Structure and Reactions of Atomic Nucleus.pptx

Structure and Reactions of Atomic Nucleus

1. Structure and Reactions of Atomic Nucleus

2. What are elementary particles? 3 Does not have a sub structure, Can not be split

3. Some Masses in Various Units 4

4. Structure of the nucleons 5

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6. 7 Fig. 1: An oversimplified vision of protons as made from two up quarks and a down quark, and neutrons as made from two down quarks and an up quark — and nothing else. Fig. 2: This figure improves on Figure 1 by emphasizing the important role of the strong nuclear force in holding the quarks in the proton. Usually (and confusingly) the drawn springs are intended to schematically indicate that there are gluons in the proton.

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9. 10 Electrons and quarks contain no discernible structure; they cannot be reduced or separated into smaller components. It is therefore reasonable to call them “ elementary ” particles, a name that in the past was mistakenly given to particles such as the proton, which is in fact a complex particle that contains quarks. The term  subatomic particle  refers both to the true elementary particles, such as quarks and electrons, and to the larger particles that quarks form.

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11. Nuclear Reactions 13

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13. 15 R adioactive emissions result in the change of one element into another when a nuclide of one element decays, it emits radiation and usually changes into a nuclide of a different element

14. 16 Nuclear stability and radioactive d ecay ( will be discussed in detail later)

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17. 23 N uclide N ucleus containing a specified number of protons and neutrons R adionuclides Nuclides that are radioactive R adioisotopes A toms containing these nuclei

18. Radioactivity 24 Many nuclides are unstable and spontaneously emit radiation , a process termed radioactive decay . - The intensity of the radiation is not affected by temperature, pressure, or other physical and chemical conditions. When a nuclide decays , it emits radiation and usually changes into a nuclide of a different element . There are three natural types of radioactive emission:

19. Types of Radioactive Emissions 25 Beta particles ( β , β - , or ) are high-speed electrons. β -1 Alpha particles ( α , , or ) are identical to helium-4 nuclei. α 4 2 He 2+ 4 2 Gamma rays (  or ) are very high-energy photons. 

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22. Nuclear Equations 28 When a nuclide decays, it becomes a nuclide of lower energy, and the excess energy is carried off by the emitted radiation and the recoiling nucleus The decay process is represented by a balanced nuclear equation. Both the total charge and the total mass must be balanced: Reactants = Products Total A Total Z Total A Total Z

23. Particles involved in nuclear reactions

24. Modes of Radioactive Decay 32 Alpha (α) decay involves the loss of an α particle from the nucleus. For each α particle emitted, A decreases by 4 and Z decreases by 2 in the daughter nuclide. This is the most common form of decay for a heavy, unstable nucleus. 226 222 4 88 86 2 Ra → Rn + α

25. 33 Beta ( β ) decay is a general class of radioactive decay that encompasses three modes: β − decay, β + emission, and electron capture. β - decay involves the ejection of a β - particle from the nucleus. A neutron is converted to a proton , which remains in the nucleus, and a β - particle is expelled: A remains the same in the daughter nuclide but Z increases by 1 unit. n → p + β 1 1 1 -1 63 63 0 28 29 -1 Ni → Cu + β

26. Positron ( β + ) emission is the emission of a β + particle from the nucleus. The positron is the antiparticle of the electron. A proton in the nucleus is converted into a neutron , and a positron is emitted: A remains the same in the daughter nuclide but Z increases by 1 unit. Electron capture occurs when the nucleus interacts with an electron in a low atomic energy level. A proton is transformed into a neutron : The effect on A and Z is the same as for positron emission. p → n + β 1 1 1 1 11 11 0 6 5 1 C → B + β p + e → n 1 1 -1 1 34

27. 35 Gamma (  ) emission involves the radiation of high-energy  photons. Gamma emission usually occurs together with other forms of radioactive decay. Several  photons of different energies can be emitted from an excited nucleus as it returns to the ground state.  emission results in no change in either A or Z since γ rays have no mass or charge.

28. 38 Modes of Radioactive Decay *

29. 39 Modes of Radioactive Decay *

30. 40 Summary

31. 41 Alpha Particle ( α) Beta Particle ( β) Gamma Radiation ( γ) In addition to the three major types of radioactive particles listed above, two additional less common types of emissions have been discovered.  These include  positron emission  and  electron capture . Positron emission ( β +   decay ) Electron capture

32. 42 Sample Problem 1 Writing Equations for Nuclear Reactions PROBLEM: Write balanced equations for the following nuclear reactions: (a) Naturally occurring thorium-232 undergoes α decay. (b) Zirconium-86 undergoes electron capture. PLAN: We first write a skeleton equation that includes the mass numbers, atomic numbers, and symbols of all the particles on the correct sides of the equation, showing the unknown product particle as X. Then, because the total of mass numbers and the total of charges on the left side and the right side of the equation must be equal, we solve for A and Z , and use Z to determine the identity of X from the periodic table. A Z

33. 43 Sample Problem 1 Writing Equations for Nuclear Reactions SOLUTION: (a) Writing the skeleton equation, with the α particle as a product: 232 A 4 90 Z 2 Th → X + α For A , 232 = A + 4, so A = 228. For Z, 90 = Z + 2, so Z = 88. The daughter nuclide produced in this reaction is Ra. (b) Writing the skeleton equation, with the captured electron as a reactant: 86 0 A 40 -1 Z Zr + e → X For A , 86+ 0 = A so A = 86. For Z, 40 -1 = Z so Z = 39. Element X is yttrium, symbol Y. 232 228 4 90 88 2 Th → Ra + α 86 0 86 40 -1 39 Zr + e → Y

34. Check Always read across superscripts and then across subscripts, with the yield arrow as an equal sign, to check your arithmetic. In part (a), for example, 232 = 228 + 4, and 90 = 88 + 2. 44

35. 45 Sample Problem 2 Which of the following produces a particle? electron capture positron alpha particle beta particle

36. 46 Sample Problem 3 Write the nuclear equation that represents the radioactive decay of radon-222 by alpha particle emission and identify the daughter isotope. SOLUTION:

37. 47 Sample Problem 4 Write the nuclear equation that represents the radioactive decay of boron-12 by beta particle emission and identify the daughter isotope. A gamma ray is emitted simultaneously with the beta particle. SOLUTION:

38. 48 Sample Problem 5 Write the nuclear equation that represents the radioactive decay of iodine-131 by beta particle emission and identify the daughter isotope. A gamma ray is emitted simultaneously with the beta particle. SOLUTION:

39. 49 Tutorial 1

40. Penetrating Power of Radiation 50 Alpha, beta, and gamma emissions have different abilities to penetrate matter.

41. 51 These extremely high energy photons can travel through most forms of matter because they have no mass.

42. Nuclear Stability 52 Many nuclei are radioactive; that is, they decompose, forming another nucleus and producing one or more particles. Example, carbon-14:

43. Nuclear Stability 53 Many nuclei are radioactive; that is, they decompose, forming another nucleus and producing one or more particles. Example, carbon-14: Can we predict how, and whether, an unstable nuclide will decay? Our knowledge of the nucleus is much less complete than that of the whole atom, but some patterns emerge by observing naturally occurring nuclides.

44. All stable nuclei except the hydrogen-1 nucleus ( 1 H) contain at least one neutron to overcome the electrostatic repulsion between protons. As the number of protons in the nucleus increases, the number of neutrons needed for a stable nucleus increases even more rapidly. Too many protons (or too few neutrons) in the nucleus result in an imbalance between forces, which leads to nuclear instability.

45. 58 The Band of Stability Two key factors determine the stability of a nuclide: The number of neutrons ( N ) , the number of protons ( Z ) , and their ratio (N/Z) , which we calculate from (A − Z)/Z . This factor relates primarily to nuclides that undergo one of the three modes of β decay. The total mass of the nuclide, which mostly relates to nuclides that undergo α decay . A plot of number of neutrons vs. number of protons for all stable nuclides produces a band of stability that gradually curves above the line for N = Z .

46. A plot of number of neutrons vs. number of protons for the stable nuclides 59 The points form a narrow band of stability that gradually curves above the line for N = Z ( N / Z = 1). The only stable nuclides with N / Z < 1 are

47. A plot of number of neutrons vs. number of protons for the stable nuclides 60 Light nuclei are most stable when N = Z . Heavy nuclei are most stable when N > Z. Above about Z = 20 As the number of protons increases, the Coulomb force increases and so more neutrons are needed to keep the nucleus stable. All nuclei with Z > 83 are unstable.

48. A plot of number of neutrons vs. number of protons for the stable nuclides 61 Light nuclei are most stable when N = Z . Heavy nuclei are most stable when N > Z. Above about Z = 20 As the number of protons increases, the Coulomb force increases and so more neutrons are needed to keep the nucleus stable. All nuclei with Z > 83 are unstable.

49. 63 n/p too large beta decay X n/p too small positron decay or electron capture Y

50. A plot of number of neutrons vs. number of protons for the stable nuclides 64

51. Predicting the Mode of Decay 65 An unstable nuclide generally decays in a mode that shifts its N / Z ratio toward the band of stability N / Z N / Z

52. 66 1. Neutron-rich nuclides Nuclides with too many neutrons for stability (a high N / Z ratio) lie above the band of stability. They undergo β − decay, which converts a neutron into a proton , thus reducing the value of N / Z . n → p + β 1 1 1 -1 Ex:

53. 67 2 . Proton-rich nuclides Nuclides with too many protons for stability (a low N/Z ratio) lie below the band of stability. They undergo β + emission and/or e − capture, both of which convert a proton into a neutron , thus increasing the value of N/Z. (The rate of e − capture increases with Z, so β + emission is more common among lighter elements and e − capture more common among heavier elements.) Ex: p + e → n 1 1 -1 1 p → n + β 1 1 1 1

54. 68 3 . Heavy nuclides Nuclides with Z > 83 are too heavy to be stable and undergo α decay , which reduces their Z and N values by two units per emission. Educated guess about mode of decay

55. The atomic mass of an element is the weighted average of its naturally occurring isotopes. The mass number A of a stable nuclide will be relatively close to the atomic mass. If an unstable nuclide of an element (given Z ) has an A value much higher than the atomic mass, it is neutron rich and will probably decay by β − emission. If, on the other hand, an unstable nuclide has an A value much lower than the atomic mass, it is proton rich and will probably decay by β + emission and/or e − capture. 69

56. Predicting the Mode of Decay Nuclide type Description Mode of Decay neutron-rich high N/Z β - decay neutron → proton, lowers N/Z proton-rich low N/Z β + emission or e - capture proton → neutron, increases N/Z heavy nuclides Z > 83 α decay reduces both Z and N Summary 71

57. 72 Ex:

58. Sample Problem 6 Predicting the Mode of Nuclear Decay PROBLEM: Predict the nature of the nuclear change(s) each of the following radioactive nuclides is likely to undergo: (a) B (b) U (c) As (d) La 12 234 81 127 5 92 33 57 PLAN: If the nuclide is too heavy to be stable ( Z > 83), it undergoes α decay. For other cases, we use the Z value to obtain its atomic mass from the periodic table. If the mass number of the nuclide is higher than the atomic mass, the nuclide has too many neutrons: N is too high and β - decay occurs. If the mass number is lower than the atomic mass, the nuclide has too many protons: Z is too high and the nuclide decays by β + emission or e - capture. 73

59. SOLUTION: (a) 12 B has Z = 5 and its atomic mass is 10.81. The nuclide’s A value of 12 is significantly higher than its atomic mass, so it is neutron rich. It will probably undergo β - decay . (b) 234 U has Z = 92, so it will undergo α decay and decrease its total mass. (c) 81 As has Z = 33 and its atomic mass is 74.92. The A value of 81 is much higher than the atomic mass, so it is neutron rich and will probably undergo β - decay . (d) 127 La has Z = 57 and its atomic mass is 138.9 The A value of 127 is much lower than the atomic mass, so it is proton rich and will probably undergo β + emission or e - capture . 74

60. 75 Ex:

61. Stability and Nuclear Structure 76 Nuclear structure is related to nuclear stability in several ways: The effect of the strong force. “Why does the nucleus stay together?” 76

62. Stability and Nuclear Structure 77 77 Protons within the nucleus experience electrostatic repulsive forces, which destabilize the nucleus. The strong force , which exists between all nucleons, counteracts the weaker repulsive forces. 77

63. Odd and even numbers of nucleons. - Elements with an even Z (number of protons) usually have a larger number of stable nuclides. - Over half the stable nuclides have both even N and even Z . Number of Stable Nuclides for Elements 48 to 54 Element Atomic No. ( Z ) No. of nuclides Cd 48 8 In 49 2 Sn 50 10 Sb 51 2 Te 52 8 I 53 1 Xe 54 9 78

64. 79 An Even-Odd Breakdown of the Stable Nuclides Z N No. of nuclides Even Even 157 Even Odd 53 Odd Even 50 Odd Odd 4 TOTAL 264

65. Nucleon energy levels. Nucleons are found in nucleon energy levels and pairing of the spins of like nucleons leads to greater stability. Certain numbers of neutrons and protons are extra stable N or Z = 2, 8, 20, 50, 82 and 126 Like extra stable numbers of electrons in noble gases (e - = 2, 10, 18, 36, 54 and 86) Magic numbers 80

66. no need to seriously think about this now 81

67. Sample Problem 7 Predicting Nuclear Stability PROBLEM: Which of the following nuclides would you predict to be stable and which radioactive? Explain. (a) Ne (b) S (c) Th (d) Ba 18 32 236 123 10 16 90 56 PLAN: In order to evaluate the stability of each nuclide, we determine the N and Z values and the N/Z ratio. We can then compare these to the values for stable nuclides. We also note whether Z and N are even or odd. SOLUTION: This nuclide has N = (18 – 10) = 8 and Z = 10, so the N/Z ratio is 18 – 10 10 = 0.8, which is too low to be stable. (a) 18 Ne is Radioactive . 82

68. This nuclide has N = Z = 16, so N/Z = 1.0. With Z < 20 and even values for N and Z , this nuclide is most likely stable. This nuclide has Z = 90, and every nuclide with Z < 83 is radioactive. This nuclide has N = 67 and Z = 56, so N/Z = 1.20. For Z values of 55 to 60, Figure 24.2A shows that N/Z ≥ 1.3, so this nuclide has too few neutrons to be stable (b) 32 S is stable . (c) 236 Th is radioactive . (d) 123 Ba is radioactive . 83

69. The 238 U decay series. A parent nuclide may undergo a series of decay steps before a stable daughter nuclide forms. The succession of steps is called a decay series. 84

70. The Interconversion of Mass and Energy 85

71. The Interconversion of Mass and Energy 86 The total quantity of mass-energy in the universe is constant. Any reaction that releases or absorbs energy also loses or gains mass . E = mc 2 or Δ E = Δ mc 2 so Δ m = Δ E c 2 Einstein’s equation: where Δ m is the mass difference between reactants and products:

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73. when 1 mol of water breaks up into its atoms, heat is absorbed: Ex: for all practical purposes, mass is conserved in chemical reactions. 90

74. In a chemical reaction, the energy changes in breaking or forming bonds is relatively small, so mass changes are negligible. In a nuclear reaction, the energy changes are enormous, and the mass changes are easily measurable. 91

75. Mass Difference in Nuclear Reactions 92 The mass of the nucleus is less than the combined masses of its nucleons. Mass always decreases when nucleons form a nucleus , and the “lost” mass is released as energy. - Energy is required to break a nucleus into individual nucleons. Read calculation on page 1116-Silberberg

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77. Two key points emerge from these calculations : The mass of the nucleus is less than the combined masses of its nucleons: there is always a mass decrease when nucleons form a nucleus. The mass change of this nuclear process (9.89×10 −2 g/mol ≈ 10.×10 −2 , or 0.10 g/mol) is nearly 10 million times what we found earlier for the chemical process of breaking the bonds in water (10.4×10 −9 g/mol). This could be measured easily on any laboratory balance. 95

78. Nuclear Binding Energy 96 The nuclear binding energy is the energy required to break 1 mol of nuclei into individual nucleons. - Binding energy is expressed using the electron volt ( eV ). Nucleus + nuclear binding energy → nucleons 1 amu = 931.5 x 10 6 eV = 931.5 MeV

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80. nuclear binding energy for carbon-12 98

81. We can compare the stability of nuclides of different elements by determining the binding energy per nucleon. For 12C, we have nuclear stability nuclear binding energy nucleon 99

82. Calculate the binding energy for the Helium atom  ( 4 2 He) Ex: 100

83. Sample Problem 8 Calculating the Binding Energy per Nucleon PROBLEM: Iron-56 is an extremely stable nuclide. Compute the binding energy per nucleon for 56 Fe and compare it with that for 12 C (mass of 56 Fe atom = 55.934939 amu; mass of 1 H atom = 1.007825 amu; mass of neutron = 1.008665 amu). PLAN: Iron-56 has 26 protons and 20 neutrons. We calculate the mass difference, Δ m , when the nucleus forms by subtracting the given mass of one 56 Fe atom from the sum of the masses of 26 1 H atoms and 30 neutrons. To find the binding energy per nucleon, we multiply Δ m by the equivalent in MeV and divide by the number of nucleons. SOLUTION: Δ m = [(26 x mass 1 H atom) + (30 x mass neutron)] – mass 56 Fe atom = [(26 x 1.007825 amu ) + (30 x 1.008665 amu )] - 55.934939 = 0.52856 amu Calculating the mass difference: 101

84. Binding energy per nucleon = 0.52846 amu x 931.5 MeV/amu 56 nucleons = 8.790 MeV/nucleon An 56 Fe nucleus would require more energy per nucleon to break up into its nucleons than would 12 C, so 56 Fe is more stable than 12 C . 102

85. The variation in binding energy per nucleon. The greater the binding energy per nucleon, the more stable the nuclide. 103

86. 104 Most of the nuclear processes we’ve considered so far involve radioactive decay, in which a nucleus emits one or a few small particles or photons to become a more stable and, usually, slightly lighter nucleus. Two other nuclear processes cause much greater mass changes: Both fission and fusion release enormous quantities of energy.

87. The variation in binding energy per nucleon. The greater the binding energy per nucleon, the more stable the nuclide. 105 two ways nuclides can increase their binding energy per nucleon:

88. 106 Fission or Fusion The binding energy per nucleon peaks at elements with mass number A ≈ 60. - Nuclides become more stable with increasing number up to around 60 nucleons, after which stability decreases. There are two ways nuclides can increase their binding energy per nucleon: A heavier nucleus can split into lighter ones by undergoing fission . Lighter nuclei can combine to form a heavier nucleus in a process called fusion .

89. Fission of 235 U caused by neutron bombardment. 107

90. 108 Nuclear Fission Nuclear fission involves the splitting of large nuclei into smaller nuclei, using neutron bombardment to start the process. Fission releases energy and generates more high-energy neutrons, which cause further fission to occur. The fission process becomes self-sustaining by a chain reaction . The mass required to achieve this is called the critical mass . The energy from nuclear fission can be harnessed and converted to other forms of energy.

91. 109

92. The mass required to achieve a chain reaction is called the critical mass . If the mass of fissionable material is too small, most of the neutrons escape before causing another fission event, and the process dies out.

93. 112 Uncontrolled Fission: The Atomic Bomb Controlled Fission: Nuclear Energy Reactors

94. An atomic bomb based on 235U 113 An atomic bomb uses an uncontrolled chain reaction to produce a powerful explosion.

95. Hiroshima and Nagasaki in 1945.

96. Controlled Fission: Nuclear Energy Reactors 115 Like a coal-fired power plant, a nuclear power plant generates heat to produce steam, which turns a turbine attached to an electric generator . But a nuclear plant has the potential to produce electric power much more cleanly than by the combustion of coal.

97. A light-water nuclear reactor 116 The dome-shaped structure is the containment shell for the nuclear reactor.

98. Schematic Diagram of a Nuclear Power Plant 117

99. 118

100. 119 Annual Waste Production 35,000 tons SO 2 4.5 x 10 6 tons CO 2 1,000 MW coal-fired power plant 3.5 x 10 6 ft 3 ash 1,000 MW nuclear power plant 70 ft 3 vitrified waste

101. 120 Revisit:

102. 121 Nuclear Fusion Nuclear fusion in the Sun is the ultimate source of nearly all the energy—and chemical elements—on Earth. In fact, all the elements heavier than hydrogen were formed in fusion and decay processes within stars. Our sun, which presently consists of 73% hydrogen, 26% helium, and 1% other elements, gives off vast quantities of energy from the fusion of protons to form helium:

103. 122 The major difficulty is that high temperatures are required to initiate fusion. The forces that bind nucleons together to form a nucleus are effective only at very small distances (  10 -13 cm). Thus, for two protons to bind together and thereby release energy, they must get very close together. But protons, because they are identically charged, repel each other electrostatically.

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105. 124 The fusion of deuterium and tritium, for example, occurs at practical rates at about 10 8 K, a temperature hotter than the Sun’s core! How can such conditions be achieved? The tokamak design for magnetic containment of a fusion plasma.

106. Comparison (as Fuels)

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108. Nuclear reactions produce much more heat energy per kg than the chemical reactions

109. Induced Nuclear Reactions ( Transmutation and Bombardment Reactions ) 129 The discovery of radioactivity in the late 19 th century showed that some nuclei spontaneously transform into nuclei with a different number of protons, thereby producing a different element. When scientists realized that these naturally occurring radioactive isotopes decayed by emitting subatomic particles, they realized that—in principle—it should be possible to carry out the reverse reaction, converting a stable nucleus to another more massive nucleus by bombarding it with subatomic particles in a nuclear transmutation reaction.

110. a nuclear reaction in which a nuclide of one element is changed into a nuclide of another element. It is also possible to cause a transmutation reaction to occur in a laboratory setting by means of a bombardment reaction . Transmutation reaction Bombardment reaction a nuclear reaction brought about by bombarding stable nuclei with small particles traveling at very high speeds.

111. The first successful nuclear transmutation reaction was carried out in 1919 by Ernest Rutherford 131

112. 132 More than 2000 bombardment-produced radionuclides that do not occur naturally are now known. nuclides of 30 elements that do not occur in nature have been produced in small quantities as the result of bombardment reactions. Four of these “synthetic” elements, produced between 1937 and 1941, filled gaps in the periodic table for which no naturally occurring element had been found. Most radioisotopes used in the field of medicine are “synthetic” radionuclides. For example, the synthetic radionuclides cobalt-60, yttrium-90, iodine-131 are used in radiotherapy treatments for cancer.

113. 133 Schematic diagram of a linear accelerator. The linear accelerator uses a series of tubes with alternating voltage. A particle is accelerated from one tube to the next by repulsion. particle accelerators were able to impart high kinetic energies to particles by placing them in an electric field, usually in combination with a magnetic field

114. 134 Schematic diagram of a cyclotron accelerator.

115. 136 Revisit: Radioactive Decay

116. The Kinetics of Radioactive Decay 137 Both chemical and nuclear systems tend toward maximum stability. N / Z stable H owever, the tendency of a chemical system to become more stable tells nothing about how long that process will take, and the same holds true for nuclear systems.

117. 138 Detection and Measurement of Radioactivity Radioactive emissions interact with surrounding atoms. To determine the rate of nuclear decay, we measure radioactivity by observing the effects of these interactions over time. Because these effects can be electrically amplified billions of times, it is possible to detect the decay of a single nucleus. Ionization counters and scintillation counters are two devices used to measure radioactive emissions.

118. 139 detects radioactive emissions as they ionize a gas. Ionization produces free electrons and gaseous cations, which are attracted to electrodes and produce an electric current. Ionization Counter Geiger-M ü ller counter

119. 140 Scintillation Counter A scintillation counter detects radioactive emissions by their ability to excite atoms and cause them to emit light. Radioactive particles strike a light-emitting substance, which emits photons. The photons strike a cathode and produce an electric current. A scinatillation “cocktail” in tubes to be placed in the counter. The light-emitting substance in the counter, called a phosphor.

120. The most versatile method for measuring radiation in the laboratory is the scintillation counter.

121. The simplest device for detecting exposure to radiation is the photographic film badge. The film is protected from exposure to light, but any other radiation striking the badge causes the film to fog.

122. Revisit: Radioactivity radioactivity: a process by which the nucleus of an atom spontaneously changes itself by emitting particles or energy. Isotopes that undergo spontaneous nuclear reactions are said to be radioactive . Decay reactions A nucleus spontaneously disintegrates by emitting particles Alpha particle

123. 145 Radiation can be dangerous because it has high enough energy to break bonds in molecules. How do we measure the intensity of radiation?

124. 146 The further we are from a source, the the amount of radiation exposure. smaller

125. intensity of radiation: the amount of energy that flows per unit area per time

126. Ionization: Effects of Nuclear Radiation on Matter 148

127. 149 The key to both types of effects is that nuclear changes cause chemical changes in surrounding matter. In other words, even though the nucleus of an atom may undergo a reaction with little or no involvement of the atom’s electrons, the emissions from that reaction do affect the electrons of nearby atoms. Virtually all radioactivity causes ionization in surrounding matter, as the emissions collide with atoms and dislodge electrons:

128. Effects of Ionizing Radiation on Living Tissue 150 Ionizing radiation has a destructive effect on living tissue, and, if the ionized atom is part of a key biological macromolecule or cell membrane, the results can be devastating to the cell and perhaps the organism. The danger from a radioactive nuclide depends on three factors: The type of radiation emitted The half-life of the radioactive nuclide Most importantly, the biological behavior of the radioactive nuclide

129. 151 Molecular Interactions with Radiation The interaction of ionizing radiation with molecules causes the loss of an electron from a bond or a lone pair. This results in the formation of free radicals , molecular or atomic species with one or more unpaired electrons. Free radicals are unstable and extremely reactive. Double bonds in membrane lipids are very susceptible to attack by free-radicals:

130. 152

131. ROS: Radiation-induced reactive oxygen species

132. 154 Units of Radiation The gray is the SI unit for energy absorption. 1 Gy = 1 J absorbed per kg of body tissue. The rad is more widely used: 1 rad = 0.01 J/kg or 0.01 Gy. T he number of cation-electron pairs produced in a given amount of living tissue is a measure of the energy absorbed by the tissue. ( r adiation- a bsorbed d ose )

133. 155 Units of Radiation

134. Typical Radiation Doses from Natural and Artificial Sources Source of Radiation Average Adult Exposure Natural Cosmic radiation 30-50 mrem /yr Radiatio n from the ground From clay soil and rocks In wooden houses In brick houses In concrete (cinder block) houses ~25-170 mrem /yr 10-20 mrem /yr 60-70 mrem /yr 60-160 mrem /yr Radiation from the air (mainly radon) Outdoors, average value In wooden houses In brick houses In concrete (cinder block) houses 20 mrem /yr 70 mrem /yr 130 mrem /yr 260 mrem /yr Internal radiation from minerals in tap water and daily intake of food. ( 40 K, 14 C, Ra) ~ 40 mrem /yr

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139. 161 Units of Radioactivity The SI unit of radioactivity is the becquerel ( Bq ), defined as one disintegration per second (d/s). The curie ( Ci ) is a more commonly used unit: 1 Ci = 3.70x10 10 d/s

140. 162 Rate of Radioactive Decay Radioactive nuclei decay at a characteristic rate, regardless of the chemical substance in which they occur. The rate of radioactive decay (also called the activity ) is proportional to the number of nuclei present. Rate = kN Radioactive decay follows first-order kinetics , and the rate constant k is called the decay constant . The larger the value of k , the higher the activity of the substance.

141. 163

142. 164 CHEM 12642 - Physical Chemistry I integrate 1 st order:

143. 165 R adioactive decay is a first-order process, but first order with respect to the number of nuclei rather than their concentration.

144. 166 Half-Life of Radioactive Decay The half-life of a nuclide is the time taken for half the nuclei in a sample to decay. - The number of nuclei remaining is halved after each half-life. - The mass of the parent nuclide decreases while the mass of the daughter nuclide increases - Activity is halved with each succeeding half-life. ln 2 k t 1/2 = T he half-life for a first-order process is constant.

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146. 168

147. 169 Decay Constants ( k ) and Half-Lives ( t 1/2 ) of Beryllium Isotopes Nuclide k t 1/2 Be 1.30x10 -2 /day 53.3 days Be 1.0x10 16 /s 6.7x10 -17 s Be Stable Be 4.3x10 -7 /yr 1.6x10 6 yr Be 5.02x10 -2 /s 13.8 s 7 4 8 4 9 4 10 4 11 4

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149. 171 Sample Problem 9 Finding the half-life of a Radioactive nuclide. PROBLEM: Technetium-99m is used to form pictures of internal organs in the body and is often used to assess heart damage. The m for this nuclide indicates an excited nuclear state that decays to the ground state by gamma emission. The rate constant for decay of is known to be 1.16 × 10 -1 /h. What is the half-life of this nuclide? 99m 43 Tc SOLUTION:

150. 172 Sample Problem 10 PROBLEM: The half-life of molybdenum-99 is 66.0 h. How much of a 1.000-mg sample of is left after 330 h? 99 42 Mo SOLUTION:

151. 173 Radioisotopic Dating Radioisotopes can be used to determine the ages of certain objects. Radiocarbon dating measures the relative amounts of 14 C and 12 C in materials of biological origin. The ratio of 14 C/ 12 C remains the same for all living organisms. Once the organism dies, the amount of 14 C starts to decrease as it decays to form 14 N. Since 14 C decays at a predictable rate, measuring the amount present indicates the time that has passed since the organism died. 40 K/ 40 Ar ratios can be used to determine the age of certain rocks.

152. 174

153. 175 Rearranging the equation and integrating over time gives an expression for finding the number of nuclei remaining, 𝒩 t , after a given time, t: where 𝒩 is the number of nuclei at t = 0 .

154. 176 Sample Problem 11 Finding the Number of Radioactive Nuclei PROBLEM: Strontium-90 is a radioactive by-product of nuclear reactors that behaves biologically like calcium, the element above it in Group 2A(2). When 90 Sr is ingested by mammals, it is found in their milk and eventually in the bones of those drinking the milk. If a sample of 90 Sr has an activity of 1.2x10 12 d/s, what are the activity and the fraction of nuclei that have decayed after 59 yr ( t 1/2 of 90 Sr = 29 yr )? PLAN: The fraction of nuclei that have decayed is the change in the number of nuclei, expressed as a fraction of the starting number. The activity of the sample ( A ) is proportional to the number of nuclei ( N ), and we are given A . We can find A t from the integrated form of the first-order rate equation, in which t is 59 yr. We need the value of k , which we can calculate from the given t 1/2 .

155. 177 Sample Problem 11 Finding the Number of Radioactive Nuclei SOLUTION: fraction decayed = A – A t A = t 1/2 = ln 2 k so k = ln 2 t 1/2 0.693 29 yr = 0.024 yr -1 N t N ln = ln A t A = kt or ln A – ln A t = kt so ln A t = - kt + ln A = -(0.024 yr -1 x 59 yr) + ln(1.2x10 12 d/s) = -1.4 + 27.81 ln A t = 26.4 A t = e 26.4 = 2.9x10 11 d/s = 1.2x10 12 d/s – 2.9x10 11 d/s 1.2x10 12 d/s = 0.76

156. 178 Sample Problem 12 Applying Radiocarbon Dating PROBLEM: The charred bones of a sloth in a cave in Chile represent the earliest evidence of human presence at the southern tip of South America. A sample of the bone has a specific activity of 5.22 disintegrations per minute per gram of carbon (d/ min·g ). If 12 C/ 14 C ratio for living organisms results in a specific activity of 15.3 d/ min·g , how old are the bones ( t 1/2 of 14 C = 5730 yr )? PLAN: We calculate k from the given half-life. Then use the first-order rate equation to find the age of the bones, using the given activities of the bones and of a living organism. SOLUTION: k = ln 2 t 1/2 = 0.693 5730 yr = 1.21x10 -4 yr -1

157. 179 The bones are about 8900 years old. t = 1 k A A t ln ln = 1 1.21x10 -4 yr -1 15.3 d/min·g 5.22 d/min·g = 8.89x10 3 yr

158. Some constructive uses of nuclear energy.

159. 181