Download Nuclear Chem Notes - Warren County Schools

Chapter 15
Nuclear Chemistry
•
•
•
•
•
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Radioactivity
Nuclear Reactions
Rates of Radioactive Decay
Medical Applications of Isotopes
Biological Effects of Radiation
Nuclear Energy
15-1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Radiation
• Radiation
– Energy that comes from a source and travels
through matter or space
– Two types of radiation:
• Electromagnetic
– Includes light, gamma rays, and X-rays
• Particulate
– Mass given off from unstable atoms with the
energy of motion
– Ionizing radiation
• Radiation of either type that can produce charged
particles in matter
15- 2
Radioactivity
• Radioactive decay
– The spontaneous emission of
electromagnetic or other types of
radiation
• Radioactive atoms
– Unstable atoms that give off excess
matter, energy, or both as ionizing
radiation
15- 3
Nucleons
• Nucleons
– General term used to describe nuclear
particles, protons, and neutrons
– Remember:
• Z signifies the atomic number, the number
of protons in the nucleus of an atom
• N signifies the neutron number, the
number of neutrons in the nucleus of an
atom
• Sum of N and Z is A (N+Z = A), the mass
number
15- 4
Nuclides
• Remember, isotopes are:
– Atoms with the same atomic number Z, but
different neutron numbers N and mass
numbers A
• Nuclides
– Isotopes that exist for a measurable length of
time and have a defined energy state
– An atom of a particular atomic number, mass
number and neutron number
15- 5
•
•
•
Of the more than 3,000
nuclides known, about
250 are stable
The rest decompose over
a period of time, emitting
radiation in the process of
creating new nuclides
The stable nuclides have
approximately equal
numbers of protons and
neutrons (N/Z ratio = 1) in
the lighter elements (Z = 1
to 20) and more neutrons
than protons in the
heavier elements (N/Z
ratio > 1).
Band of
Stability
Figure 15.4
15- 6
Radiation
• In a nuclear reaction, an emission of
radiation usually accompanies changes
in the composition of the nucleus.
• Natural radiation associated with
radioactive decay can be placed into
three classes:
– Alpha particles
– Beta particles
– Gamma rays
15- 7
Radiation
• The three classes of natural radiation
behave differently in an electric field,
as shown below:
Figure 15.5
15- 8
Table 15.1 Properties of Types
of Radiation
Radiation
Notation Mass Charge
Type
a , 42 He 2+
Alpha
4
2
Beta
(electron)
0
-1
Beta
(positron)
0
1
Gamma
b-
b
γ
+
Penetration
into Al
4
2+
0.01 mm
~0
1-
0.5-1.0 mm
~0
1+
(Reacts with
electrons)
0
0
50-110 mm
15- 9
Types of Radiation
•
Alpha particles
– Nuclei of helium-4 atoms
– Contain 2 protons and 2
neutrons
– Least harmful to animal
and human tissue
•
Gamma rays
– High energy
electromagnetic radiation:
energy without charge or
mass
– Highest energy and most
penetrating type of
radiation
15-10
Types of Radiation
• Beta particles
– Small, charged particle
that can be emitted from
unstable atoms at speeds
approaching the speed of
light
– Penetrate through skin
into tissue
– 2 types of beta particles:
• Positron
– Same mass as an
electron with an
opposite charge
• Electron
15- 11
Nuclear Reactions
• Two conditions must be met to
balance a nuclear equation:
1. Conservation of mass number
2. Conservation of nuclear charge (atomic
number)
• Examples:
Th ®
228
88
Th ®
231
91
232
90
231
90
238
92
Ra + a
Pa + -10b -
U+ a ®
4
2
4
2
239
94
Pu + 3 n
1
0
15-12
Alpha Particle Emission
• When a nucleus emits an alpha particle, it loses
2 protons and 2 neutrons, so its atomic number
decreases by 2 and its mass number decreases
by 4. Happens with nuclei of about 80+ amu.
232
228
4
90
88
2
Th ®
Ra + a
Figure 15.7
15-13
Beta Particle (Electron) Emission
• When a nucleus emits a beta particle (electron),
its atomic number increases by 1 and its mass
number remains unchanged.
Th ®
231
90
231
91
Pa + b
0
-1
-
Figure 15.8
15-14
Beta Particle (Positron) Emission
• When a nucleus emits a beta particle (positron),
its atomic number decreases by 1 and its mass
number remains unchanged.
23
12
Mg ® Na + b
23
11
0
1
+
Figure 15.9
15-15
Electron Capture
• A proton and an electron combine to
form a neutron. The mass number
stays the same, but the atomic
number decreases by 1.
• Very few nuclides undergo this
transformation.
7
4
Be + e ® Li
0 -1
7
3
15-16
Gamma Ray Emission
• In all nuclear reactions, the nucleus changes from a
state of higher energy to a state of lower energy.
• Gamma rays are pure electromagnetic energy.
• Results in no change in mass or atomic number.
99mTc
99Tc + γ
Figure 15.10
15-17
Practice – Nuclear Reactions
• Fill in the appropriate nuclide for the
X in the following nuclear reactions:
1. X ®
2.
214
82
Pb + a
4
2
Th ® X + b
234
90
0
-1
3. X ® B + b
11
5
0
1
-
+
15-18
Practice Solutions – Nuclear
Reactions
• Fill in the appropriate nuclide for the
X in the following nuclear reactions:
Po ®
1.
218
84
2.
234
90
3.
11
6
Th ®
214
82
Pb + a
234
91
4
2
Pa + b
0
-1
C® B+ b
11
5
0
1
-
+
15-19
Nuclear Bombardment Reactions
•
•
•
•
Nuclei are hit with a beam of nuclei or nuclear
particles to trigger a nuclear reaction
Occurs when a nuclear reaction is not
spontaneous and is produced intentionally by
artificial means
Used to synthesize transuranium elements,
those following uranium on the periodic table
Some examples:
238
92
97
42
U+ n®
1
0
239
92
U®
239
93
Np + b
0
-1
-
Mo + H ® Tc + 2 n
209
83
2
1
Bi + a ®
4
2
97
43
211
85
1
0
At + 2 n
1
0
15-20
Practice – Nuclear Bombardment
Reactions
• Bombarding a bismuth-209 target
with a beam of another nuclide
produces bohrium-262 and a
neutron. Identify the nuclide used in
the bombardment, and write a
balanced equation to describe this
nuclear reaction.
15-21
Practice – Nuclear Bombardment
Reactions
• Bombarding a bismuth-209 target with a
beam of another nuclide produces
bohrium-262 and a neutron. Identify the
nuclide used in the bombardment, and
write a balanced equation to describe
this nuclear reaction.
209
83
Bi + X ®
262
107
Bh + n
1
0
15-22
Practice Solutions – Nuclear
Bombardment Reactions
209
83
Bi + X ®
262
107
Bh + n
1
0
For the mass number:
209 + A = 262 + 1
A = 54
For the atomic number:
83 + Z = 107 + 0
Z = 24
209
83
Bi + Cr ®
54
24
262
107
Bh + n
1
0
15-23
Particle Accelerators
• Particle accelerators are used for nuclear
bombardment reactions.
• The synchroton, perhaps the most
successful accelerator, uses a circular
path for the accelerating particles.
Figure 15.13
15-24
•
•
•
Spontaneous Nuclear Decay
Reactions
The tendency for the
neutron/proton (N/Z) ratio to
move toward the band of
stability, explains the
nuclear reactions of
naturally radioactive
nuclides.
For every process except γ
emission, the change that
occurs for an unstable
nuclide takes it closer to the
observed band of stability.
Radioactive nuclides convert
spontaneously over time to
form stable nuclides.
Figure 15.14
15-25
Table 15.2 Nuclear Instability
Reason for
Nuclear
Instability
Radioactive
Process
Excess Mass
Alpha decay
4
2
N/Z too high
Beta decay
0
-1
N/Z too low
Positron
emission
N/Z too low
Electron
capture
-
Increase
Energetically
excited
Gamma
emission
γ ray
None
Emitted
Radiation
Change in
N/Z Ratio
a
Slight
increase
0
1
b
b
-
+
Decrease
Increase
15-26
Practice – Predicting the Method of Decay
• Predict the method of radioactive decay
of the unstable nuclide Neon-18.
The N/Z ratio in this nuclide is :
18 - 10 8
=
= 0.8
10
10
This ratio is lower than the ideal ratio of 1, so the
nuclide undergoes radioactiv e decay to increase
the value. The emission of a positron converts a
proton to a neutron and a positron.
18
0 +
Ne
®
10
1b
+
18
9F
15-27
Radioactive Decay Series
•
•
•
In heavier elements,
often the product of
radioactive decay is
itself radioactive.
In such cases, a
series of alpha and
beta decay steps
ultimately leads to a
stable nuclide.
Accounts for most of
the radioactive decay
among elements 83
through 92.
Figure 15.15
15-28
Detecting Radiation
• Various instruments have been
developed to give speedier and more
accurate measures of radiation intensity:
– Geiger-Muller counter
– Scintillation counter
Figure 15.16
15-29
Half-Life
• The time required for half of a sample of
a nuclide to decay to a different nuclide
• It takes the same time for a fresh sample
to decay to one-half the original number
of atoms of that nuclide as it does onehalf to decay to one-fourth and so on.
• The shorter the half-life of a nuclide, the
more intense the radiation that it emits.
http://www.eserc.stonybrook.edu/ProjectJava/Radiation/
15-30
Half-Life
15-31
Archeological Dating
• Radio-carbon dating
– Using carbon-14 to measure time on an
archeological scale.
– As long as a plant or animal is alive, its
carbon-14 content should match that in the
atmosphere.
– After it dies, its carbon-14 content
decreases through beta decay:
14
14
0 6
7
-1
– The half-life of the process is 5730 years.
C® N+ b
15-32
Medical Applications
• Many medical applications exists
that use radioactivity:
– Power generators
• Example: 238Pu is used to power
pacemakers
– Medical diagnoses
• Radioactive nuclides are used as tracers
to track movements of substances in
chemical or biological systems
• Example: 99mTc is used to help doctors
locate tumors
15-33
Medical Applications
– Positron Emission Topography
• A PET scan detects abnormalities in living
tissues without disrupting the tissue.
15-34
Medical Applications
– Cancer therapy
• Radioactive nuclides, in much higher
doses than those used for imaging, are
used to treat cancerous tumors.
• Cancer cells absorb nutrients containing
gamma-emitting components, the gamma
radiation becomes concentrated in the
cancerous cells, destroying them in
greater numbers than normal cells.
• Examples: 131I destroys thyroid tumors,
198Au used to treat lung cancer, 32P used
for eye tumors.
15-35
Biological Effects of Radiation
•
Radiation can have one of four effects on
the functioning of a cell:
1. The radiation can pass through the cell with no
damage.
2. The cell can absorb the radiation and be
damaged, but it can subsequently repair the
damage and resume normal functioning.
3. The cell can be damaged so severely that it
cannot repair itself. New cells formed from this
cell will be abnormal. This mutant cell can
ultimately cause cancer if it continues to
proliferate.
4. The cell can be so severely damaged that it dies.
15-36
Biological Effects of Radiation
•
Radon
– A rare noble gas which has also
been implicated as a possible cause
of lung cancer.
– Accumulates in houses from
particular kinds of soils or rock
strata.
15-37
Nuclear Energy
• Fission
– Splitting of a heavy nucleus into two or more
lighter nuclei and some number of neutrons
– Example:
235
92
235
92
U + n ® Kr +
141
56
235
92
U + n ® Sr +
143
54
1
0
92
36
1
0
90
38
U + n ® Zr +
1
0
94
40
140
58
Ba + 3 n
1
0
Xe + 3 n
1
0
Ce + 2 n + 6 b
1
0
0
-1
15-38
-
Fission of Uranium-235
Figure 15.22
15-39
Chain Reactions
•
•
•
•
A reaction in which the product of one step is
the reactant in another step.
In order for a chain reaction to sustain itself,
the amount and shape of the sample of
fissionable material must be such that the
neutrons will not escape due to energy that is
higher than optimum for inducing further
fission.
A chain reaction should maintain a constant
rate.
Critical mass
– The smallest amount of fissionable material
necessary to support a continuing chain reaction.
15-40
Plutonium
When nonfissionable U-238 captures a fast
neutron, it eventually forms the fissionable
nuclide of plutonium, Pu-239, which can
support a chain reaction. Plutonium is a
transuranium element, meaning that it
has an atomic number greater than the 92
of uranium. The fissionable plutonium
produced in a uranium-fueled reactor can
be used as a fuel or in nuclear weapons.
41
Fission Reactors
• Nuclear power plants
use fission to
produce electric
energy
• If the chain reaction
is going too quickly,
movable control rods
made of these
elements are
inserted into a core
of uranium fuel in
fission reactors
Figure 15.23
15-42
Fission Reactors
15-43
A Nuclear World?
Nuclear energy generates about 21 percent of the electricity
produced in the United States. Questions of safety, costs, and
nuclear waste disposal have halted construction of nuclear
reactors in the United States.
44
A Nuclear World?
Nuclear Power plants locations throughout the world.
45
Fusion Reactions
• Fusion
– Combination of light nuclei to form heavier
nuclei
– A major fusion reaction occurs continuously
in the Sun and other stars:
4 11H ® 42 He + 2 10b +
This process occurs in several steps :
1
1
H+ H® H+ b
1
1
1
1
1
1
2
1
0
1
+
H + 21H ® 23 He
H + He ® He + b
3
2
4
2
0
1
+
15-46
Fusion Reactor
Figure 15.26
15-47
•
Fusion Reaction Terms
Ignition temperature
– Temperature required to initiate a fusion reaction
•
Breeder reactors
– A reactor that produces fuel that can be used in
other reactors
•
Plasma
– An ionized gas that must created and controlled at
temperatures of about 108 K
– Melts most container material
•
•
Until recently, fusion in reactors required more
energy than was given off
In order to achieve fusion, the gaseous
reactants must be condensed to a small volume
at high temperatures.
15-48
Nuclear Fusion.
Nuclear fusion produces tremendous quantities of energy and
has the potential of becoming the ultimate source of energy on
earth.
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