1. Nuclear Chemistry Chapter 21

2. Stable vs. Unstable Nuclei 1.Most nuclei are stable – do not change 2.Some nuclei are unstable (radioactive) Change into a different nucleus Spontaneous process – happens naturally, by itself Releases radiation Only nuclear reactions can change a nucleus. No chemical process can

3. Radium  Radon + Radiation 1.The radium was unstable (radioactive) 2.Turned into a different element (decayed) 3.The lost mass was turned into radiation

4. Nuclear Radiation Is spontaneously emitted from a radioactive nucleus Can not be seen, smelled, heard Can be detected using a Geiger counter or photographic film

5. Uses of Radiation 1.Nuclear fuel ( 235 U and 239 Pu) 2.Nuclear Weapons 3.Irradiated Food 4.Smoke Alarms (Amercium-241) 5.Cancer treatment (Cobalt-60) 6.Medical Tracers

8. Types of Nuclear Radiation Alpha particle  ( 4 2 He) Helium nucleus Beta particle  ( 0 -1 e) fast-moving electron. Gamma rays     high energy form of electromagnetic radiation 2 p + 2 n e-e-

9. Light RadioRadar MicroIR Visible Light UVX- rays Gamma The Electromagnetic Spectrum Safe radiation (non-ionizing) Dangerous (ionizing) Produced by nuclear decay

10. What Stops Radiation Paper Al Foil Wood Lead. Iron, Concrete Alpha (  ) Beta (  ) Gamma (  )

11. Decay Equations Alpha Decay 238 92 U  4 2 He + 234 90 Th Beta Decay 234 90 Th  0 -1 e + 234 91 Pa

12. Decay Equations Gamma Decay Occurs with alpha and beta decay No change in atomic mass (gamma radiation has no mass 0 0  )

13. Decay: Ex 1 What product is formed when radium-226 undergoes alpha decay? 226 88 Ra  4 2 He +

14. Decay: Ex 2 What element undergoes alpha decay to form lead-208?  4 2 He + 208 82 Pb

15. Decay: Ex 3 What isotope is produced when thorium-231 beta decays? 231 90 Th  0 -1 e +

16. Positron Emission –Same mass an electron, but opposite charge –Form of anti-matter 0 1 e Electron Capture –Nucleus captures a core electron –electron is added rather than lost

17. Common Particles ParticleSymbol Alpha 4 2 He Beta 0 -1 e Positron 01e01e Electron 0 -1 e Proton 1 1 H or 1 1 p Neutron 10n10n

18. Decay: Ex 4 Write the equation that describes oxygen-15 undergoing positron emission. Write the equation that describes mercury-201 undergoing electron capture

19. Which nuclei are radioactive (unstable) 1.All elements have at least one radioactive isotope 2.All isotopes of elements heavier than Lead (element 82) are radioactive 3.All elements heavier than 92 (U) are man- made and radioactive 82 Pb 207.2 At least one radioactive isotope All isotopes are radioactive

21. Belt of stability – based on neutron:proton ratio –Below ~20 = 1:1 ratio stable –Ratio increases with increasing # protons –Isotopes outside the belt try to decay and get on the belt

22. Decay Modes Atomic # >84 –Alpha Decay Above belt –Too many neutrons –Beta emission Below belt –Too few neutrons –electron capture or positron emission

24. Most heavy isotopes (above 84) decay by alpha emission Slide down to lead- 206

25. Decay Modes: Ex 1 Predict the decay mode for carbon-14 8n : 6pToo many n’s, prefers 1:1 14 6 C 

26. Decay Modes: Ex 2 Predict the decay mode for xenon-118 64n : 54p=1.2Too few n’s (check graph) 118 54 Xe Or 118 54 Xe

27. Decay Modes: Ex 3 Predict the decay mode for plutonium-239 Predict the decay mode for indium-120

28. Further Observations Magic #’s - Nuclei with 2, 8, 20, 28, 50 or 82 protons or 2, 8, 20, 28, 50 or 126 neutrons are especially stable. Nuclei with even #s of both protons and neutrons are more stable than those with odds numbers. Ex: 63 Cu and 65 Cu are abundant, but 64 Cu is not. Why?

29. Transmutation Rutherford(1919) – First successful alchemist 14 7 N + 4 2 He  17 8 O + 1 1 H 14 7 N(  p) 17 8 O Modern methods –Particle Accelerators (Cyclotrons) –Use neutrons or other elements (creation of transuranium elements)

30. Transmutation: Ex 1 Write the balanced nuclear equations for the process : 27 13 Al(n,  ) 24 11 Na

31. Transmutation: Ex 2 Write the shorthand notation for: 16 8 O + 1 1 H  13 7 N + 4 2 He

32. Transmutation: Neutrons Neutrons produced from radioactive decay Cobalt-60 is used in radiation therapy 58 26 Fe + 1 0 n  59 26 Fe 59 26 Fe  59 27 Co + 0 -1 e 59 27 Co + 1 0 n  60 27 Co

33. Transmutation: Transuranium Elements 238 92 U + 1 0 n  239 92 U  239 93 Np + 0 -1 e 239 94 Pu + 4 2 He  242 96 Cm + 1 0 n 209 83 Bi + 64 28 Ni  272 111 Rg + 1 0 n

34. Half-Life Half-life - The time during which one-half of a radioactive sample decays –Ranges from fraction of a second to billions of years. –You can’t hurry half-life.

35. Half-Life IsotopeHalf-life Uranium-2384.51x10 9 years Lead-21020.4 years Polonium-2141.6x10 -4 seconds The polonium-214 will decay much sooner than the uranium. The uranium will be radioactive pretty much until the earth is destroyed when our sun goes out in 10 billion years.

36. Carbon-14 dating Upon death, 14 C radioactively decays. (half-life = 5730 y) Reasonable to up to 50,000 years. 15% margin of error Mummies, the Dead Sea Scrolls, Shroud of Turin

38. Half-life: Example 1 Carbon-14 has a half-life of 5730 years and is used to date artifacts. How much of a 26 g sample will exist after 3 half-lives? How long is that?

39. Half-life: Example 2 Tritium undergoes beta decay and has a half life of 12.33 years. How much of a 3.0 g sample of tritium remains after 2 half-lives?

40. Half-life: Example 3 Radon-226 has a half-life of 1600 years? How much of a 30 gram sample remains after 6400 years?

41. Half-life: Example 4 Cesium-137 has a half-life of 30 years. If you start with a 200 gram sample, and you now have 25 grams left, how much time has passed?

42. Half-life: Example 5 Calcium-45 has a half-life of 160 days. If you start with a 500 gram sample, and you now have 31.25 grams left, how much time has passed?

43. Rate Law First order rate law Rate = kN (N is the initial concentration) Rate = -  N=dN= -kN  tdt dN= -kN dt dN= -kdt N

44. ∫dN = ∫-kdt N ∫dN = -k∫dt(Integrate left from N 0 to N t N and time from 0 to t) lnN t = -ktorN t = N o e -kt N 0

45. Calculating k or the half-life lnN t = -kt N 0 ln1 = -kt ½ 2 k = 0.693 t ½

46. Rate Law: Ex 1 Uranium-238 has a half-life of 4.5 X 10 9 yr. If 1.000 mg of a 1.257 mg sample of uranium- 238 remains, how old is the sample? k = 0.693 t ½ k = 0.693=1.5 x10 -10 yr -1 4.5 X 10 9 yr

47. lnN t = -kt N 0 ln 1.000 = -(1.5 x10 -10 yr)t 1.257 t = 1.5 X 10 9 yr

48. Rate Law: Ex 2 A wooden object is found to have a carbon-14 activity of 11.6 disintegrations per second. Fresh wood has 15.2 disintegrations per second. If the half-life of 14 C is 5730 yr, how old is the object?

49. Rate Law: Ex 2 A wooden object is found to have a carbon-14 activity of 11.6 disintegrations per second. Fresh wood has 15.2 disintegrations per second. If the half-life of 14 C is 5730 yr, how old is the object? ANS: 2230 yr

50. Rate Law: Ex 3 After 2.00 yr, 0.953 g of a 1.000 g sample of strontium-90 remains. How much remains after 5.00 years? x =0.887 g

51. Ex 4 A sample for medical imaging contains 18 F (1/2 life = 110 minutes). What percentage of the original sample remains after 300 minutes? ANS: 15.1%

52. E = mc 2 Energy changes in chemical reactions –Exothermic – gives off energy, products mass less than reactants –Endothermic – absorbs energy, products mass more than reactants –THESE MASS CHANGES ARE WAY TOO SMALL TO MEASURE

53. Energy Changes in nuclear decay –Mass loss from nuclei –Energy always released –This energy is additional kinetic energy given to the products (products move faster than reactants) c = 3.00 X 10 8 m/s

55. E = mc 2 : Ex1 238 92 U  234 90 Th+ 4 2 He 238.0003 amu233.9942 amu4.0015amu 238.0003 amu237.9957 amu  m = -0.0046 g/mol = -4.6 X 10 -6 kg/mol E = mc 2 E = (4.6 X 10 -6 kg/mol)(3.00X10 8 m/s) 2 E = 4.1 X 10 11 J/mol (can power a 60-W light bulb for 217 years)

56. E = mc 2 : Ex 2 Calculate the energy released from the following decay. 60 27 Co  0 -1 e+ 60 28 Ni 60 27 Co59.933819 amu 0 -1 e0.00054858 amu 60 28 Ni59.930788 amu ANS: 2.23 X 10 11 J/mol

57. E = mc 2 : Ex 3 The following decay produces 2.87 X 10 11 J/mol of 11 6 C. What is the mass change in this decay? 11 6 C  11 5 B+ 0 1 e ANS: -3.19 X 10 -3 g/mol

58. Binding Energy The mass of nuclei are ALWAYS less than the masses of individual protons and neutrons (nucleons). Mass defect

60. Nuclear Binding Energy – energy needed to separate nucleus into p & n –The larger the binding energy, the more stable the isotope –Iron-56 has the highest binding energy –Stars only make up to Iron-56 (unless supernova)

61. The Four Forces ForceRangeDescription Strong Nuclear ForceShort Range (nucleus) Strongest, holds nucleus together (gluons) ElectromagneticInfinite Range Between positive and negative charges (virtual photons) Weak Nuclear ForceShort Range (nucleus) Involved in some nuclear decay and fusion(quark to quark transmutations, J particle) GravityInfinite Range Weakest, between any object with mass, even dark matter (gravitons)

62. Strong Nuclear Force –Short-range force – operates only within nuclear distances –Force between p and n that overcomes proton- to-proton repulsion

63. Binding Energy: Ex 1 Calculate the binding energy for a helium-4 nucleus given the following information: 4 2 He4.00150 amu proton1.00728 amu neutron1.00866 amu

64. Mass of individual nucleons protons2(1.00728 amu)2.01456 amu neutrons2(1.00866 amu)2.01732 amu total4.03188 amu Mass defect 4.03188 amu -4.00150 amu 0.03038 amu

65. Mass defect = 0.03038 g/mol 0.03038 g 1 kg1 mol 1mol 1000 g6.022X10 23 atoms = 5.045 X 10 -29 kg/atom E=mc 2 E = (5.045 X 10 -29 kg/atom)(3.00 X 10 8 m/s) 2

66. E = 4.534 X10 -12 J/atom or E = 4.534 X 10 -12 J/ 4 nucleons E = 1.13X10 -12 J/nucleon

67. Binding Energy: Ex 2 Calculate the binding energy for an iron-56 nucleus given the following information: 56 26 Fe55.92068 amu proton1.00728 amu neutron1.00866 amu ANS: 1.41 X 10 -12 J/nucleon

69. Fission: Chain Reaction Must absorb some of those neutrons or fission continues unchecked (explosion?)

70. Uranium Fuel Rods Control Rods Moderator (water) Turbine Steam

72. Nuclear Fission Power Uses 235 U First commercial nuclear power - 1957 at Shippingport, PA People living near a nuclear power plant = 1/10 radiation of a coast-to-coast jet plane trip (cosmic radiation). Three-Mile Island (1979) - partial meltdown due. No fatalities, no serious release of radiation. Chernobyl, Ukraine (1986) – full meltdown. 31 deaths, 260,000 exposed to high levels of radiation.

73. Nuclear Fission: Bombs Nuclear bombs (uranium or plutonium) Critical Mass – minimum mass required for a chain reaction –Subcritical mass –Critical mass (1 kg)

77. Fusion Fusion: Combining 2 nuclei of lighter element Thermonuclear fusion occurs at high temperatures like in the sun (3 to 40 million K). –657 million tons of hydrogen is fused to 653 million tons of helium each second –Energy released = sunlight Not yet feasible for commercial reactors

79. Sources of Exposure to Radiation Natural Exposure (~80%) 1.The atmosphere (Radon and carbon-14) 2.Particles that come from outer space 3.Rocks, soil and bricks (Uranium and Thorium) 4.Foods (carbon-14)

80. Technological Sources (~20%) 1.Nuclear weapons testing 2.High-altitude plane flights 3.X-rays (even though they are not alpha, beta or gamma) 4.Fossil fuel and nuclear electrical generation 5.Disturbances in rocks from mining, building 6.Smoking (VERY high levels)

82. Measuring Exposure to Radiation 1.Units rad – total exposure rem – [roentgen equivalent man] – total damaging exposure millirem (mrem) – 1/1000 th of a rem 2.mrem is the unit used to measure possible damage to human tissue. 3.U.S. Average = 360 mrem/year

83. Ionizing Radiation UV light and X-rays  and  from nuclear decay Produces “free radicals” Affects bone marrow, blood, lymph nodes

84. Danger of Radon 1.Radon-222 gas passes in and out of the lungs. 2.Produced by decay of radium-226 from rocks, soil, and building materials. 3.Radon has a half-life of 3.825 days and decays into solid polonium-218. 4.Polonium-218 emits alpha particles which can damage lung tissue.

85. 222 86 Rn  218 84 Po + 4 2 He 218 84 Po  214 82 Pb + 4 2 He

88. 12.a) 191 79 Au + 0 -1 e  191 78 Pt b) 201 79 Au  201 80 Hg + 0 -1 e c) 198 79 Au  198 80 Hg + 0 -1 e d) 188 79 Au  188 78 Pt + 0 1 e 14. a) 24 11 Na  24 12 Mg + 0 -1 e b) 188 80 Hg  188 79 Au + 0 1 e c) 122 53 I  122 54 Xe + 0 -1 e d) 242 94 Pu  238 92 U + 4 2 He

89. 18.a) Positron emission, electron capture b) Beta c) Beta d) Positron emission, electron capture 20.a) Even, even – more abundant b) odd, even – more abundant c) even, even – more abundant d) even, even – more abundant

90. 28.a) 32 15 Pb) 7 3 Lic) 187 75 Re d) 99 43 Tce) 99 38 Sr 34. 2.6 min 36.85 d 40. 3520 y 46. 1.6143 X 10 13 J/mol 48.a) 1.20 X 10 -12 J/nucleon b) 1.40 X 10 -12 J/nucleon c) 1.35 X 10 -12 J/nucleon 50.a) -1.697 X 10 12 J/mol b) -3.13 X 10 11 J/mol c) -1.773 X 10 12 J/mol