta emissions, to finally end up as Lead-206
  22. 22. Sec 11.7 Chemical Effects In general, electrons of molecules are effected by radiation  One, the electrons can be excited to a higher energy state  Or two, the electrons can be ionized to actually make them leave the atom or molecule entirely  Examples of radiation capable of causing ionization are X rays and Ultraviolet light
  23. 23. Sec 11.7 Chemical Effects The radiation can strike the atom and cause ionization leading to an ion pair Fig 11.7 Page 277
  24. 24. Sec 11.7 Chemical Effects Usually the ion pair formation is accompanied by the formation of a free radical  A free radical is a molecule or ion that has an unpaired electron, note that this is not common with normal molecules  Free radicals are dangerous and pose problems to cellular activity
  25. 25. Sec 11.8 Biochemical Effects The three main types of radioactive particles (alpha, beta, gamma) have different amounts of penetrating power  An alpha particle is slow and normally do not penetrate the skin (ie stopped by a sheet of paper)  The primary danger from alpha particles arises from ingesting a substance that emits alpha particles
  26. 26. Sec 11.8 Biochemical Effects  Beta particles are more penetrating than alpha particles and can be stopped by a thick sheet of aluminum  Prolonged exposure to beta particles can cause harm, and once again ingesting a substance that emits beta radiation is harmful  Gamma radiation is the highest penetration of the three types and readily passes through the skin into tissues and organs  Gamma radiation can be stopped by thick lead
  27. 27. Sec 11.8 Biochemical Effects Figure 11.8 Page 279
  28. 28. Sec 11.9 Detection of Radiation Low levels of radiation cannot be felt, tasted, heard, seen, or smelled  However, there are methods to detect radiation levels, most famous is the Geiger Counter  The Geiger counter is relatively portable and can display the levels of radiation  Another way to detect radiation is by the use of photographic film that will darken when exposed
  29. 29. Sec 11.10 Sources of Radiation Most sources of radiation are not the high energy dangerous sources referred to previously  Humans are exposed to natural low level dosages of radiation on a daily basis from the world around us  The levels of these radiation sources are much smaller than those generally thought to cause the health issues of radiation sickness
  30. 30. Sec 11.10 Sources of Radiation Figure 11.11 Page 281
  31. 31. Sec 11.11 Nuclear Medicine There are two main classes of uses of radiation in medicine:  Diagnosis  Therapy
  32. 32. Sec 11.11 Nuclear Medicine Diagnosis – radioactive isotopes are used to create an image of target tissues Medical Imaging requires three things:  Radioactive element that goes into the tissue to be imaged  Detection and mapping of the tissue to see concentration levels  Computer to translate the detection map into a visual image
  33. 33. Sec 11.11 Nuclear Medicine Not all radioactive materials are suitable choices for use in medicine Some criteria used are:  Detectable at low concentrations  Short half-life to limit the time of exposure  The radioactive material must have a known mechanism for elimination from the body  The chemical properties must be mostly compatible with normal body biochemistry. It should be selective for the desired body tissue
  34. 34. Sec 11.11 Nuclear Medicine Common choices for medical imaging Table 11.4 Page 284
  35. 35. Sec 11.11 Nuclear Medicine Alternately, sometimes radioactive isotopes are used in therapy to selectively destroy diseased tissue The radiation kills both cancer and normal tissue but the cancer cells are more effected because they are faster dividing. This is why people often have hair loss or stomach problems, fast dividing cells
  36. 36. Sec 11.11 Nuclear Medicine Common Choices for Therapy Table 11.5 Page 284
  37. 37. Sec 11.12 Fission and Fusion Fission – nuclear fission is the opposite of fusion and involves causing an element to fragment into other elements, which also can release energy. Example:
  38. 38. Sec 11.12 Fission and Fusion Fission reactions when controlled can be used to create atomic energy in power plants (“nuclear power”) Fission reactions when uncontrolled can be used in atomic weapons or nuclear explosions.
  39. 39. Sec 11.12 Fission and Fusion Example of a Chain Reaction of Uranium Figure 11.14 Page 285
  40. 40. Sec 11.12 Fission and Fusion Fusion – nuclear fusion is the process of smaller elements colliding and forming a larger element, which gives off a large amount of energy Example:
  41. 41. Sec 11.12 Fission and Fusion Fusion reactions are occurring in the sun and stars, giving off tremendous amounts of energy Fusion reactions are also responsible for the hydrogen bomb The elements that are “man-made” were discovered by controlled fusion reactions
  42. 42. Sec 11.13 Comparison of Reactions Table 11.6 Outlines the differences between chemical and nuclear reactions
  43. 43. Problems Assigned problems from pages 289 - 292  11.5, 11.9, 11.11, 11.15  11.19, 11.23, 11.26, 11.36, 11.41, 11.42  11.52, 11.53, 11.55, 11.63  Practice Test page 292