Download Chapter #20 Nuclear Chemistry

Chapter 20
Nuclear Chemistry
HISTORY
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Radioactivity was discovered by Henry Bequerel
in 1896 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
Identification of Radiation
Penetrating Effects
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.
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)
Stability radioactive decay
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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.
Magic Numbers
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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
Stability radioactive decay
Number of stable nuclides related to numbers of
protons and neurons
Nuclear Stability
Stability radioactive decay
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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.
Decay Series
The
HalfLives of
Nuclides
in the
238 U
92
Decay
Series
Carbon Radioactive Decay Products
Name
Carbon-10
Carbon-11
Carbon-12
Carbon-13
Carbon-14
Carbon-15
Carbon-16
Mass
(amu)
12.00000
13.00335
Mode(s) of
Natural
Half-Life
Decay
Abundance
Positron Emis. 19.45 s
Positron Emis. 20.3 min
(Stable)
Decay
Decay
Decay
(Stable)
5730 y
2.4 s
0.74 s
98.89 %
1.11 %
Types of Radioactive Decay
Decay processes
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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
Decay Types
Alpha particle emitters (Mass number
changes)
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Nuclei with atomic mass number>200
The daughter nuclei contains two fewer
protons and two fewer neutrons than the
parent
U-238, Th-230
Types of Radioactive Decay
Decay processes
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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
Decay Types
Beta particle decay
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Too many neutrons
Atomic number increases, thus more
protons
 Neutron splits into a proton and electron
called transmutation.
1
o
1
 0n → 1 P + -1 β
 Examples Th-234, I-131
234
Th
90
234
91 Pa +
0
-1 e
Decay Types
Electron Capture
Neutron-poor nuclides
Electron in an inner shell reacts with a proton
1 P+
0 β → 1 n
1
-1
0
A X
Z
+
0 β
-1
→
A X’
Z-1
+ x-ray
No change in mass number
Example iron-55
Decay Types
Positron Emission
Neutron-poor nuclides
Same mass as an electron, but opposite charge,
the positron emission is opposite beta decay
1 P
1
A X
Z
→
→
1
0n
A X’
Z-1
Example C-11
+
+
0
+1β
0
+1β
Decay Types
Electron Capture
Neutron-poor nuclides
Electron in an inner shell reacts with a proton
1 P+
0 β → 1 n
1
-1
0
A X +
0 β → A X’ + x-ray
Z
-1
Z-1
No change in mass number
Example iron-55
Decay Types
Gamma Emission 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
98
43Tc* →
98
43Tc +
0
0γ
Decay Types
Spontaneous Fission
 Very massive nuclei Z > 103
 Usually large amounts of energy are
released
 Usually neutrons are released
 Example:
 25498Cf → 11846Pd + 13252Te + 4 10n
Decay Types
Various Types of Radioactive Processes Showing
the Changes That Take Place in the Nuclides
Radioactive Decay
Radiochemical Dating
• n = t/t1/2
 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.
Radiocarbon Dating of Artifacts
Calibration Curves
Kinetics of Radioactive Decay
•
Radioactive decay is a first order process, but
using atoms instead of concentration
Radioactive decay rates
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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
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
Sample Problem Solution
• First calculate the rate constant k from the half-life:
k=0.693/5730 = 0.000121 yr-1
• Substitute into the first order rate equation.
• ln(N/N0) = kt
t = ln(N/N0)/k = ln(15/5.1)/0.000121
t = 8910 years old sandals
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 2001.
The 14C/12C ratio in the tusk was only 1.19% of
that in modern elephant tusks. How old is the
mammoth tusk?
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?
Nuclear Transformations
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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
Cyclotron
Nuclear reactions can be induced by accelerating a
particle and colliding it with the nuclide.
Cyclotron
An Aerial View of Fermilab, a High Energy Particle
Accelerator Cyclotron.
The
Accelerator
Tunnel at
Fermilab
Linear Accelerator
Linear Accelerator
Cyclotron
Detection and Uses of Radioactivity
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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
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
Geiger Counter
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
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Protium is the only exception, since there is no defect
The other isotopes of hydrogen deuterium and tritium have
defects
Mass of neutron = 1.008665 amu
Mass of proton = 1.007276 amu
Mass of electron = 0.0005446623 amu , note mass of
electron is not really necessary in calculations since it
subtracts out when finding the difference
Subatomic Particles
Particle
Mass(g)
Charge
Electron(e)
9.11 x 10-28
-1
Proton(p)
1.67 x 10-24
+1
Neutron(n)
1.67 x 10-24
0
Particle
6.64 x 10-24
+2
Positron
9.11 x 10-28
+1
Thermodynamic Stability
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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 10-13 J OR amu = 931 MeV
All nuclei have different relative stabilities, see
figure 18.9
Sample problem:
• 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 = (234.43601 + 4.002603) - 238.050788 =
-0.004584 amu or -4.584X10-6kg
ΔE = mC2 ↔ ΔE =( -4.584X10-6kg)(2.998X108m/s)2
=-4.120X1011 j/mole
ΔE = -0.004584 amu X 931 MeV/amu
Divide by the mass number to get energy per nucleon, called
binding energy
Practice
What is the binding energy of 60Ni? The mass of a
60Ni atom is 59.9308 amu. The mass of an electron
is 9.10939 x 10-31 kg and 1 amu is 1.66054 x 10-27
kg.
Thermodynamic Stability
Revisiting the graph on page 988
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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.
Thermodynamic Stability
• For lighter elements, fusion processes lead to
nuclei with greater binding energy, whereas
heavy elements are formed through other
processes.
Artificial Elements
• Scientists have been transmuting elements
since 1919 when oxygen-17 and hydrogen-1
were produced from nitrogen-14 and 
particles.
14
7N +
4
2He
17
8O +
1
1H
• 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?
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.
Energy in Nuclear Reactions
For example, the mass change for the decay of 1
mol of uranium-238 is −0.0046 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
Fission Process
•
Discovered in the 1930’s when U-235 was
bombarded with neutrons
Neutrons, due to their neutral charge do not require
accelerators
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11n + 23592U → 14156Ba + 9236Kr + 3 11n
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
Fission Process
•
Since neutrons are produced, then it is
possible to have a self-sustaining reaction
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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
Nuclear Fission
• How does one tap all that energy?
• Nuclear fission is the type of reaction
carried out in nuclear reactors.
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.
Nuclear Fission
If there are not enough radioactive nuclides in the path
of the ejected neutrons, the chain reaction will die out.
Nuclear Fission
Therefore, there must be a certain minimum amount of
fissionable material present for the chain reaction to be
sustained: critical mass.
Nuclear Reactors
•
In nuclear reactors the heat generated by
the reaction is used to produce steam that turns
a turbine connected to a generator
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.
Fusion Process
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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?
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
Hydrogen Fusion
• Heavier elements formed through the process
of fusion.
1
1
H 
 H
H +
1
1
H +
2
1
1
1
3
2
2 He
2
1
0
1
+
e (positron)
H 

He


3
2
4
2
He + 2 H
1
1
Effects of Radiation
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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
Quantities of Radiation
Unit
Parameter
Description
Curie (Ci)
Level of
Radioactivity
3.7x1010 nuclear
disintegrations/s
Becquerel (B)*
Level of
Radioactivity
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
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
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•
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
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Gamma highly penetrating, since electromagnetic
energy consisting of photons
Beta particles penetrate up to one cm
Alpha particles are stopped by the skin
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
About REM
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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.
Acute Effects of Single Whole-Body Doses of
Ionizing Radiation
Dose(REM)
0.05-0.25
0.25-1.0
1.0-2.0
2.0-4.0
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
4.0-10.0
Death, usually through infection, within
weeks
>10.0
Death within hours
Typical Radiation Exposures
for a Person Living in the
United States (1 millirem =
10-3 rem)
Sources of Radiation
Biological Effect of Radiation
Radon Gas Release from Rocks
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
192Ir

74 d
Prostate cancer therapy
Coronary disease
Medical Imaging Radionuclides
Nuclide
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
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
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
Red Giant
Supernova
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.
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.
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.
ChemTour: Balancing Nuclear
Reactions
Click to launch animation
PC | Mac
This quantitative exercise teaches nuclear equation
balancing through worked examples and Practice
Exercises.
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.
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.
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.
ChemTour: Balancing Nuclear
Reactions
Click to launch animation
PC | Mac
This quantitative exercise teaches nuclear equation
balancing through worked examples and Practice
Exercises.
End Chapter #20
Nuclear Chemistry