1 Chapter 26 Nuclear Chemistry. 2 Chapter Goals 1. The Nucleus 2. Neutron-Proton Ratio and Nuclear...
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Transcript of 1 Chapter 26 Nuclear Chemistry. 2 Chapter Goals 1. The Nucleus 2. Neutron-Proton Ratio and Nuclear...
2
Chapter Goals
1. The Nucleus2. Neutron-Proton Ratio and Nuclear Stability3. Nuclear Stability and Binding Energy4. Radioactive Decay5. Equations for Nuclear Reactions6. Neutron-Rich Nuclei (Above the Band of
Stability)7. Neutron-Poor Nuclei (Below the Band of
Stability)8. Nuclei with Atomic Number Greater than 839. Detection of Radiation
3
Chapter Goals
10. Rates of Decay and Half-Life
11. Disintegration Series
12. Uses of Radionuclides
13. Artificial Transmutations of Elements
14. Nuclear Fission
15. Nuclear Fission Reactors
16. Nuclear Fusion
4
Comparison Of Chemical and Nuclear Reactions
Nuclear Reactions1 Elements may be
converted from one element to another.
2 Particles within the nucleus, such as protons and neutrons, are involved in reactions.
Chemical Reactions1 No new elements can
be produced, only new chemical compounds.
2 Usually only the outer most electrons participate in reactions.
5
Comparison Of Chemical and Nuclear Reactions
Nuclear Reactions3 Release or absorb
immense amounts of energy, typically 1000 times more.
4 Rates of reaction are not influenced by external factors.
Chemical Reactions3 Release or absorb
much smaller amounts of energy.
4 Rates of reaction depend on factors such as concentration, pressure, temperature, and catalysts.
6
Beginning of Nuclear Science In 1896, Henri Becqurel accidentally
discovered radioactivity in U salts. In 1898, Marie and Pierre Curie discovered
two new radioactive elements in U mine residue. Po and Ra
In 1898, Ernest Rutherford discovered that radioactivity has two distinct forms. and radiation
7
Fundamental Particles of Matter
PARTICLE MASS (amu) CHARGE
Electron(e-)
0.0005458 1-
Proton(p or p+)
1.0073 1+
Neutron(n or n0)
1.0087 none
8
The Nucleus
The nucleus consists of protons and neutrons in a very small volume. Protons and neutrons are made of other
fundamental particles called quarks. Nuclei have a diameter of approximately 10-12 cm Nuclei have a density of approximately 2 x 1014 g/cm3. The strong nuclear force binds the nucleus together at
extremely short distances of 10-13 cm
9
Neutron-Proton Ratio and Nuclear Stability
Terminology used in nuclear chemistry.
1. Nuclides denotes different nuclei.
2. Isotopes are nuclei that have the same number of protons but different neutron numbers.
Isotopes are the same element. Experimentally, it can be shown that nuclei have a
preference for even numbers of protons and neutrons The next table is all of the nonradioactive nuclides broken
into various combinations of protons and neutrons.
10
Neutron-Proton Ratio and Nuclear Stability
Proton Number Neutron Number Number of NuclidesEven Even 157Even Odd 52Odd Even 50Odd Odd 4
11
Neutron-Proton Ratio and Nuclear Stability
Special stability is associated with certain proton and neutron numbers due to shell effects in nuclei similar to the
s, p, d, and f shells in atoms These proton and neutron numbers
are called “Magic Numbers.” Magic numbers are:
2 8 20 28 50 82 126
12
Neutron-Proton Ratio and Nuclear Stability
Example nuclides with magic numbers of nucleons includes:
nucleus magicdoubly a O
nucleus magicdoubly a He
816
8
242 nucleus magicdoubly a He242
nucleus magicdoubly a Ca
nucleus magicdoubly a O
nucleus magicdoubly a He
204020
816
8
242
nucleus magicdoubly a Ca
nucleus magicdoubly a Ca
nucleus magicdoubly a O
nucleus magicdoubly a He
284820
204020
816
8
242
nucleus magicsingly a Sn
nucleus magicdoubly a Ca
nucleus magicdoubly a Ca
nucleus magicdoubly a O
nucleus magicdoubly a He
70120
50
284820
204020
816
8
242
nucleus magicdoubly a Pb
nucleus magicsingly a Sn
nucleus magicdoubly a Ca
nucleus magicdoubly a Ca
nucleus magicdoubly a O
nucleus magicdoubly a He
12620882
7012050
284820
204020
816
8
242
13
Nuclear Stability and Binding Energy
The mass deficiency or mass defect of a nucleus is determined in this fashion.
The mass defect is the mass of the nuclear particles that has been used to bind the nucleus in the nuclear binding energy or strong nuclear force.
atom of mass actuale andn ,p all of masses of summ -
14
Nuclear Stability and Binding Energy
Due to the Einstein relationship, we can calculate the nuclear binding energy for a nucleus.
2
2
2
cm Energy Binding
or
cmE
mc E
15
Nuclear Stability and Binding Energy
Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom.
amu 0.0005458 of mass a haselectron 1
amu 1.0087 of mass a hasneutron 1
amu 1.0073 of mass a hasproton 1
electrons 19 and neutrons 20 protons, 19 hasK 39
16
Nuclear Stability and Binding Energy
Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom.
amu 32307.39
amu 0104.0 amu 1740.20 amu 1387.19
amu 0.000545819 amu 1.008720 amu 1.0073 19
is electrons and neutrons, protons, theof masses theof sum The
amu 0.000545819 amu 1.008720 amu 1.0073 19
:is electrons and neutrons, protons, of masses theof sum The
amu 00110.0m
amu 39.32197-amu 32307.39 m
17
Nuclear Stability and Binding Energy
Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2.
amu
g24mol
amu20
molatoms23
atomamu
10661.110624.6
10022.6 0.00110=m
18
Nuclear Stability and Binding Energy
Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2.
molkg6
molg
amug24
molamu20
molatoms23
atomamu
1010.1 00110.0
10661.110624.6
10022.6 0.00110=m
19
Nuclear Stability and Binding Energy
Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2.
E = mc2 kgmol
ms
kgmol
ms
kg ms mol
Jmol
2
2
2
2
110 10 3 00 10
110 10 9 00 10
9 90 10 9 90 10
6 8 2
6 16
10 10
. .
. .
. .
20
Radioactive Decay
Nuclei whose neutron-to-proton ratio lies outside the belt of stability experience spontaneous radioactive decay.
Decay type depends on where the nuclei is positioned relative to the band of stability.
Radioactive particles are emitted with different kinetic energies. Energy change is related to the change in binding
energy from reactant to products.
22
Equations for Nuclear Reactions
Two conservation principles hold for nuclear reaction equations.
The following principles hold for all nuclear reactions.
1. The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products.
2. The sum of the atomic numbers of the reactants equals the sum of the atomic numbers of the products.
23
Equations for Nuclear Reactions
For the general reaction:
The two conservation principles demand
1. M1 = M2 + M3
and
2. Z1 = Z2 + Z3
Where the M's are mass numbers, And the Z's are atomic numbers.
YRQ 3
3
2
2
1
1
MZ
MZ
MZ
24
Neutron Rich Nuclei (Above the Band of Stability)
These nuclei have too high a ratio of neutrons to protons.
Decays must lower this ratio and include: beta emission neutron emission
Beta emission is associated with the conversion of a neutron to a proton;
01n p1
110
25
Neutron Rich Nuclei (Above the Band of Stability)
Beta emission simultaneously decreases the number of neutrons (by one) and increases the number of protons (by one). Efficiently changes the neutron to proton ratio.
Examples of beta emission:
614
714
-10C N+ 6
14 714
-10
89226
-10
C N +
Ra Ac +
88226
26
Neutron Rich Nuclei (Above the Band of Stability)
Neutron emission does not change the atomic number, but it decreases the number of neutrons. The product isotope is less massive by the mass of 1
neutron. Examples of neutron emission
nI+I
n+ NN10
13653
13753
10
167
177
n+ NN 10
167
177
27
Neutron Poor Nuclei (Below the Band of Stability)
These nuclides have too low a ratio of neutrons to protons.
Nuclear radioactive decays must raise this ratio
The possible decays include: 1. electron capture
2. positron emission
29
Neutron Poor Nuclei (Below the Band of Stability)
Electron capture involves the capture of an electron in the lowest energy level in the atom by the nucleus. conversion of a proton to a neutron
CleAr
n e p3717
01-
3718
10
01-
11
n e p 10
01-
11
30
Neutron Poor Nuclei (Below the Band of Stability)
A positron has the mass of an electron but has a positive charge. The symbol is 0
+1e. Positron emission is associated with the
conversion of a proton into a neutron.
n e p 10
01
11
Ar eK
n e p3918
01
3919
10
01
11
N eO
Ar eK
n e p
157
01
158
3918
01
3919
10
01
11
31
Nuclei with Atomic Number Greater than 83
Alpha emission occurs for some nuclides, especially heavier ones.
Alpha () particles are helium nuclei, 42He, containing two protons and two
neutrons. Alpha emission increases the neutron-to-
proton ratio.
He HgPb 42
20080
20482
32
Nuclei with Atomic Number Greater than 83
All nuclides having atomic numbers greater than 83 are beyond the belt of stability and are radioactive. Many of these isotopes decay by emitting
alpha particles.
He ThU 42
23490
23892
33
Nuclei with Atomic Number Greater than 83
The transuranium elements (Z>92) also decay by nuclear fission in which the heavy nuclide splits into nuclides of intermediate mass and neutrons.
n4 Mo BaCf 10
10642
14256
25298
34
Detection of Radiation
Present radiation detection schemes depend on the fact that particles and radiations emitted by radioactive decay are energetic and some carry charges.
1. Photographic Detection Radioactivity affects photographic plates or
film as does ordinary light. Medical and dental x-ray photographs are
made using this technique.
35
Detection of Radiation
2. Fluorescence Detection Fluorescent substances absorb energy from
high energy rays and then emit visible light. A scintillation counter is an instrument
using this principle.
36
Detection of Radiation
3. Cloud Chambers contain air saturated with a vapor.
Radioactive decay particles emitted ionize the air molecules in the chamber.
The vapor condenses on these ions. Then the ion tracks are photographed.
39
Detection of Radiation
4. Gas Ionization Counters The ions produced by ionizing radiation are
passed between high voltage electrodes causing a current to flow between the electrodes and the current is amplified.
This is the basis of operation of gas ionization counters such as the Geiger-Mueller counter.
42
Rates of Decay and Half-Life
The rates of all radioactive decays are independent of temperature and obey first order kinetics.
The same relationships developed in Chapter 16 apply here as well.
k t aA
Aln
or Ak decay of Rate
0
43
Rates of Decay and Half-Life
For counting radioactive decay the relationship changes just slightly:
k t aN
Nln
or Ak decay of Rate
0
44
Rates of Decay and Half-Life
The half-life, t1/2, is related to the rate constant by the simple relationship:
k a
0.693
k a
2ln t
21
45
Rates of Decay and Half-Life
Example 26-3: How much 60Co remains 15.0 years after it is initially made? 60Co has a half-life of 5.27 years.
k t
0
0
0
A
A
k tA
Aln
case for thisk t k t aA
Aln
e1-y 132.0k
y 27.5
0.693
t
0.693 k
k a
0.693
k a
2ln t
21
21
97.1
y 15.0y 132.0
k t0
0.1A
0.1A
AA1-
e
e
eremains 13.9%or 0.139 A
.139)0( 0.1A
46
Disintegration Series
Some nuclides are so far away from the belt of stability, that it takes many nuclear disintegrations (a series of them) to attain nuclear stability.
Table 26-4 in the textbook outlines in detail three of these disintegration series: The 238U, 235U and 232Th series:
48
Uses of Radionuclides
Radioactive Dating Radiocarbon dating can be used to
estimate the ages of items of organic origin.
14C is produced continuously in the upper atmosphere by the bombardment of 14N by cosmic-ray neutrons:
p C n N 11
146
10
14
49
Uses of Radionuclides
14C atoms react with O2 to form CO2
The CO2 then is incorporated into plant life by photosynthesis.
After the organism dies the 14C content decreases via radioactive decay The 14C half-life is 5730 years.
01-147
146 N C
50
Uses of Radionuclides
The potassium-argon and uranium-lead methods are used for dating older objects.
Potassium-argon method relies on the electron capture decay of 40K to 40Ar
y103.1t
Ar K 9
01
4018
4019
21
e
51
Uses of Radionuclides
The uranium-lead method relies on the alpha decay of 238U to 234Th.
y105.4t
He Th U9
42
23490
23892
21
52
Uses of Radionuclides
Example 26-4: Estimate the age of an object whose 14C activity is only 55% that of living wood.
1. Determine the rate constant for 14C.
14 y1021.1y 5730
693.0
t
0.693k
k
693.0
k a
693.0t
21
21
53
Uses of Radionuclides
2. Determine the age of the object.
ty1021.1 1.82ln
ty1021.1 55%
100%ln
case in thisk t k t aA
Aln
14
14
0
y 4940 t
y1021.1
0.598t
ty1021.1 0.598
14
14
54
Artificial Transmutations of Elements
Bombardment of a nuclide with a nuclear particle can make an unstable compound nucleus that decays to a new nuclide by emission of a different particle.
The rules for balancing equations for nuclear reactions which were presented in the section on radioactivity still hold.
55
Artificial Transmutations of Elements
Bombardment with Positive Ions If the bombarding particle is positively charged, it must be accelerated with sufficient energy to overcome
the coulomb repulsion of the positive nucleus bombarding particles penetrate the nucleus
Particle accelerators such as cyclotrons or linear accelerators are used for this.
58
Artificial Transmutations of Elements
? n 3 H Th
At n 3He Bi
n TcH Mo
10
11
23090
21085
10
42
20983
10
9743
21
9642
59
Artificial Transmutations of Elements
Pa n 3 H Th
At n 3He Bi
n TcH Mo
22891
10
11
23090
21085
10
42
20983
10
9743
21
9642
60
Artificial Transmutations of Elements
Neutron Bombardment Because neutrons have no charge, there is no coulomb repulsion to their nuclear penetration,
so they do not have to be accelerated. Nuclear reactors are often used as neutron sources.
61
Artificial Transmutations of Elements
Neutrons with large kinetic energy are called fast neutrons. Slow neutrons ("thermal neutrons") have had their excess energy decreased by collisions with moderators
Common moderators are hydrogen, deuterium, or the hydrogen atoms in paraffin. Slow neutrons are more likely to be captured by target nuclei.
63
Artificial Transmutations of Elements
reaction n, He H n Li
reaction n, Hg n Hg42
31
10
63
00
20180
10
20080
64
Nuclear Fission Some nuclides with atomic numbers greater than 80 are
able to undergo fission. These nuclei split into nuclei of intermediate masses and emit
one or more neutrons.
Some fissions are spontaneous while others require activation by neutron bombardment.
Enormous amounts of energy are released in these fissions. Some of the numerous possible fission paths for 235U (after
bombardment by a neutron) are:
65
Nuclear Fission
energy n 2 Sr Xe
energy n 2 Rb Cs
energy n 3 Kr Ba
energy n 3 Br La
energy n 4 Zn Sm
Un U
10
9038
14454
10
9037
14455
10
9336
14056
10
8735
14657
10
7230
16062
23692
10
23592
66
Nuclear Fission
Fission is energetically favorable for elements with Z greater than 80 The product nuclides are more stable (near
the high part of the nuclear binding energy curve).
67
Nuclear Fission Reactors
Electricity can be generated from steam heated by nuclear fission reactions.
Greatest danger of nuclear reactors is core meltdown.
There have been two very serious nuclear reactor accidents:
1. Three Mile Island in PA.
2. Chernobyl in the Ukraine.
68
Nuclear Fission ReactorsDescription of Nuclear Reactors
Light Water Reactors use normal water as the coolant and moderator.
Typical Reactor Fuels are: 235UO2
239Pu
Moderator is the material that slows neutrons from fast to thermal. Commonly used moderators are graphite, water, heavy
water.
69
Nuclear Fission Reactors Control Rods are usually made of boron which is an
efficient neutron absorber. Control rods remove neutrons and slow the chain reaction.
Cooling Systems The reactor core must be cooled to remove the heat from
the nuclear reactions. Some possible coolants are:
water - both normal and heavy helium liquid sodium
70
Nuclear Fission Reactors Shielding provides workers and public
with protection from radiation. Lead and concrete are commonly used in
commercial reactors.
71
Nuclear Fusion Fusion, the merging of light nuclei to make
heavier nuclei, is favorable for very light atoms. Extremely high energies or temperatures are
necessary to initiate fusion reactions. The energy source for stars is fusion.
The fusion reaction in main sequence stars is:
energy n He H H 10
42
31
21
72
Nuclear Fusion
Fusion is the most energetic process in nature. Fusion has produced all of the chemical
elements beyond H and He up to Fe. Fusion is a potential energy source for
humans. Thermonuclear or hydrogen bombs
have been in existence since the 1950’s.
73
Nuclear Fusion
Controlled nuclear fusion is a very real possibility. Nuclear fusion must occur at temperatures of 10
million oC. Fusion reactors must contain this temperature
and not melt! Some fusion reactors exist around the world
However at present none can generate a sustainable fusion reaction.
Potential energy source for the 21st Century.
74
Synthesis Question
How are thermonuclear or fusion reactors designed so that the hot plasma, temperature of approximately 10 x 106 degrees, does not touch the sides of the reactor? The reactor would melt if the plasma were to touch the sides.
75
Synthesis Question
Most fusion reactors use intense magnetic fields to confine the hot plasma in the center of the reactor away from the walls.
76
Group Question
Stars are enormous thermonuclear fusion reactors generating enormous amounts of heat and energy. What keeps stars from blowing themselves apart? How do they remain stable for millions and billions of years?