Chapter 19

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Chapter 19
The Nucleus: A
Chemist’s View
Chapter 19
Table of Contents
19.1
19.2
19.3
19.4
19.5
19.6
19.7
Nuclear Stability and Radioactive Decay
The Kinetics of Radioactive Decay
Nuclear Transformations
Detection and Uses of Radioactivity
Thermodynamic Stability of the Nucleus
Nuclear Fission and Nuclear Fusion
Effects of Radiation
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Section 19.1
Nuclear Stability and Radioactive Decay
Review
• Atomic Number (Z) – number of protons
• Mass Number (A) – sum of protons and
neutrons
A
Z
X
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Section 19.1
Nuclear Stability and Radioactive Decay
Radioactive Decay
• Nucleus undergoes decomposition to form a
different nucleus.
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Section 19.1
Nuclear Stability and Radioactive Decay
Radioactive Stability
• Nuclides with 84 or more protons are unstable.
• Light nuclides are stable when Z equals A – Z
(neutron/proton ratio is 1).
• For heavier elements the neutron/proton ratio
required for stability is greater than 1 and
increases with Z.
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Section 19.1
Nuclear Stability and Radioactive Decay
Radioactive Stability
• Certain combinations of protons and neutrons
seem to confer special stability.
 Even numbers of protons and neutrons are
more often stable than those with odd
numbers.
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Section 19.1
Nuclear Stability and Radioactive Decay
Radioactive Stability
• Certain specific numbers of protons or neutrons
produce especially stable nuclides.
 2, 8, 20, 28, 50, 82, and 126
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Section 19.1
Nuclear Stability and Radioactive Decay
The Zone of Stability
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Section 19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Alpha production ( ):
• Beta production ( ):
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Section 19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Gamma ray production ( ):
• Positron production:
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Section 19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Electron capture:
Inner-orbital electron
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Section 19.1
Nuclear Stability and Radioactive Decay
Decay Series (Series of
Alpha and Beta Decays)
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Section 19.1
Nuclear Stability and Radioactive Decay
Concept Check
Which of the following produces a  particle?
a)
68
31
b)
62
29
c)
212
87
d)
129
51
Ga +
0
1
Cu 
Fr 
Sb 
e 
0
1
4
2
e+
He +
0
1
e+
68
30
62
28
Zn
electron capture
Ni
positron
208
85
At
alpha particle
Te
beta particle
129
52
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Section 19.2
Atomic
The
Kinetics
Masses
of Radioactive Decay
Rate of Decay
Rate = kN
• The rate of decay is proportional to the number
of nuclides. This represents a first-order
process.
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Section 19.2
Atomic
The
Kinetics
Masses
of Radioactive Decay
Half-Life
• Time required for the number of nuclides to
reach half the original value.
t1/ 2
ln  2  0.693
=
=
k
k
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Section 19.2
Atomic
The
Kinetics
Masses
of Radioactive Decay
Nuclear Particles
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Section 19.2
Atomic
The
Kinetics
Masses
of Radioactive Decay
Half-Life of Nuclear Decay
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Section 19.2
Atomic
The
Kinetics
Masses
of Radioactive Decay
Exercise
A first order reaction is 35% complete at the
end of 55 minutes. What is the value of k?
k = 7.8 x 10-3 min-1
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Section 19.3
The MoleTransformations
Nuclear
Nuclear Transformation
• The change of one element into another.
27
13
1
Al + 42 He  30
P
+
15
0n
249
98
Cf + O 
18
8
263
106
1
0
Sg + 4 n
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Section 19.3
The MoleTransformations
Nuclear
A Schematic Diagram of a Cyclotron
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Section 19.3
The MoleTransformations
Nuclear
A Schematic Diagram of a Linear Accelerator
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Section 19.4
Detection and Uses of Radioactivity
Measuring Radioactivity Levels
• Geiger counter
• Scintillation counter
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Section 19.4
Detection and Uses of Radioactivity
Geiger Counter
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Section 19.4
Detection and Uses of Radioactivity
Carbon–14 Dating
• Used to date wood and cloth artifacts.
• Based on carbon–14 to carbon–12 ratio.
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Section 19.4
Detection and Uses of Radioactivity
Radiotracers
• Radioactive nuclides that are introduced into
organisms in food or drugs and whose
pathways can be traced by monitoring their
radioactivity.
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Section 19.4
Detection and Uses of Radioactivity
Radiotracers
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Section 19.5
Thermodynamic Stability of the Nucleus
Energy and Mass
• When a system gains or loses energy it
also gains or loses a quantity of mass.
E = mc2
m = mass defect
E = change in energy
• If E is negative (exothermic), mass is lost
from the system.
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Section 19.5
Thermodynamic Stability of the Nucleus
Mass Defect (Δm)
4
2
• Calculating the mass defect for He :

Since atomic masses include the masses of the
electrons, we must account for the electron mass.
4.0026 = mass of
1.0078 = mass of
•
4
2 He
1
1H
atom = mass of
atom = mass of
4
2 He
1
1H
nucleus + 2me
nucleus + me
He nucleus is “synthesized” from 2 protons
and two neutrons.
4
2
m =  4.0026  2me  
m =  0.0304 amu
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2 1.0078  me  + 2 1.0087
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28
Section 19.5
Thermodynamic Stability of the Nucleus
Binding Energy
• The energy required to decompose the nucleus
into its components.
• Iron-56 is the most stable nucleus and has a
binding energy of 8.97 MeV.
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Section 19.5
Thermodynamic Stability of the Nucleus
Binding Energy per Nucleon vs. Mass Number
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Section 19.6
Nuclear Fission and Nuclear Fusion
Nuclear Fission and Fusion
• Fusion – Combining two light nuclei to form a
heavier, more stable nucleus.
• Fission – Splitting a heavy nucleus into two
nuclei with smaller mass numbers.
1
0
n+
235
92
U
142
56
91
36
1
0
Ba + Kr + 3 n
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Section 19.6
Nuclear Fission and Nuclear Fusion
Nuclear Fission
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Section 19.6
Nuclear Fission and Nuclear Fusion
Fission Processes
• A self-sustaining fission process is called a
chain reaction.
Neutrons
Causing
Fission
Event
Event
subcritical
<1
critical
=1
supercritical
>1
Result
reaction stops
sustained reaction
violent explosion
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Section 19.6
Nuclear Fission and Nuclear Fusion
Schematic Diagram of a Nuclear Power Plant
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Section 19.6
Nuclear Fission and Nuclear Fusion
Schematic Diagram
of a Reactor Core
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Section 19.6
Nuclear Fission and Nuclear Fusion
Nuclear Fusion
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Section 19.7
Effects of Radiation
Biological Effects of Radiation
Depend on:
1.
2.
3.
4.
Energy of the radiation
Penetrating ability of the radiation
Ionizing ability of the radiation
Chemical properties of the radiation source
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Section 19.7
Effects of Radiation
rem (roentgen equivalent for man)
• The energy dose of the radiation and its
effectiveness in causing biologic damage must
be taken into account.
Number of rems = (number of rads) × RBE
rads
RBE
=
=
radiation absorbed dose
relative effectiveness of the
radiation in causing biologic
damage
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Section 19.7
Effects of Radiation
Effects of Short-Term Exposures to Radiation
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