1. Chapter 20
Nuclear Chemistry

2. HISTORY
Radioactivity was discovered by Henry Bequerel in by observing uranium salts emit energy.
Madame Curie and her husband extended Bequerel’s work on radioactivity
Curie was the first to use Radioactivity to describe the spontaneous emission of alpha, beta, and gamma particles from an unstable nucleus.
Both Curies suffered the effects of radiation poisoning.
Rutherford took over and bombarded gold with alpha particles

3. Identification of Radiation

4. Penetrating Effects

5. Nuclear Chemistry
Nuclear chemistry is the study of reactions that involve changes in the nuclei of atoms.
Radioactive decay is the spontaneous disintegration of alpha, beta, and gamma particles.
Radioactive decay follows first-order kinetics.

6. NUCLEAR STABILITY
Kinetic stability describes the probability that a nucleus will undergo decomposition to form a different nucleus (radioactive decay)
Thermodynamic Stability - the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons. (Binding energy)

7. Stability radioactive decay
Light elements are stable if the neutrons and protons are equal, i.e. 1:1 ration, heavier nuclides require a ratio of 1:1.5
Magic numbers of protons and neutrons seem to exist, much like 8 electrons to make elements and ions Nobel.

8. Magic Numbers
Even number protons and neutrons are stable compared to odd ones
Magic numbers (protons or neutrons) 2,8,20,28,50,82 and 126
For example tin has 10 stable isotopes, even number, but on either side of elemental tin indium and antimony have only two stable isotopes
Nuclei with magic numbers of both protons and neutrons are said to be “doubly magic” and even more stable i.e. Helium-4 two protons and two neutrons and Pb-208 with 82 protons and 126 neutrons
Could be shells for nucleons, like electrons

9. Number of stable nuclides related to numbers of protons and neurons
Stability radioactive decay
Number of stable nuclides related to numbers of protons and neurons

10. Nuclear Stability

11. Stability radioactive decay
Nucleons are protons and neutrons
The strong nuclear force keeps the nucleus together by overcoming the repulsive force of the protons.
Neutrons are present to help dissipate the repulsive forces between the protons
As the atomic number (number of protons) increases, so does the number of neutrons to shield the repulsion of the protons
All nuclides with 84 or more protons are unstable and radioactive. This means the strong force is only strong enough neutralize the force of 84 protons.

12. Decay Series

13. The Half-Lives of Nuclides in the 23892U Decay Series

14. Carbon Radioactive Decay Products
Name
Mass (amu)
Mode(s) of Decay
Half-Life
Natural Abundance
Carbon-10
Positron Emis.
19.45 s
Carbon-11
20.3 min
Carbon-12
(Stable)
98.89 %
Carbon-13
1.11 %
Carbon-14
Decay
5730 y
Carbon-15
2.4 s
Carbon-16
0.74 s

15. Radioactive Decay

16. Half-Life n = t/t1/2 At/Ao = (1/2)n
t - time, t1/2 - time for a half-life, and n - the number of half-lives
At/Ao = (1/2)n
Ao is the amount initially present, At is the amount at time t, and n is the number of half-lives

17. Types of Radioactive Decay
Decay processes
Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration
Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio
Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons

18. Decay Types Alpha particle emitters (Mass number changes)
Nuclei with atomic number>200
The daughter nuclei contains two fewer protons and two fewer neutrons than the parent
U-238, Th-230

19. Decay Types Beta particle decay Too many neutrons
Atomic number increases, thus more protons
Neutron splits into a proton and electron
n → P β
Examples Th-234, I-131 1 1 o 1 -1
234
234 Th Pa e + 91 -1 90

20. Decay Types Electron Capture Neutron-poor nuclides
Electron in an inner shell reacts with a proton 11P β → 10n
AZX β → AZ-1X’ + x-ray
No change in mass number
Example iron-55

21. Decay Types Positron Emission Neutron-poor nuclides
Same mass as an electron, but opposite charge, the positron emission is opposite beta decay
11P → 10n β
AZX → AZ-1X’ β
Example C-11

22. Decay Types Electron Capture Neutron-poor nuclides
Electron in an inner shell reacts with a proton 11P β → 10n
AZX β → AZ-1X’ + x-ray
No change in mass number
Example iron-55

23. Decay Types Gamma Emission 00γ 9843Tc* → 9843Tc + 00γ
Many nuclear decay daughters are in an elevated, or excited, energy state
These meta stable isotopes emit gamma rays to lower their potential energy
This emission can be instantaneous, or delayed for sever hours
Te-99m has a half life of about 6 hours
9843Tc* → 9843Tc γ

24. Decay Types Spontaneous Fission Very massive nuclei Z > 103
Usually large amounts of energy are released
Usually neutrons are released
Example:
25498Cf → Pd Te n

25. Decay Types
Various Types of Radioactive Processes Showing the Changes That Take Place in the Nuclides

26. Radiocarbon Dating of Artifacts26

27. Calibration Curves 27

28. Radiochemical Dating n = t/t1/2 At/Ao = 0.5n
t - time, t1/2 - time for a half-life, and n - the number of half-lives
At/Ao = 0.5n
Ao - amount initially present, At - amount at time t, and n - the number of half-lives
If we know what fraction of sample is left (At/Ao) and its half-life (t1/2), we can calculate how much time has elapsed. 28

29. Kinetics of Radioactive Decay
Radioactive decay is a first order process, but using atoms instead of concentration
Radioactive decay rates
Activity is defined as the number of nuclei that decay per unit time
A = -ΔN/Δt, the units are usually disintegrations per second or minute (dps), dpm
The activity is directly proportional to the number of atoms, thus A(Rate)=kN
From Che162 we know the first order rate law is lnN/N0 = -kt
Also t1/2 ln1/2N0/N0 = -kt1/2 → t1/2 = 0.693/k

30. Example problem
Fort Rock Cave in Oregon is the site where archaeologists discovered several Indian sandals, the oldest ever found in Oregon. Analysis of the 14C/12C ratio of the sandals gave an average decay rate of 5.1 dpm per gram of carbon. Carbon found in living organisms has a C-14/C-12 ratio of 1.3 X 10-12, with a decay rate of 15 dpm/g C. How long ago was the sage brush in the sandals cut? The half life of carbon-14 is 5730 years. Note dpm is disintegrations per minute

31. Sample Problem Solution
First calculate the rate constant k from the half-life: k=0.693/5730 = yr-1
Substitute into the first order rate equation.
ln(N/N0) = kt
t = ln(N/N0)/k = ln(15/5.1)/
t = 8910 years old sandals

32. Practice
A mammoth tusk containing grooves made by a sharp stone edge (indicating the presence of humans or Neanderthals) was uncovered at an ancient camp site in the Ural Mountains in The 14C/12C ratio in the tusk was only 1.19% of that in modern elephant tusks. How old is the mammoth tusk? 32

33. Practice
Radioactive radon-222 decays with a loss of one  particle. The half-life is 3.82 days. What percentage of the radon in a sealed vial would remain after 7.0 days? 33

34. Nuclear Transformations
Rutherford (1919) was the first to carry out a bombardment reaction, when he combined an alpha particle with nitrogen-14, creating oxygen-17 and a proton
The next successful bombardment reaction was done 14 years later when Aluminum-27 to make phosphorus-30 and a neutron
If the bombarding particle has a positive charge then repulsion by the nucleus hinders the process, thus particle accelerators are required.
Cyclotron and linear accelerator pg850
Neutrons, do not suffer from the repulsive effect
Synthetic elements have been made, called transuranium elements

35. Cyclotron
Nuclear reactions can be induced by accelerating a particle and colliding it with the nuclide.

36. Cyclotron
An Aerial View of Fermilab, a High Energy Particle Accelerator Cyclotron.

37. The Accelerator Tunnel at Fermilab

38. Linear Accelerator

39. Linear Accelerator Cyclotron39

40. Detection and Uses of Radioactivity
Geiger counter, high energy from radioactive substances ionizes the Ar, thus allowing a current to flow. The more ions the more current, thus more radioactive
Scintillation counter, measures the amount of light given off by a phosphor such as ZnS, which is measured by a photometer
Badges

41. Geiger Counter
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

42. Geiger Counter

43. Thermodynamic Stability
This is done by comparing the mass of the individual protons and neutrons to the mass of the nucleus itself. The difference in mass is called the mass defect (Δm), which when plugged into E = mC2, or ΔE = ΔmC2 for change in energy
The mass of an atom is always less than the mass of the subatomic particles
Protium is the only exception, since there is no defect
The other isotopes of hydrogen deuterium and tritium have defects
Mass of neutron = amu
Mass of proton = amu
Mass of electron = amu , note mass of electron is not really necessary in calculations since it subtracts out when finding the difference

44. Subatomic Particles Particle Mass(g) Charge
Electron(e) x
Proton(p) x
Neutron(n) x
Particle x
Positron x 44

45. Thermodynamic Stability
Just like a molecule is more stable that its atoms, an nucleus in more stable than its individual atoms.
Energy changes for nuclear process are extremely large when compared to normal chemical and physical changes, thus very valuable energy source.
Normal units are expressed per nucleon, in MeV (million electron volts)
MeV = 1.60 X J OR amu = 931 MeV
All nuclei have different relative stabilities, see figure 18.9

46. Sample problem: Solution (Note: AMU = g/mole)
Calcualte the changes in mass (in amu) and energy (in J/mol and eV/atom) that accompany the radioactive decay of 238U to 234Th and an alpha particle. The alpha particle absorbs two electrons from the surrounding matter to form a helium atom.
Solution (Note: AMU = g/mole)
Δm = mass prod. – mass react.
Δm = (mass 234Th + mass 42He) - mass238U
Δm = ( ) =
amu or X10-6kg
ΔE = mC2 ↔ ΔE =( X10-6kg)(2.998X108m/s)2
=-4.120X1011 j/mole
ΔE = amu X 931 MeV/amu
Divide by the mass number to get energy per nucleon, called binding energy

47. Practice
What is the binding energy of 60Ni? The mass of a 60Ni atom is amu. The mass of an electron is x kg and 1 amu is x kg. 47

48. Thermodynamic Stability
Revisiting the graph on page 988
Notice that Iron is the most stable nuclide
∆E is negative when a process goes from a less stable to a more stable state
In nuclear reactions more stable nuclei can be achieved by combining nuclei (fusion) or splitting a nucleus (fission)
Lighter elements typically undergo fusion, while elements heavier than iron undergo fission.

49. Thermodynamic Stability
For lighter elements, fusion processes lead to nuclei with greater binding energy, whereas heavy elements are formed through other processes.

50. Artificial Elements
Scientists have been transmuting elements since 1919 when oxygen-17 and hydrogen-1 were produced from nitrogen-14 and  particles.
147N He O H
Artificial transmutation requires bombardment with high velocity particles.
Alpha particles are positivly charged so how do they strike the nucleus, since the nucleus is positivly charged? 50

51. Energy in Nuclear Reactions
In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal.
However, these energies are many thousands of times greater in nuclear reactions.

52. Energy in Nuclear Reactions
For example, the mass change for the decay of 1 mol of uranium-238 is − 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

53. Fission Process
Discovered in the 1930’s when U-235 was bombarded with neutrons
Neutrons, due to their neutral charge do not require accelerators
11n U → Ba Kr n
This process delivers 2.1X1013J/mole, compared to 8.0X105j of energy for the combustion of methane
About 26 million times more energy
Another splitting process produces the elements Te-137 and Zr-97, with two neutrons
There are 200 different isotopes of 35 different element produced, thus the nucleus fragments in many different ways

54. Fission Process
Since neutrons are produced, then it is possible to have a self-sustaining reaction
If the average production of neutrons is less than one, the reaction is called subcritical
If the neutron production is equal to one then it is called critical
If the neutron production is greater than one then the reaction is called super-critical
To achieve the a critical state, then a critical mass is required
If the mass is too small then the neutrons escape before splitting other nuclei

55. Nuclear Fission How does one tap all that energy?
Nuclear fission is the type of reaction carried out in nuclear reactors.

56. Nuclear Fission
Bombardment of the radioactive nuclide with a neutron starts the process.
Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

57. Nuclear Fission
If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

58. Nuclear Fission
Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: critical mass.

59. Nuclear Reactors
In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator

60. 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 reaching a dangerous supercritical mass.

61. Fusion Process
Combining of nuclei, such as the reaction the occurs on the sun
Problem is that the nuclei are positive in charge, thus high temperatures (4X 107K) necessary to give the nuclei the correct amount of kinetic energy to overcome the repulsion
Electric current heat
Laser heat
Because to the high temperature then what about containment?

62. Nuclear Fusion Fusion would be a superior method of generating power.
The good news is that the products of the reaction are not radioactive.
The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins

63. Hydrogen Fusion
Heavier elements formed through the process of fusion.

64. Effects of Radiation What happens when one is exposed to radiation?
Somatic damage is damage to the organism itself
Genetic damage is damage to the genetic machinery, RNA DNA for example
Damage depends on the following factors

65. Quantities of Radiation
Unit
Parameter
Description
Curie (Ci)
Level of Radioactivity
3.7x1010 nuclear disintegrations/s
Becquerel (B)*
1 disintegration/s
Gray (Gy)
Ionizing Energy Absorbed
1 Gy = 1 J/kg of tissue mass
Sievert (Sv)
Amount of Tissue Damage
1Sv = 1Gy x RBE**
*SI unit of radioactivity **Relative Biological Effectiveness 65

66. Damage Factors
The energy of the radiation, measured in rads ( radiation absorbed dose), where one rad = 10-2 J of energy deposited per kg of tissue
Since different radioactive particles do different kinds of damage the rad is not the best way to consider the effects
Penetrating ability of the radiation
Gamma highly penetrating, since electromagnetic energy consisting of photons
Beta particles penetrate up to one cm
Alpha particles are stopped by the skin

67. Damage Factors Ionizing ability of the radiation
Gamma radiation only occasionally ionize
Alpha particles, highly ionizing and leave a trail of damage, since it is an ion itself, it will strip electrons from other substances
Chemical properties of the radiation source.
Inert nuclides such as the noble gases pass through the body
A radioactive substance such as iodine, can be concentrated in a specific location of the body. For iodine it is the thyroid.
rem = rads X RBE

68. About REM rem is the radiation equivalent in man
rbe is the relative effectiveness of the radiation in causing biologic damage, which is one for betta and gamma, and 20 for alpha
Alpha particles have a higher rbe than beta and gamma, since the helium nuclei is much larger.

69. Acute Effects of Single Whole-Body Doses of Ionizing Radiation
Dose(REM)
Toxic Effect
No acute effect, possible carcinogenic or mutagenic damage to DNA
Temporary reduction in white blood cell count
Radiation sickness: fatigue, vomiting, diarrhea, impaired immune system
Severe radiation sickness: intestinal bleeding, bone marrow destruction
Death, usually through infection, within weeks
>10.0
Death within hours 69

70. Typical Radiation Exposures for a Person Living in the United States (1 millirem = 10-3 rem)

71. Sources of Radiation 71

72. Biological Effect of Radiation72

73. Radon Gas Release from Rocks73

74. Radiation Therapy Nuclide Radiation Half-Life Treatment 32P  14.3 d
Leukemia Therapy
60Co

5.3 yr
External Cancer Therapy
123I 
13.3 yr
Thyroid Therapy
131Cs
9.7 days
Prostate cancer therapy
192Ir
74 d
Coronary disease 74

75. Medical Imaging Radionuclides
Radiation Emitted
Half-Life (hr)
Use
99mTc 
6.0
Bones, Circulatory system, Various Organs
67Ga 78
Tumors in the Brain and Other Organs
201Tl 73
Coronary Arteries, Heart Muscle
123I
13.3
Thyroxin Production in Thyroid Gland 75

76. Synthesis of the elements in stars
Stars are formed from the gravitational attraction of interstellar dust, mostly hydrogen
The density gradually increases reaching a density of about 100g/mL, with a temperature of about 1.5X107 K
At this point hydrogen begins to fuse into He-4, releasing energy, like our sun
The overall reaction is 4 protium atoms combining to make helium and 2 beta particles plus 2 photons of gamma radiation
The helium then concentrates in the core of the star, thus increasing the density and temperature, thus becoming a red giant star

77. Synthesis of the elements in stars
At a temperature of about 2X108 K, the helium nuclei begin to fuse producing Be-8
Be-8 is unstable due to low neutron numbers, and absorbs alpha particles creating C, O, Ne, Mg
The next stage is formation of a red supergiant star, where Na, Si, S, Ar, and Ca are produced
Next in the progressions is the formation of a massive red supergiant star, where Fe and Ni are formed by proton-neutron exchange reactions
Finally the supernova is produced where elements with Z>28 being formed by multiple neutron captures

78. Red Giant

79. Supernova

80. Click to launch animation
ChemTour: Half-Life
Click to launch animation
PC | Mac
Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.

81. ChemTour: Fusion of Hydrogen
Click to launch animation
PC | Mac
This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.

82. ChemTour: Modes of Radioactive Decay
Click to launch animation
PC | Mac
This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.

83. ChemTour: Balancing Nuclear Reactions
Click to launch animation
PC | Mac
This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.

84. Click to launch animation
ChemTour: Half-Life
Click to launch animation
PC | Mac
Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.

85. ChemTour: Fusion of Hydrogen
Click to launch animation
PC | Mac
This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.

86. ChemTour: Modes of Radioactive Decay
Click to launch animation
PC | Mac
This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.

87. ChemTour: Balancing Nuclear Reactions
Click to launch animation
PC | Mac
This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.

88. End Chapter #20 Nuclear Chemistry