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Chapter 21. Nuclear Chemistry
Media Resources
Figures and Tables in Transparency Pack: Section:
Figure 21.2 Stable and Radioactive Isotopes as a 21.2 Patterns of Nuclear Stability
Function of Numbers of Neutrons and Protons
Activities: Section:
Uranium-238 Decay Series 21.2 Patterns of Nuclear Stability
Radioactive Decay 21.4 Rates of Radioactive Decay
Animations: Section:
Separation of Alpha, Beta, and Gamma Rays 21.1 Radioactivity
First-Order Process 21.4 Rates of Radioactive Decay
Other Resources
Further Readings: Section:
Radioactivity in the Classroom 21.1 Radioactivity
Identifying Students’ Misconceptions about 21.1 Radioactivity
Nuclear Chemistry
Nuclear Chemistry: State of the Art for Teachers 21.1 Radioactivity
Radioactivity: A Natural Phenomenon 21.1 Radioactivity
Chapter 21
302
California Earthquakes: Predicting the Next Big 21.4 Rates of Radioactive Decay
One Using Radiocarbon Dating
Searching for Real Time 21.4 Rates of Radioactive Decay
How Radioactive Is your Banana? 21.5 Detection of Radioactivity
Development and Proliferation of 21.5 Detection of Radioactivity
Radioimmunoassay Technology
Radioactivity in the Service of Many 21.5 Detection of Radioactivity
Positron Emission Tomography Merges 21.5 Detection of Radioactivity
Chemistry with Biological Imaging
Nuclear Chemistry
303
Chapter 21. Nuclear Chemistry
Common Student Misconceptions
• Initially, many students think that atoms of one element cannot be transformed into atoms of another
element.
Teaching Tips
• This is new territory for students who have not taken advanced courses in physics.
Lecture Outline
21.1 Radioactivity
1
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• Nuclear reactions involve changes in the atomic nuclei.
• Nuclear chemistry is the study of nuclear reactions (their uses in chemistry and their impact on
biological systems).
• When nuclei change spontaneously, emitting energy, they are said to be radioactive.
They may also be written in a manner that shows the mass number as a superscript and the atomic
number as a subscript:
• A nuclide is a nucleus containing a specified number of protons and neutrons.
Chapter 21
304
Nuclear Equations
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4
• Most nuclei are stable.
• Radionuclides are unstable and spontaneously emit particles and/or electromagnetic
radiation.
• Example: Uranium-238 is radioactive.
• It emits alpha () particles.
• These are helium-4 particles.
Types of Radioactive Decay
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• There are three types of radiation which we will consider:
• -Radiation is the loss of (alpha particles) from the nucleus.
• electron capture
• The nucleus captures an electron from the electron cloud surrounding the nucleus.
• Representations:
• In nuclear chemistry, to ensure the conservation of nucleons we write all particles with their
3
“Nuclear Chemistry: State of the Art for Teachers” from Further Readings
4
“Radioactivity: A Natural Phenomenon” from Further Readings
5
“Beta Decay Diagram” from Further Readings
6
“Separation of Alpha, Beta, and Gamma Rays” Animation from Instructor’s Resource CD/DVD
7
“Scientists Honor Centennial of the Discovery of Radioactivity” from Further Readings
He
4
2
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305
21.2 Patterns of Nuclear Stability
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Neutron-to-Proton Ratio
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• The proton has high mass and high charge.
• Therefore, the proton-proton repulsion is large.
contains all stable nuclei.
• All nuclei with 84 or more protons are radioactive.
• Nuclei above the belt of stability undergo -emission.
• When an is lost; the number of neutrons decreases and the number of protons increases.
• Nuclei below the belt of stability undergo +-emission or electron capture.
• This results in the number of neutrons increasing and the number of protons decreasing.
• Nuclei with atomic numbers greater than 83 usually undergo -emission.
• The number of protons and neutrons decreases (in steps of 2).
Radioactive Series
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• A nucleus usually undergoes more than one transition on its path to stability.
• The series of nuclear reactions that accompany this path is the radioactive series, or the nuclear
disintegration series.
Further Observations
14
• Magic numbers are 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons.
• Nuclei with a “magic number” of nucleons are more stable than nuclei that do not have the magic
21.3 Nuclear Transmutations
15
• Nuclear transmutations are nuclear reactions resulting from the collisions between nuclei or
between a nucleus and a neutron.
• For example, nuclear transmutations can occur using high-velocity -particles:
e
0
1-
Chapter 21
306
Accelerating Charged Particles
• To overcome electrostatic forces, charged particles need to be accelerated before they react.
Reactions Involving Neutrons
• Most synthetic isotopes used in medicine and research are made using neutrons as projectiles.
• An example is the preparation of cobalt-60 for use in cancer radiation therapy.
Transuranium Elements
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• Transuranium elements follow uranium in the periodic table.
21.4 Rates of Radioactive Decay
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• Radioactive decay is a first-order process.
• Each isotope has a characteristic half-life.
Radiometric Dating
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• The half-life of any particular nuclide is constant.
• Thus, half –life may be used as a nuclear clock to determine the age of objects.
• Dating of objects based on their isotopes and isotope abundances is called radiometric dating.
billion years.
Calculations Based on Half-life
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• Radioactive decay is a first-order process:
16
“Heavy Stuff” from Further Readings
17
“Nucleogenesis! A Game with Natural Rules for Teaching Nuclear Synthesis and Decay” from Further
Readings
Nuclear Chemistry
307
• In radioactive decay the constant, k, is called the decay constant.
• The rate of decay is called activity (disintegrations per unit time).
• There are several units used to express activity or radioactivity.
21.5 Detection of Radioactivity
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• Matter is ionized by radiation.
• A Geiger counter determines the amount of ionization by detecting an electric current.
• A thin window is penetrated by the radiation and causes the ionization of Ar gas.
Radiotracers
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• Photosynthesis has been studied using 14C:
• The carbon dioxide is said to be 14C labeled.
• The presence of 14C in the intermediates or products of photosynthesis can be determined.
• 14C is detected as it moves from carbon dioxide to ultimately become incorporated into glucose.
• Thus the path of the carbon atoms may be traced.
• Radiotracers are used to follow an element through a chemical reaction.
24
“How Radioactive Is Your Banana?” from Further Readings
25
“Development and Proliferation of Radioimmunoassay Technology” from Further Readings
26
“Radioactivity in the Service of Many” from Further Readings
Chapter 21
308
21.6 Energy Changes in Nuclear Reactions
• Einstein showed that mass and energy are proportional:
E = mc2
• If a system loses mass, it loses energy (exothermic).
• If a system gains mass, it gains energy (endothermic).
• Since c2 is a large number, small changes in mass cause large changes in energy.
• Mass and energy changes in nuclear reactions are much greater than in chemical reactions.
• To calculate the energy change per mole of 238U :
Nuclear Binding Energies
33
• The mass of a nucleus is less than the mass of its nucleons.
• Mass defect is the difference between the mass of a nucleus and the masses of its nucleons.
• Nuclear binding energy is the energy required to separate a nucleus into its nucleons.
• Since E = mc2, the binding energy is related to the mass defect.
21.7 Nuclear Power: Fission
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• Nuclear power plants and most forms of nuclear weapons utilize nuclear fission.
• Splitting of heavy nuclei is exothermic for large mass numbers.
• Consider a neutron bombarding a 235U nucleus.
• The heavy 235U nucleus can split in several different ways such as:
s
Nuclear Chemistry
309
• Each neutron produced can cause the fission of another 235U nucleus.
• The number of fissions and the resulting energy increase rapidly.
• Reactions that multiply this way are called chain reactions.
• Without controls, an explosion results.
• Consider the fission of a nucleus that results in the production of neutrons.
• Each neutron can cause another fission.
Nuclear Reactors
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• Use fission as a power source.
• Use a subcritical mass of 235U (238U is enriched with about 3% 235U).
• Fuel elements contain enriched 235UO2 pellets are encased in Zr or stainless steel tubes.
• Control rods are composed of Cd or B, which absorb neutrons.
• They help to regulate the flux of neutrons.
• Boiling water reactors: generates steam by boiling the primary coolant (no secondary coolant
is needed).
• Heavy water reactor: used D2O as moderator and primary coolant.
• Gas-cooled reactors: used a gas such as CO2 as primary coolant and graphite as moderator.
• High-temperature pebble-bed reactor: not yet in commercial use.
Nuclear Waste
• Fission products accumulate as a reactor operates.
• These reduce the reactor efficiency.
36
Figure 21.19 from Transparency Pack
37
“Nuclear Power for the Future” from Further Readings
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“Lise Meitner and the Discovery of Nuclear Fission” from Further Readings
39
“Aspects of Nuclear Waste Disposal of Use in Teaching Basic Chemistry” from Further Readings
Chapter 21
310
• The potential for environmental contamination by long-lived isotopes is a serious consideration.
21.8 Nuclear Power: Fusion
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• Light nuclei can fuse to form heavier nuclei.
• Most reactions in the Sun are fusion reactions.
• Fusion products are not usually radioactive, so fusion is a good energy source.
• Also, the hydrogen required for the reaction can easily be supplied by seawater.
• However, high energies are required to overcome repulsion between nuclei before the reaction
can occur.
• Research continues.
FORWARD REFERENCES
• Hydrogen as a fuel used by the Sun and other stars to produce energy will be mentioned in
Chapter 22 (section 22.2).
21.9 Radiation in the Environment and Living Systems
• Ionizing radiation involves ionization that occurs when radiation removes an electron from an atom
or molecule.
• This is generally more harmful to biological systems than nonionizing radiation.
• Radiation absorbed by tissue causes excitation (nonionizing radiation) or ionization (ionizing
radiation).
Radiation Doses
• Absorbed radiation is measured in:
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• Gray: 1 Gy is the SI unit for absorption of 1 J of energy per kg of tissue.
• Rad is the radiation absorbed dose.
• One rad is the absorption of 10−2 J of radiation per kg of tissue.
Radon
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• The nucleus is which is a decay product of
• Radon exposure accounts for more than half of the 360 mrem annual exposure to ionizing radiation.
• Rn is a noble gas; it is extremely stable.
• Therefore, it is inhaled and exhaled without any chemical reactions occurring.
• The half-life of is 3.82 days.
• Radon testing kits are readily available in many areas of the country.
FORWARD REFERENCES
• Group 8A elements will be the subject of section 22.3 in Chapter 22.
Rn
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U
238
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Rn
222
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Chapter 21
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Further Readings:
1. Enrique A. Hughes and Anita Zalts, “Radioactivity in the Classroom,” J. Chem. Educ., Vol. 77, 2000,
613−614.
6. Stu Borman, “Scientists Honor Centennial of The Discovery of Radioactivity,” Chemical and
Engineering News, April 29, 1996, 55−65.
7. S. G. Hutchinson and F. I. Hutchinson, “Radioactivity in Everyday Life,” J. Chem. Educ., Vol. 74,
1997, 501−505.
8. V. E. Viola, “Teaching Nuclear Science: A Cosmological Approach,” J. Chem. Educ., Vol. 71, 1994,
840−944.
13. Donald J. Olbris and Judith Herzfeld, “Nucleogenesis! A Game with Natural Rules for Teaching
Nuclear Synthesis and Decay,” J. Chem. Educ., Vol. 76, 1999, 349−352.
14. M. W. Rowe, “Archaeological Dating,” J. Chem. Educ., Vol. 63, 1986, 16−20.
15. M. W. Rowe, “Radioactive Dating: A Method for Geochronology,” J. Chem. Educ., Vol. 62, 1985,
580−584.
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19. Rosalyn S. Yalow, “Development and Proliferation of Radioimmunoassay Technology,” J. Chem.
Educ., Vol. 76, 1999, 767−768.
23. Marcus E. Raichle, “Visualizing the Mind,” Scientific American, April 1994, 64.
24. Timothy J. McCarthy, Sally W. Schwarz, and Michael J. Welch, “Nuclear Medicine and Positron
Emission Tomography: An Overview,” J. Chem. Educ., Vol. 71, 1994, 830−836.
25. Helen Carmichael, “Special Agents,” Chemistry in Britain, Vol. 34(8), August 1998, 30−33. An
introduction to contrast agents for medical imaging.
29. Ruth Lewin Sime, “Lise Meitner and the Discovery of Nuclear Fission,” J. Chem. Educ., Vol. 66,
1989, 373–378. This issue of Journal of Chemical Education includes several papers on radiochemistry.
30. Gregory R. Choppin, “Aspects of Nuclear Waste Disposal of Use in Teaching Basic Chemistry,” J.
Chem. Educ., Vol. 71, 1994, 826−829.
