Nuclear Chemistry
Chapter 21
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Radioactivity
• _________________—unstable atomic nuclei
spontaneously emit particles, electromagnetic
radiation (EMR), or both
• ________________________—results from
bombarding nuclei with neutrons, protons,
or other nuclei
______________ (Z) = number of protons in nucleus
______________ (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
proton
1p
1H
or
1
1
neutron
1n
0
A
ZX
electron
0b
0e
or
-1
-1
Element Symbol
positron
0b
0e
or
+1
+1
a particle
4He
4a
or
2
2
A
Z
21.1
What is the difference between
0e
+1
0b
+1
p.673
0
+1e
and
0b
+1
represents an electron in or from an atomic orbital
represents an electron that is physically identical
to an electron in or from an atomic orbital, but this
electron comes from the decay of a neutron to a
proton and an electron—it is also called a beta
particle or ray
21.1
Comparison of Chemical Reactions and Nuclear Reactions
21.1
Balancing Nuclear Equations
1. Conserve mass number (A).
The sum of protons plus neutrons in the products must equal
the sum of protons plus neutrons in the reactants.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
+ 2 10n
235 + 1 = 138 + 96 + 2x1
2. Conserve atomic number (Z) or nuclear charge.
The sum of nuclear charges in the products must equal the
sum of nuclear charges in the reactants.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
92 + 0 = 55 + 37 + 2x0
+ 2 10n
21.1
212Po
decays by alpha emission. Write the balanced
nuclear equation for the decay of 212Po.
4
alpha particle - 42He or 2a
212Po
84
+ AZX
212 = 4 + A
A = 208
84 = 2 + Z
Z = 82
212Po
84
Ex 21.1, p.673
4He
2
4He
2
+ 208
82Pb
21.1
Nuclear Stability and Radioactive Decay
Beta decay
+-10b + n
14C
6
14N
7
40K
19
40Ca
20
________ # of ________ by 1
+ -10b + n
1n
0
________ # of ________ by 1
1p
1
+ -10b + n
Positron decay
++10b + n
________ # of ________ by 1
++10b + n
________ # of ________ by 1
11C
6
11B
5
38
19K
38Ar
18
1p
1
1n
0
++10b + n
n and n have A = 0 and Z = 0
21.2
Nuclear Stability and Radioactive Decay
Electron capture decay
+n
37Ar
18
+ -10e
37Cl
17
55Fe
26
+ -10e
55Mn
25
1p
1
+n
+ -10e
________ # of ________ by 1
________ # of ________ by 1
1n
0
+n
Alpha decay
212Po
84
4He
2
+ 208
82Pb
________ # of ________ by 2
________ # of ________ by 2
Spontaneous fission
252Cf
98
1n
2125
In
+
2
49
0
21.2
Nuclear Stability
•
Certain numbers of neutrons and protons are extra stable
•
n or p = 2, 8, 20, 50, 82 and 126
•
Like extra stable numbers of electrons in noble gases
(e = 2, 10, 18, 36, 54 and 86)
•
Nuclei with even numbers of both protons and neutrons
are more stable than those with odd numbers of neutron
and protons
•
All isotopes of the elements with atomic numbers higher
than 83 are radioactive
•
All isotopes of Tc and Pm are radioactive
Number of Stable Isotopes with Even and Odd Numbers of Protons and Neutrons
21.2
n/p too large
beta decay
X
Y
n/p too small
positron decay or electron capture
p.675
21.2
______________________ is the energy required
to break up a nucleus into its component protons
and neutrons.
BE is an indication of the stability of a nucleus.
In order to compare nuclei of two different
isotopes/elements, we must take into account the
fact that they have different numbers of _________.
For this reason, nuclear binding energy per nucleon
is more useful.
p.676
21.2
Mass Defect
The difference between
the ________________ of an atom
and
the ________________ of the masses of
protons, neutrons, and electrons
What does the mass defect tell us? How much mass was
changed to energy in the formation of the atom.
p.676
The Law of Charges tells us…?
So how can all those positively charged protons be
crammed into the tiny space of the nucleus?
We call it the “strong nuclear force”
or just the “strong force.”
Some of the mass of the nucleons is
converted to energy and lost.
This is the general idea behind fusion:
Build new, larger nuclei and release
great amounts of energy!
Nuclear binding energy per nucleon vs Mass number
note
nuclear binding energy
nucleon
p.678
nuclear stability
21.2
Which element has the greatest net attractive
forces among its nucleons? (graph)
Radioactivity: unstable nuclei spontaneously emit
particles, electromagnetic radiation (EMR), or both
Main types of radioactivity:
• particles (He2+)
• particles (e-)
• rays (short-wavelength emr)
•positron emission
•electron capture
Often it involves a multi-step sequence, a series….
and all obey first-order kinetics.
The Uranium Decay Series
Kinetics of Radioactive Decay
N
daughter
DN
rate = Dt
rate = lN
DN
= lN
Dt
N = N0exp(-lt)
lnN = lnN0 - lt
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
l is the decay or rate constant
ln2
l =
t½
21.3
Kinetics of Radioactive Decay
ln[N] = ln[N]0 - lt
ln [N]
[N]
[N] = [N]0exp(-lt)
l is the first order rate constant and
N is the number of radioactive nuclei present at time t
p.679f
21.3
Radiometric Assumptions
The method measures the parent/daughter
ratio of the elements.
1. The system must initially contain none of
____________________________________.
2. The decay rate must _______________________.
3. The amounts of the parent element and
the daughter products must be affected by
___________________________________.
p.681f
Radiocarbon Dating
14N
7
+ 01n
14C
6
14C
6
14N
7
+ 11H
+ -10b + n
t½ = 5730 years
Uranium-238 Dating
238U
92
206Pb
82
+ 42a + 6-10b
t½ = 4.51 x 109 years
Potassium-40 Dating
40
19K
+-10e
40Ar
18
t½ = 1.2 x 109 years
21.3
Nuclear Transmutation
14N
7
27Al
13
14N
7
+ 24a
+ 24a
+ 11p
17O
8
+ 11p
30P
15
+ 01n
11C
6
+ 42a
Cyclotron Particle Accelerator
p.683
21.4
Nuclear Transmutation
The Transuranium Elements
21.4
Nuclear Fission
235U
92
+ 01n
90Sr
38
1n + Energy
+ 143
Xe
+
3
0
54
Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2
Energy = 3.3 x 10-11J per 235U
= 2.0 x 1013 J per mole 235U
Combustion of 1 ton of coal = 5 x 107 J
p.685f
21.5
Nuclear Fission
Representative fission reaction
235U
92
p.686
+ 01n
90Sr
38
1n + Energy
+ 143
Xe
+
3
0
54
21.5
Nuclear Fission
A ________________________ is a self-sustaining sequence
of nuclear fission reactions.
The minimum mass of fissionable material required to
generate a self-sustaining nuclear chain reaction is the
________________________.
Non-critical
Critical
21.5
Nuclear Fission
Schematic
diagram of a
nuclear
fission reactor
21.5
Nuclear Fission
Annual Waste Production
35,000 tons SO2
4.5 x 106 tons CO2
3.5 x 106
ft3 ash
1,000 MW coal-fired
power plant
70 ft3
vitrified
waste
1,000 MW nuclear
power plant
21.5
Nuclear Fission
Hazards of the
radioactivities in spent
fuel compared to
uranium ore
From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG
21.5
Nuclear Fusion
Fusion Reaction
2
2
3
1
1 H + 1H
1 H + 1H
2H
1
+ 13H
6Li
3
+ 12H
4He
2
2
+ 10n
4He
2
Energy Released
6.3 x 10-13 J
2.8 x 10-12 J
3.6 x 10-12 J
Tokamak magnetic
plasma
confinement
21.6
Radioisotopes in Medicine
•
1 out of every 3 hospital patients will undergo a nuclear
medicine procedure
•
24Na,
•
131I,
t½ = 14.8 hr, b emitter, ________________________
•
123I,
t½ = 13.3 hr, g-ray emitter, _____________________
•
18F,
t½ = 1.8 hr, b+ emitter, ________________________
•
99mTc,
t½ = 14.8 hr, b emitter, _______________________
t½ = 6 hr, g-ray emitter, _____________________
Brain images
with 123I-labeled
compound
21.6
Biological Effects of Radiation
Radiation absorbed dose (rad)
1 rad = 1 x 10-5 J/g of material
Roentgen equivalent for man (rem)
1 rem = 1 rad x Q
Average Yearly Radiation Doses for Americans
Quality Factor
g-ray = 1
b=1
a = 20
21.6
Biological Effects of Radiation
Formation of _______________ and/or ________________
that attack membranes, enzymes, or DNA.
Damage can be ________________ or _______________.
p.695f