chapter 4 nuclear chemistry: the heart of matter
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Chapter 4 Nuclear Chemistry: The Heart of Matter. Radioisotopes. Radioactive decay – Many isotopes are unstable Nuclei that undergo radioactive decay May produce one or more types of radiation. Natural Radioactivity. Background radiation What occurs from natural sources - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 4
Nuclear Chemistry: The Heart of Matter
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Radioisotopes
• Radioactive decay – Many isotopes are unstable
– Nuclei that undergo radioactive decay
– May produce one or more types of radiation
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Natural Radioactivity• Background
radiation– What occurs from
natural sources– Accounts for >80%
of radioactivity exposure
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Types of Radiation
• Ionizing radiation – knocks electrons out of atoms or groups of atoms
– Produces charged species – ions– Charged species that cause damage
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Alpha Decay
• Nucleus loses particle– Mass decreases by 4 and atomic number decreases by 2
He42
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Beta Decay• Nucleus loses particle
– No change in mass but atomic number increases
e01
Positron Emission• Loses a positron
– Equal mass but opposite charge of an electron– Decrease in atomic number and no change in mass +
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Electron Capture• Nucleus absorbs an
electron and then releases an X-ray
• Mass number stays the same and atomic number decreases
Gamma Radiation
• Release of high-energy photon
• Typically occurs after another radioactive decay• No change in mass number or atomic number
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Nuclear Equations
• Elements may change in nuclear reactions• Total mass and sum of atomic numbers must be
the same• MUST specify isotope
Po He Rn 21884
42
22286
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Differences Between Chemical and Nuclear Reactions
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Example 4.1 Balancing Nuclear Equations
Write balanced nuclear equations for each of the following processes. In each case, indicate what new element is formed.a. Plutonium-239 emits an alpha particle when it decays.b. Protactinium-234 undergoes beta decay.c. Carbon-11 emits a positron when it decays.d. Carbon-11 undergoes electron capture.
Solutiona. We start by writing the symbol for plutonium-239 and a partial equation showing that one of the products is an alpha particle (helium nucleus):
23994 Pu 4
2 He + ?
Mass and charge are conserved. The new element must have a mass of 239 – 4 = 235 and a charge of 94 – 2 = 92. The nuclear charge (atomic number) of 92 identifies the element as uranium (U):
23994 Pu 4
2 He + 23592 U
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Half-Life• Period for one-half of the
original elements to undergo radioactive decay
• Characteristic for each isotope
• Fraction remaining =
n = number of half-lives
n21
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You obtain a new sample of cobalt-60, half-life 5.25 years, with a mass of 400 mg. How much cobalt-60 remains after 15.75 years (three half-lives)?
Example 4.2 Half-Lives
SolutionThe fraction remaining after three half-lives is
1
2n1
23
1
2 x 2 x 2
1
8===
The amount of cobalt-60 remaining is ( ) (400 mg) = 50 mg.1
8
You have 1.224 mg of freshly prepared gold-189, half-life 30 min. How much of the gold-189 sample remains after five half-lives?
Exercise 4.2A
What percentage of the original radioactivity remains after five half-lives?
Exercise 4.2B
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You obtain a 20.0-mg sample of mercury-190, half-life 20 min. How much of the mercury-190 sample remains after 2 hr?
Example 4.3
There are 120 min in 2 hr. There are ( ) = 6 half-lives in 2 hr. The fraction remainingafter six half-lives is
The amount of mercury-190 remaining is ( ) (20.0 mg) = 0.313 mg.
Solution
1
2n1
26
1
2 x 2 x 2 x 2 x 2 x 2
1
64===
164
12020
A sample of 16.0 mg of nickel-57, half-life 36.0 hr, is produced in a nuclear reactor. How much of the nickel-57 sample remains after 7.5 days?
Exercise 4.3A
Tc-99 decays to Ru-99 with a half-life of 210,000 years. Starting with 1.0 mg of Tc-99, how long will it take for 0.75 mg of Ru-99 to form?
Exercise 4.3B
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Radioisotopic Dating• Use certain isotopes to estimate the age of various
items• 235U half-life = 4.5 billion years
– Determine age of rock• 3H half-life = 12.3 years
– Used to date aged wines
Carbon-14 Dating• 98.9% 12C• Produce 14C in upper atmosphere• Half-life of 5730 years• ~50,000 y maximum age for dating
H Cn N 11
146
10
147
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A piece of fossilized wood has carbon-14 activity one-eighth that of new wood. How old is the artifact? The half-life of carbon-14 is 5730 years.
Example 4.4
SolutionThe carbon-14 has gone through three half-lives:
It is therefore about 3 x 5730 = 17,190 years old.
1
8==
1
2
1
2x
1
2x( )1
2
3
How old is a piece of cloth that has carbon-14 activity that of new cloth fibers? The half-life of carbon-14 is 5730 years.
Exercise 4.41
16
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Shroud of Turin• Alleged burial shroud of Jesus Christ
– Contains faint human likeness– First documented in Middle Ages
• Carbon-14 dating done in 1988– Three separate labs– Shroud ~800 years old– Unlikely to be burial shroud
Artificial Transmutation• Transmutation changes one element into another
– Middle Ages: change lead to gold• In 1919 Rutherford established protons as fundamental particles
– Basic building blocks of nuclei
H O He N 11
178
42
147
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Uses of Radioisotopes• Tracers
– Easy to detect– Different isotopes have similar chemical and physical properties– Physical, chemical, or biological processes
• Agriculture– Induce heritable genetic alterations – mutations– Preservative
– Destroys microorganisms with little change to taste or appearance of the food
Nuclear Medicine• Used for two purposes• Therapeutic – treat or cure disease using radiation• Diagnostic – obtain information about patient’s health
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Radiation Therapy• Radiation most lethal to dividing cells• Makes some forms of cancer susceptible• Try to destroy cancer cells before too much damage to healthy cells
– Direct radiation at cancer cells– Gives rise to side effects
Diagnostic Uses• Many different isotopes used
– See Table 4.6• Can measure specific things
– Iodine-131 to locate tumors in thyroid– Selenium-75 to look at pancreas– Gadolinium-153 to determine bone mineralization
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Imaging• Positron emission tomography (PET)• Uses an isotope that emits a positron• Observe amount of radiation released
e B C 01
115
116
Penetrating Power of Radiation• The more mass the particle has, the less penetrating it is• The faster the particle is, the more penetrating it is
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Prevent Radiation Damage• To minimize
damage – Stay a distance
from radioactive sources
– Use shielding; need more with more penetrating forms of radiation
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Example 4.6
One of the isotopes used for PET scans is oxygen-15, a positron emitter. What new element is formed when oxygen-15 decays?
Phosphorus-30 is a positron-emitting radioisotope suitable for use in PET scans. What new element is formed when phosphorus-30 decays?
Exercise 4.6
SolutionFirst write the nuclear equation
0+1 e + ?15
8O
The nucleon number does not change, but the atomic number becomes 8 – 1, or 7; and sothe new product is nitrogen-15:
0+1 e +15
8O 157N
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Energy from Nucleus• E = mc2
• Lose mass, gain energy– For chemical
reactions, mass changes are not measurable
– For nuclear reactions, mass changes may be measurable
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Binding Energy• Holds protons and neutrons together in
the nucleus• The higher the binding energy, the more
stable the element
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Nuclear Fission• “Splitting the atom”• Break a large
nucleus into smaller nuclei
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Nuclear Chain Reaction• Neutrons from one
fission event split further atoms
• Only certain isotopes, fissile isotopes, undergo nuclear chain reactions
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Manhattan Project• How to sustain the nuclear reaction?• How to enrich uranium to >90% 235U?
– Only 0.7% natural abundance• How to make 239Pu (another fissile isotope)?• How to make a nuclear fission bomb?
Radioactive Fallout
• Nuclear bomb detonated; radioactive materials may rain down miles away and days later– Some may be unreacted U or Pu– Radioactive isotopes produced during the explosion
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Nuclear Power Plants• Provide ~20% U.S. electricity
– France >70% • Slow controlled release of energy• Need 2.5–3.5% 235U• Problem with disposal of radioactive waste
Nuclear Fusion• Reaction takes smaller nuclei and builds larger ones
– Also called thermonuclear reactions
e2 He H4 01
42
11
• Releases tremendous amounts of energy–1 g of H would release same as 20 tons of coal