Transcript Chapter 19 Nuclear Chemistry

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
Review
• Atomic Number (Z) – number of protons
• Mass Number (A) – sum of protons and
neutrons
A
Z
X
Radioactive Decay
• Nucleus undergoes decomposition (or
decay) to form a different nucleus.
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.
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.
Radioactive Stability
• Certain specific numbers of protons or
neutrons produce especially stable nuclides.
 2, 8, 20, 28, 50, 82, and 126
The Band of Stability
Types of Radioactive Decay
• Alpha production ():
• Beta production ():
Types of Radioactive Decay
• Gamma ray production ():
• Positron production:
Types of Radioactive Decay
• Electron capture:
Inner-orbital electron
Decay Series
(Series of Alpha and Beta Decays)
Concept Check
Which of the following produces a  particle?
a)
68
31
b)
62
29
c)
212
87
d)
129
51
Ga +
0
1
e 
Cu 
0
1
Fr 
4
2
Sb 
e+
68
30
62
28
He +
0
1
e+
Zn
electron capture
Ni
positron
208
85
At
alpha particle
Te
beta particle
129
52
Rate of Decay
Rate = kN
• The rate of decay is proportional to the
number of nuclides. This represents a firstorder process.
Half-Life
• Time required for the number of nuclides to
reach half the original value.
t1/ 2
ln  2  0.693
=
=
k
k
Nuclear Particles
Half-Life of Nuclear 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
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
A Schematic Diagram of a Cyclotron
A Schematic Diagram of a Linear
Accelerator
Measuring Radioactivity Levels
• Geiger counter
• Scintillation counter
Geiger Counter
Carbon–14 Dating
• Used to date wood and cloth artifacts.
• Based on carbon–14 to carbon–12 ratio.
Radiotracers
• Radioactive nuclides that are introduced into
organisms in food or drugs and whose
pathways can be traced by monitoring their
radioactivity.
Radiotracers
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.
Mass Defect (m)
• Calculating the mass defect for 42 He :
 Since atomic masses include the masses of the
electrons, we must account for the electron mass.
4.0026 = mass of
4
2 He
1.0078 = mass of
1
1H
atom = mass of
atom = mass of
4
2 He
1
1H
nucleus + 2me
nucleus + me
• 42 He nucleus is “synthesized” from 2 protons and two
neutrons.
m =  4.0026  2me  
m =  0.0304 amu
2 1.0078  me  + 2 1.0087
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.
Binding Energy per Nucleon vs. Mass Number
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
142
91
1
n + 235
U

Ba
+
Kr
+
3
92
56
36
0n
Nuclear Fission
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
Schematic Diagram of a Nuclear Power Plant
Schematic
Diagram
of a Reactor
Core
Nuclear Fusion
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
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 = radiation absorbed dose
RBE = relative effectiveness of the radiation
in causing biologic damage
Effects of Short-Term Exposures to Radiation