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Chapter 19 Radioactivity and Nuclear Chemistry

Chapter 19 Radioactivity and Nuclear Chemistry

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Chapter 19 Radioactivity and Nuclear Chemistry

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  1. Chemistry: A Molecular Approach, 2nd Ed.NivaldoTro Chapter 19Radioactivity and Nuclear Chemistry Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

  2. Nuclear Medicine • Changes in the structure of the nucleus are used in many ways in medicine • Nuclear radiation can be used to visualize or test structures in your body to see if they are operating properly • e.g. labeling atoms so their intake and output can be monitored • Nuclear radiation can also be used to treat diseases because the radiation is ionizing, allowing it to attack unhealthy tissue Tro: Chemistry: A Molecular Approach

  3. The Discovery of Radioactivity • Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-rays • phosphorescence is the long-lived emission of light by atoms or molecules that sometimes occurs after they absorb light • X-rays are detected by their ability to penetrate matter and expose a photographic plate Tro: Chemistry: A Molecular Approach

  4. The Discovery of Radioactivity • Becquerel discovered that certain minerals were constantly producing energy rays that could penetrate matter • Becquerel determined that 1. all the minerals that produced these rays contained uranium 2. the rays were produced even though the mineral was not exposed to outside energy • He called them uranic raysbecause they were emitted from minerals that contained uranium • like X-rays • but not related to phosphorescence • Energy apparently being produced without energy input?? Tro: Chemistry: A Molecular Approach

  5. The Curies • Marie Curie broke down these minerals and used an electroscope to detect where the uranic rays were coming from • She discovered the rays were emitted from specific elements • She also discovered new elements by detecting their rays • radium named for its green phosphorescence • polonium named for her homeland • Because these rays were no longer just a property of uranium, she renamed it radioactivity Tro: Chemistry: A Molecular Approach

  6. When charged, the metal foils spread apart due to like charge repulsion When exposed to ionizing radiation, the radiation knocks electrons off the air molecules, which jump onto the foils and discharge them, allowing them to drop down +++ +++ Electroscope Tro: Chemistry: A Molecular Approach

  7. Other Properties of Radioactivity • Radioactive rays can ionize matter • cause uncharged matter to become charged • basis of Geiger Counter and electroscope • Radioactive rays have high energy • Radioactive rays can penetrate matter • Radioactive rays cause phosphorescent chemicals to glow • basis of scintillation counter Tro: Chemistry: A Molecular Approach

  8. What Is Radioactivity? • Radioactivity is the release of tiny, high-energy particles or gamma rays from an atom • Particles are ejected from the nucleus Tro: Chemistry: A Molecular Approach

  9. Types of Radioactive Decay • Rutherford discovered three types of rays • alpha (a) rays • have a charge of +2 c.u. and a mass of 4 amu • what we now know to be helium nucleus • beta (b) rays • have a charge of −1 c.u. and negligible mass • electron-like • gamma (g) rays • form of light energy (not a particle like a and b) • In addition, some unstable nuclei emit positrons • like a positively charged electron • Some unstable nuclei will undergo electron capture • a low energy electron is pulled into the nucleus Tro: Chemistry: A Molecular Approach

  10. g b a Rutherford’s Experiment ++++++++++++ -------------- Tro: Chemistry: A Molecular Approach

  11. a g b Penetrating Ability of Radioactive Rays 0.01 mm 1 mm 100 mm Pieces of Lead Tro: Chemistry: A Molecular Approach

  12. Facts About the Nucleus • Very small volume compared to volume of the atom • Essentially entire mass of atom • Very dense • Composed of protons and neutrons that are tightly held together • the particles that make up the nucleus are called nucleons Tro: Chemistry: A Molecular Approach

  13. Facts About the Nucleus • Every atom of an element has the same number of protons • atomic number (Z) • Atoms of the same elements can have different numbers of neutrons • isotopes • different atomic masses • Isotopes are identified by their mass number (A) • mass number = number of protons + neutrons Tro: Chemistry: A Molecular Approach

  14. Facts About the Nucleus • The number of neutrons is calculated by subtracting the atomic number from the mass number • The nucleus of an isotope is called a nuclide • less than 10% of the known nuclides are non-radioactive, most are radionuclides • Each nuclide is identified by a symbol • Element – Mass Number = X – A Tro: Chemistry: A Molecular Approach

  15. Radioactivity • Radioactive nuclei spontaneously decompose into smaller nuclei • radioactive decay • we say that radioactive nuclei are unstable • decomposing involves the nuclide emitting a particle and/or energy • The parent nuclide is the nucleus that is undergoing radioactive decay • The daughter nuclide is the new nucleus that is made • All nuclides with 84 or more protons are radioactive Tro: Chemistry: A Molecular Approach

  16. Important Atomic Symbols Tro: Chemistry: A Molecular Approach

  17. Transmutation • Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element – transmutation • showing that statement 3 of Dalton’s Atomic Theory is not valid all the time, only for chemical reactions • For one element to change into another, the number of protons in the nucleus must change! Tro: Chemistry: A Molecular Approach

  18. Chemical Processes vs. Nuclear Processes • Chemical reactions involve changes in the electronic structure of the atom • atoms gain, lose, or share electrons • no change in the nuclei occurs • Nuclear reactions involve changes in the structure of the nucleus • when the number of protons in the nucleus changes, the atom becomes a different element Tro: Chemistry: A Molecular Approach

  19. Nuclear Equations • We describe nuclear processes with nuclear equations • Use the symbol of the nuclide to represent the nucleus • Atomic numbers and mass numbers are conserved • use this fact to predict the daughter nuclide if you know parent and emitted particle Tro: Chemistry: A Molecular Approach

  20. Alpha Emission • An  particle contains 2 protons and 2 neutrons • helium nucleus • Most ionizing, but least penetrating of the types of radioactivity • Loss of an alpha particle means • atomic number decreases by 2 • mass number decreases by 4 Tro: Chemistry: A Molecular Approach

  21. Tro: Chemistry: A Molecular Approach

  22. Beta Emission • A  particle is like an electron • moving much faster • produced from the nucleus • About 10 times more penetrating than a, but only about half the ionizing ability • When an atom loses a  particle its • atomic number increases by 1 • mass number remains the same • In beta decay, a neutron changes into a proton Tro: Chemistry: A Molecular Approach

  23. Tro: Chemistry: A Molecular Approach

  24. Gamma Emission • Gamma (g) rays are high energy photons of light • No loss of particles from the nucleus • No change in the composition of the nucleus • same atomic number and mass number • Least ionizing, but most penetrating • Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange Tro: Chemistry: A Molecular Approach

  25. Positron Emission • Positron has a charge of +1 c.u. and negligible mass • anti-electron • Similar to beta particles in their ionizing and penetrating ability • When an atom loses a positron from the nucleus, its • mass number remains the same • atomic number decreases by 1 • Positrons result from a proton changing into a neutron Tro: Chemistry: A Molecular Approach

  26. Tro: Chemistry: A Molecular Approach

  27. Electron Capture • Occurs when an inner orbital electron is pulled into the nucleus • No particle emission, but atom changes • same result as positron emission • Proton combines with the electron to make a neutron • mass number stays the same • atomic number decreases by one Tro: Chemistry: A Molecular Approach

  28. Particle Changes Tro: Chemistry: A Molecular Approach

  29. Tro: Chemistry: A Molecular Approach

  30. What Kind of Decay and How Many Protons and Neutrons Are in the Daughter? ? + = proton 11 p+ 9 n0 = neutron Alpha emission giving a daughter nuclide with nine protons and seven neutrons Tro: Chemistry: A Molecular Approach

  31. What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued ? + = proton 9 p+ 12 n0 = neutron = electron Beta emission giving a daughter nuclide with 10 protons and 11 neutrons Tro: Chemistry: A Molecular Approach

  32. What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued ? + = proton 5 p+ 4 n0 = neutron = positron Positron emission giving a daughter nuclide with four protons and five neutrons Tro: Chemistry: A Molecular Approach

  33. Nuclear Equations • In the nuclear equation, mass numbers and atomic numbers are conserved • We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay Tro: Chemistry: A Molecular Approach

  34. Example 19.2b: Write the nuclear equation for positron emission from K–40 1. Write the nuclide symbols for both the starting radionuclide and the particle Tro: Chemistry: A Molecular Approach

  35. Examle 19.2b: Write the nuclear equation for positron emission from K–40 2. Set up the equation • emitted particles are products • captured particles are reactants Tro: Chemistry: A Molecular Approach

  36. Example 19.2b: Write the nuclear equation for positron emission from K–40 3. Determine the mass number and atomic number of the missing nuclide • mass and atomic numbers are conserved Tro: Chemistry: A Molecular Approach

  37. Example 19.2b: Write the nuclear equation for positron emission from K–40 4. Identify and determine the symbol of the element from the atomic number Tro: Chemistry: A Molecular Approach

  38. Practice – Write a nuclear equation for each of the following alpha emission from U–238 beta emission from Ne–24 positron emission from N–13 electron capture by Be–7 Tro: Chemistry: A Molecular Approach

  39. What Causes Nuclei to Decompose? • The particles in the nucleus are held together by a very strong attractive force only found in the nucleus called the strong force • acts only over very short distances • The neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but don’t repel each other like the protons do Tro: Chemistry: A Molecular Approach

  40. N/Z Ratio • The ratio of neutrons : protons is an important measure of the stability of the nucleus • If the N/Z ratio is too high, neutrons are converted to protons via b decay • If the N/Z ratio is too low, protons are converted to neutrons via positron emission or electron capture • or via a decay – though not as efficiently Tro: Chemistry: A Molecular Approach

  41. Valley of Stability for Z = 1  20, stable N/Z ≈ 1 for Z = 20  40, stable N/Z approaches 1.25 for Z = 40  80, stable N/Z approaches 1.5 for Z > 83, there are no stable nuclei Tro: Chemistry: A Molecular Approach

  42. Example 19.3b: Predict the kind of radioactive decay that Mg−22 undergoes • Mg–22 • Z = 12 • N = 22 – 12 = 10 • N/Z = 10/12 = 0.83 • From Z = 1  20, stable nuclei have N/Z ≈ 1 • Because Mg–22 N/Z is low, it should convert p+into n0, therefore it will undergo positron emission or electron capture Tro: Chemistry: A Molecular Approach

  43. Practice – Predict whether Kr–85 is stable or radioactive. If radioactive, predict the mode of radioactive decay and the daughter nuclide. Tro: Chemistry: A Molecular Approach

  44. Practice – Predict whether Kr–85 is stable or radioactive. If radioactive, predict the mode of radioactive decay and the daughter nuclide. Kr–85 has Z = 36 and N = (85 − 36) = 49 Because most stable isotopes with Z between 20 and 40 have N/Z ratios between 1 and 1.25, we expect Kr–85 to be radioactive Because the N/Z ratio of Kr–85 is greater than 1.25, it has too many neutrons and will undergo b decay to reduce them (Kr–85 is a byproduct of nuclear fission and was released into the atmosphere when atom bombs were tested in the 1940’s to 1960’s) Tro: Chemistry: A Molecular Approach

  45. Magic Numbers • Besides the N/Z ratio, the actual numbers of protons and neutrons affects stability • Most stable nuclei have even numbers of protons and neutrons • Only a few have odd numbers of protons and neutrons • If the total number of nucleons adds to a magic number, the nucleus is more stable • same principle as stability of the noble gas electron configuration • most stable when N or Z = 2, 8, 20, 28, 50, 82; or N = 126 Tro: Chemistry: A Molecular Approach

  46. Decay Series • In nature, often one radioactive nuclide changes into another radioactive nuclide • i.e. the daughter nuclide is also radioactive • All of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay series • To determine the stable nuclide at the end of the series without writing it all out • count the number of a and b decays • from the mass no. subtract 4 for each a decay • from the atomic no. subtract 2 for each a decay and add 1 for each b Tro: Chemistry: A Molecular Approach

  47. U-238 Decay Series a b b a a a a or b a b a b b a a b a b b a b or other combinations Tro: Chemistry: A Molecular Approach

  48. Detecting Radioactivity To detect something, you need to identify what it does • Radioactive rays can expose light-protected photographic film • We may use photographic film to detect the presence of radioactive rays – film badge dosimeters Tro: Chemistry: A Molecular Approach

  49. Detecting Radioactivity • Radioactive rays cause air to become ionized • An electroscope detects radiation by its ability to penetrate the flask and ionize the air inside • AGeiger-Müller counterworks by counting electrons generated when Ar gas atoms are ionized by radioactive rays Tro: Chemistry: A Molecular Approach

  50. Detecting Radioactivity • Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical • A scintillation counteris able to count the number of flashes per minute Tro: Chemistry: A Molecular Approach