Nuclear Chemistry
Chapter 19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
A
ZX
Element Symbol
proton
1p
1H
or
1
1
neutron
1n
0
electron
0β
0e
or
-1
-1
positron
0β
0e
or
+1
+1
α particle
4He
4α
or
2
2
A
1
1
0
0
4
Z
1
0
-1
+1
2
23.1
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
23.1
212Po
decays by alpha emission. Write the balanced
nuclear equation for the decay of 212Po.
4
alpha particle - 42He or 2α
212Po
84
4He
2
+ AZX
212 = 4 + A
A = 208
84 = 2 + Z
Z = 82
212Po
84
4He
2
+ 208
82Pb
23.1
23.1
Nuclear Stability and Radioactive Decay
Beta decay
+-10β + ν
14C
6
14N
7
40K
19
40Ca
20
Decrease # of neutrons by 1
+ -10β + ν
1n
0
Increase # of protons by 1
1p
1
+ -10β + ν
Positron decay
++10β + ν
Increase # of neutrons by 1
++10β + ν
Decrease # of protons by 1
11C
6
11B
5
38
19K
38Ar
18
1p
1
1n
0
++10β + ν
ν and ν have A = 0 and Z = 0
23.2
Nuclear Stability and Radioactive Decay
Electron capture decay
+ν
37Ar
18
+ -10e
37Cl
17
55Fe
26
+ -10e
55Mn
25
1p
1
Increase # of neutrons by 1
+ν
Decrease # of protons by 1
+ -10e
1n
0
+ν
Alpha decay
212Po
84
4He
2
+ 208
82Pb
Decrease # of neutrons by 2
Decrease # of protons by 2
Spontaneous fission
252Cf
98
1n
2125
In
+
2
49
0
23.2
n/p too large
beta decay
X
Y
n/p too small
positron decay or electron capture
23.2
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
23.2
Nuclear binding energy (BE) is the energy required to break
up a nucleus into its component protons and neutrons.
BE + 199F
911p + 1010n
E = mc2
BE = 9 x (p mass) + 10 x (n mass) – 19F mass
BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984
BE = 0.1587 amu
1 amu = 1.49 x 10-10 J
BE = 2.37 x 10-11J
binding energy
binding energy per nucleon =
number of nucleons
2.37 x 10-11 J
= 1.25 x 10-12 J
=
19 nucleons
23.2
Nuclear binding energy per nucleon vs Mass number
nuclear binding energy
nucleon
nuclear stability
23.2
Kinetics of Radioactive Decay
N
daughter
∆N
rate = ∆t
rate = λN
∆N
= λN
∆t
N = N0exp(-λt)
lnN = lnN0 - λt
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
λ is the decay constant
ln2
λ =
t½
23.3
Kinetics of Radioactive Decay
ln[N] = ln[N]0 - λt
ln [N]
[N]
[N] = [N]0exp(-λt)
23.3
Fig. 24.4
Radiocarbon Dating
14N
7
+ 01n
14C
6
14C
6
14N
7
+ 11H
+ -10β + ν
t½ = 5730 years
Uranium-238 Dating
238U
92
206Pb
82
+ 8 24α + 6-10β
t½ = 4.51 x 109 years
3 billion metric tons/year mined lead
6,306,300 tons U total resources on the planet
that have been identified.
23.3
Fig. 24.5
Dr. Willard Libby, the founder of the carbon-14 dating method, assumed the ratio of 14/12 to be constant. In
Dr. Libby’s original work, he noted that the atmosphere did not appear to be in equilibrium. Geologic time
scales suggest that the world is billions of years old and enough time had passed to achieve equilibrium.
Dr. Libby’s calculations showed that if the earth started with no 14C in the atmosphere, it would take up to
30,000 years to build up to a steady state (equilibrium).
Percent 14C Remaining Percent 12C Remaining
100
100
50
100
25
100
12.5
100
6.25
100
3.125
100
Ratio
1 to 1T
1 to 2T
1 to 4T
1 to 8T
1 to 16T
1 to 32T
Number of Half-Lives
0
1
2
3
4
5
Years Dead(Age of Fossil)
0
T = Trillion
5,730
11,460
17,190
22,920
28,650
Maximum number of half-lives that are possible indicates only 80,000 years of measurable data can exist.
The Specific Production Rate (SPR) of 14C is understood to be 18.8 atoms per gram of total carbon per
minute.
The Specific Decay Rate (SDR) is known to be only 16.1 disintegrations per gram per minute.3
This indicates that if it takes about 30,000 years to reach equilibrium and 14C is still out of equilibrium,
then the earth is not as geologically old as we assume.
Every sample of diamond and coal measured- the impervious and oldest mineral on the planet- contains
radioactive carbon 14 in a measurable level.
Nuclear Transmutation
14N
7
27
13 Al
14N
7
+ 24α
+ 24α
+ 11p
17O
8
+ 11p
30P
15
+ 01n
11C
6
+ 42α
Cyclotron Particle Accelerator
23.4
Page 1040
Nuclear Transmutation
23.4
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
23.5
Nuclear Fission
Nuclear chain reaction 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
critical mass.
Non-critical
Critical
23.5
Nuclear Fission
Schematic
diagram of a
nuclear
fission reactor
23.5
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
23.5
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
23.5
Radioisotopes in Medicine
•
1 out of every 3 hospital patients will undergo a nuclear
medicine procedure
•
24Na,
•
131I,
t½ = 14.8 hr, β emitter, thyroid gland activity
•
123I,
t½ = 13.3 hr, γ−ray emitter, brain imaging
•
18F,
t½ = 1.8 hr, β+ emitter, positron emission tomography
•
99mTc,
t½ = 14.8 hr, β emitter, blood-flow tracer
t½ = 6 hr, γ−ray emitter, imaging agent
Brain images
with 123I-labeled
compound
23.7
Geiger-Müller Counter
23.7
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
Quality Factor
γ-ray = 1
β=1
α = 20
23.8
Chemistry In Action: Food Irradiation
Dosage
Effect
Up to 100 kilorad
Inhibits sprouting of potatoes, onions, garlics.
Inactivates trichinae in pork. Kills or prevents insects
from reproducing in grains, fruits, and vegetables.
100 – 1000 kilorads
Delays spoilage of meat poultry and fish. Reduces
salmonella. Extends shelf life of some fruit.
1000 to 10,000 kilorads
Sterilizes meat, poultry and fish. Kills insects and
microorganisms in spices and seasoning.