Transcript Chapter 26

Chapter 26

Nuclear Chemistry
1
Chapter Goals
1.
2.
3.
4.
5.
6.
7.
8.
9.
The Nucleus
Neutron-Proton Ratio and Nuclear Stability
Nuclear Stability and Binding Energy
Radioactive Decay
Equations for Nuclear Reactions
Neutron-Rich Nuclei (Above the Band of
Stability)
Neutron-Poor Nuclei (Below the Band of
Stability)
Nuclei with Atomic Number Greater than 83
Detection of Radiation
2
Chapter Goals
10.
11.
12.
13.
14.
15.
16.
Rates of Decay and Half-Life
Disintegration Series
Uses of Radionuclides
Artificial Transmutations of Elements
Nuclear Fission
Nuclear Fission Reactors
Nuclear Fusion
3
Comparison Of Chemical
and Nuclear Reactions

1
2
Nuclear Reactions
Elements may be
converted from one
element to another.
Particles within the
nucleus, such as
protons and neutrons,
are involved in
reactions.

1
2
Chemical Reactions
No new elements can
be produced, only new
chemical compounds.
Usually only the outer
most electrons
participate in reactions.
4
Comparison Of Chemical
and Nuclear Reactions

3
4
Nuclear Reactions
Release or absorb
immense amounts of
energy, typically 1000
times more.
Rates of reaction are
not influenced by
external factors.

3
4
Chemical Reactions
Release or absorb
much smaller amounts
of energy.
Rates of reaction
depend on factors such
as concentration,
pressure, temperature,
and catalysts.
5
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
6
Fundamental Particles
of Matter
PARTICLE
MASS (amu)
CHARGE
Electron
(e )
Proton
(p or p+)
Neutron
(n or n0)
0.0005458
1-
1.0073
1+
1.0087
none
7
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
8
Neutron-Proton Ratio
and Nuclear Stability

1.
2.

Terminology used in nuclear chemistry.
Nuclides denotes different nuclei.
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
9
neutrons.
Neutron-Proton Ratio
and Nuclear Stability
Proton Number Neutron Number Number of Nuclides
Even
Even
157
Even
Odd
52
Odd
Even
50
Odd
Odd
4
10
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
11
Neutron-Proton Ratio
and Nuclear Stability

Example nuclides with magic numbers of
nucleons includes:
4
4
4
4
He2222
2
2
2 He
2
doubly
magic
nucleus
doubly
magic
nucleus
nucleus
doubly
magic
nucleus
aaa
doubly
magic
nucleus
16
16
16
O
888O 88
doubly
magic
nucleus
doubly
magic
nucleus
aaa
doubly
magic
nucleus
doubly
magic
nucleus
He
He
O
40
40
20
20
doubly
magic
nucleus
Ca2020a a
doubly
magic
nucleus
Ca
doubly
magic
nucleus
48
20
doubly magic
magic nucleus
nucleus
Ca 28 a doubly
120
50
Sn 70 a singly
singly magic
magic nucleus
nucleus
208
82
Pb126 a doubly magic nucleus
12
Nuclear Stability and
Binding Energy


The mass deficiency or mass defect of a
nucleus is determined in this fashion.

m  sum of masses of all p  , n and e-  actual mass of atom 

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.
13
Nuclear Stability and
Binding Energy

Due to the Einstein relationship, we can
calculate the nuclear binding energy for a
nucleus.
E  mc
2
E  m c
or
2
Binding Energy  m c
2
14
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.
39
K has 19 protons, 20 neutrons and 19 electrons
1 proton has a mass of 1.0073 amu
1 neutron has a mass of 1.0087 amu
1 electron has a mass of 0.0005458 amu
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.
Thesum
sumofofthe
themasses
massesofofthe
protons,
neutrons,
electrons
The
protons,
neutrons,and
and
electronsis is:
19
0.0005458amu
amu
191.0073
1.0073 amu   20 1.0087 amu   19  0.0005458
 19.1387 amu  20.1740 amu  0.0104 amu
 39.32307 amu
m  39.32307 amu - 39.32197 amu
m  0.00110 amu
16
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.
m = 0.00110

amu
 6.624 10
atom
20 amu
6.022 10
mol
23 atoms
1.66110
mol
 24 g


amu
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.
m = 0.00110 amu atom 6.022 10 23 atoms mol





 6.624 10 20 amu mol 1.66110  24 g amu
 0.00110
g
mol
 1.10 10
 6 kg
mol
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.

E = mc  110
.  10
2

 110
.  10
 9.90  10
6 kg
mol
6 kg

9.00  10
10 kg m2
s2 mol
16 m2


8m 2
mol 3.00  10 s
s2

10 J
9.90  10
mol
19
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.
20
Radioactive Decay
21
Equations for Nuclear
Reactions

Two conservation principles hold for
nuclear reaction equations.
The following principles hold
for all nuclear reactions.
1.
2.
The sum of the mass numbers of the reactants
equals the sum of the mass numbers of the
products.
The sum of the atomic numbers of the reactants
equals the sum of the atomic numbers of the
products.
22
Equations for Nuclear
Reactions

For the general reaction:
M1
Z1

1.
Q R  Y
M3
Z3
M2
Z2
The two conservation principles demand
M1
=
M2 + M3
and
2.
Z1


=
Z2
+ Z3
Where the M's are mass numbers,
And the Z's are atomic numbers.
23
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;
1
1
0
0 n1 p  1

24
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:
14
14
0
14
6
77
-1
C
+ 
 N
N+
226
226
88 Ra  89 Ac
0
+-1

25
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
17
7
N N+ n
137
53
16
7
1
0
I I+ n
136
53
1
0
26
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.
2.
electron capture
positron emission
27
Neutron Poor Nuclei (Below the
Band of Stability)
28
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

1
1
p e n
37
18
0
-1
1
0
Ar  e Cl
0
-1
37
17
29
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.
0
11
11
0
1

1
0
1
1
0
p
 ee 
nn
p

1
1
0
39
39
19
K e 
0
0
1
39
39
18
Ar
K

e

Ar
19
01
18
15
15
8 O 1 e  7 N
30
Nuclei with Atomic Number
Greater than 83


Alpha emission occurs for some nuclides,
especially heavier ones.
Alpha () particles are helium nuclei,
4 He, containing two protons and two
2
neutrons.

Alpha emission increases the neutron-toproton ratio.
204
82
Pb Hg  He
200
80
4
2
31
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.
238
92
U Th  He
234
90
4
2
32
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.
252
98
Cf  Ba 
142
56
106
42
Mo  4 n
1
0
33
Detection of Radiation

1.
Present radiation detection schemes
depend on the fact that particles and
radiations emitted by radioactive decay
are energetic and some carry charges.
Photographic Detection


Radioactivity affects photographic plates or
film as does ordinary light.
Medical and dental x-ray photographs are
made using this technique.
34
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.
35
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.
36
Detection of Radiation

Diagram of a Simple Cloud Chamber
37
Detection of Radiation

A Cloud Chamber Photo from a Large Detector.
38
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 GeigerMueller counter.
39
Detection of Radiation

Schematic of Geiger Counter
40
Detection of Radiation

Picture of a Geiger Counter
41
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.
Rate of decay  kA  or
 A0 
ln 
akt
 A
42
Rates of Decay and Half-Life

For counting radioactive decay the
relationship changes just slightly:
Rate of decay  kA  or
 N0 
ln 
akt
 N
43
Rates of Decay and Half-Life

The half-life, t1/2, is related to the rate
constant by the simple relationship:
ln 2 0.693
t 12 

ak
ak
44
Rates of Decay and Half-Life

Example 26-3: How much 60Co remains 15.0
years after it is initially made? 60Co has a halflife of 5.27 years.
A
 0 (20.139)
A
t case
ln  01.ln
 a k t  k0.693
t
forkthis
tA



A
e
1 A

0
2
a k or 13.9%
a k remains
A  0.139
A 
-1
 0.693
ln 
 k t



0
.
132
y
15.0 y 
0.693
A
0  1.0 e

Ak 
5.27 y
 A  t1k2t

  e
0 
kA
0.132
A  1.0 ye
1.97
-1
45
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:
46
Disintegration Series
47
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:
14
1
14
1
0
6
1
N  n  C p
48
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.
14
6
C N 
14
7
0
-1
49
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
40
40
0
19
18
1
K  Ar  e
t 12  1.3  10 y
9
50
Uses of Radionuclides

The uranium-lead method relies on the
alpha decay of 238U to 234Th.
238
92
U
Th  He
234
90
4
2
t 12  4.5  10 y
9
51
Uses of Radionuclides

1.
Example 26-4: Estimate the age of an
object whose 14C activity is only 55% that
of living wood.
Determine the rate constant for 14C.
0.693 0.693
t 12 


ak
k
0.693 0.693
 4 1
k

 1.2110 y
t 12
5730 y
52
Uses of Radionuclides
2.
Determine the age of the object.


4 1
A


0
0.598
ln   1.21
a kt10
k ty
in thist case
 A 
0.598
 100%

t

4
1

4

1
ln 

1
.
21

10
y
t
1.21 10 y
 55% 
t  4940
  1.21y10 4 y 1 t
ln 1.82




53
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.
54
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.
55
Artificial Transmutations
of Elements
Example
96
42
reactions are:
Mo  H Tc  n
2
1
97
43
1
0
56
Artificial Transmutations
of Elements
96
42
Mo  H Tc  n
209
83
2
1
97
43
1
0
Bi  He 3 n 
4
2
1
0
210
85
At
57
Artificial Transmutations
of Elements
96
42
Mo  H Tc  n
209
83
2
1
97
43
1
0
Bi  He 3 n 
210
85
Th  H  3 n 
?
4
2
230
90
1
1
1
0
1
0
At
58
Artificial Transmutations
of Elements
96
42
Mo  H Tc  n
209
83
2
1
97
43
1
0
Bi  He 3 n 
210
85
Th  H  3 n 
228
91
4
2
230
90
1
1
1
0
1
0
At
Pa
59
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.
60
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.
61
Artificial Transmutations
of Elements
200
80
Hg  n Hg   n,   reaction
1
0
201
80
0
0
62
Artificial Transmutations
of Elements
200
80
6
3
Hg  n 
1
0
Hg   n,   reaction
201
80
0
0
Li  n  H  He n,   reaction
1
0
3
1
4
2
63
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:
64
Nuclear Fission
235
92
U  01n 
 U 
236
92
160
62
Sm  Zn  4 n  energy
146
57
La  Br  3 n  energy
140
56
93
Ba  36
Kr  301 n  energy
144
55
Cs  Rb  2 n  energy
144
54
Xe  Sr  2 n  energy
72
30
87
35
90
37
90
38
1
0
1
0
1
0
1
0
65
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).
66
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.
2.
Three Mile Island in PA.
Chernobyl in the Ukraine.
67
Nuclear Fission Reactors


Description of Nuclear Reactors
Light Water Reactors use normal water as the
coolant and moderator.
Typical Reactor Fuels are:



235UO
2
239Pu
Moderator is the material that slows neutrons
from fast to thermal.

Commonly used moderators are graphite, water, heavy
water.
68
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
69
Nuclear Fission Reactors

Shielding provides workers and public
with protection from radiation.

Lead and concrete are commonly used in
commercial reactors.
70
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:
2
1
H  H  He  n  energy
3
1
4
2
1
0
71
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.
72
Nuclear Fusion

Controlled nuclear fusion is a very real
possibility.



Fusion reactors must contain this temperature
and not melt!
Some fusion reactors exist around the world


Nuclear fusion must occur at temperatures of 10
million oC.
However at present none can generate a
sustainable fusion reaction.
Potential energy source for the 21st Century.
73
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.
74
Synthesis Question

Most fusion reactors use intense
magnetic fields to confine the hot
plasma in the center of the reactor
away from the walls.
75
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?
76
End of Chapter 26

Nuclear science has been one of the
driving forces of science in the 20th
Century.
77