Nuclear Chemistry
Chapter 21
Stable vs. Unstable Nuclei
1. Most nuclei are stable – do not change
2. Some nuclei are unstable (radioactive)
•
•
•
Change into a different nucleus
Spontaneous process – happens naturally, by
itself
Releases radiation
Only nuclear reactions can change a nucleus.
No chemical process can
Radium

Radon
+ Radiation
1. The radium was unstable (radioactive)
2. Turned into a different element (decayed)
3. The lost mass was turned into radiation
Nuclear Radiation
• Is spontaneously emitted from a radioactive
nucleus
• Can not be seen, smelled, heard
• Can be detected using a Geiger counter or
photographic film
Uses of Radiation
1.
2.
3.
4.
5.
6.
Nuclear fuel (235U and 239Pu)
Nuclear Weapons
Irradiated Food
Smoke Alarms (Amercium-241)
Cancer treatment (Cobalt-60)
Medical Tracers
Types of Nuclear Radiation
Alpha particle Helium nucleus
2 p+
a (42He)
Beta particle
b
(0-1e)
Gamma rays
g (00g)
2n
fast-moving electron.
e-
high energy form of
electromagnetic radiation
The Electromagnetic Spectrum
Light
Dangerous (ionizing)
Safe radiation (non-ionizing)
Radio Radar Micro
IR
Visible
Light
UV
Xrays
Gamma
Produced
by nuclear
decay
What Stops Radiation
Paper
Alpha (a)
Beta (b)
Gamma (g)
Al Foil
Lead.
Wood
Iron,
Concrete
Decay Equations
Alpha Decay
238 U  4 He +
92
2
Beta Decay
234 Th  0 e +
90
-1
234
234
90Th
91Pa
Decay Equations
Gamma Decay
Occurs with alpha and beta decay
No change in atomic mass (gamma radiation
has no mass 00g)
Decay: Ex 1
What product is formed when radium-226
undergoes alpha decay?
226
88U 
4
2He
226
88U 
4
2He +
+ ?
222
86Rn
Decay: Ex 2
What element undergoes alpha decay to form
lead-208?
?

4
212
84Po 
4
2He +
2He +
208
82Pb
208
82Pb
Decay: Ex 3
What isotope is produced when thorium-231
beta decays?
231
90Th 
0
-1e
231
90Th 
0
-1e +
+ ?
231
91Pa
Positron Emission
– Same mass an electron, but opposite charge
– Form of anti-matter
0 e
1
Electron Capture
– Nucleus captures a core electron
– electron is added rather than lost
Common Particles
Particle
Symbol
Apha
4 He
2
Beta
0 e
-1
Positron
0 e
1
Electron
0 e
-1
Proton
Neutron
1 H
1
or 11p
1 n
0
Decay: Ex 4
Write the equation that describes oxygen-15
undergoing positron emission.
Write the equation that describes mercury-201
undergoing electron capture
Which nuclei are radioactive (unstable)
1. All elements have at least one radioactive
isotope
2. All isotopes of elements heavier than Lead
(element 82) are radioactive
3. All elements heavier than 92 (U) are manmade and radioactive
82
Pb
At least one
radioactive isotope
207.2
All isotopes are
radioactive
• Belt of stability – based on neutron:proton
ratio
– Below ~20 = 1:1 ratio stable
– Ratio increases with increasing # protons
– Isotopes outside the belt try to decay and get on
the belt
Decay Modes
• Above belt
– Too many neutrons
– Beta emission
• Below belt
– Too few neutrons
– electron capture or positron emission
• Atomic # >84
– Alpha Decay
• Most heavy isotopes
(above 84) decay by
alpha emission
• Slide down to lead206
Decay Modes: Ex 1
Predict the decay mode for carbon-14
Too many n’s, prefers 1:1
8n : 6p
14
0 e + 14 N
C

6
-1
7
(ratio now 1:1)
Decay Modes: Ex 2
Predict the decay mode for xenon-118
64n : 54p =1.2
Too few n’s (check graph)
0 e  118 I
Xe
+
54
-1
53
or
118 Xe  0 e + 118 I
54
-1
53
118
Decay Modes: Ex 3
Predict the decay mode for plutonium-239
Predict the decay mode for indium-120
Further Observations
• Magic #’s - Nuclei with 2, 8, 20, 28, 50 or 82
protons or 2, 8, 20, 28, 50 or 126 neutrons
are especially stable.
• Nuclei with even #s of both protons and
neutrons are more stable than those with
odds numbers.
Ex: 63Cu and 65Cu are abundant, but 64Cu is
not. Why?
Transmutation
• Rutherford(1919) – First successful
alchemist
14 N + 4 He  17 O + 1 H
7
2
8
1
14 N(a,p) 17 O
7
8
• Modern methods
– Particle Accelerators (Cyclotrons)
– Use neutrons or other elements (creation of
transuranium elements)
Transmutation: Ex 1
Write the balanced nuclear equations for the
process : 2713Al(n, a) 2411Na
Transmutation: Ex 2
Write the shorthard notation for:
16
1 H 
O
+
8
1
13
4 He
N
+
7
2
Transmutation: Neutrons
• Neutrons produced from radioactive decay
• Cobalt-60 is used in radiation therapy
58 Fe
26
59 Fe
26
59 Co
27


+ 10n 
+ 10n
59 Fe
26
59 Co
27
60 Co
27
+ 0-1e
Transmutation: Transuranium
Elements
 23992U  23993Np + 0-1e
238
1 n
U
+
92
0
230
4 He  242 Cm + 1 n
Pu
+
94
2
96
0
209
64 Ni  272
1 n
Bi
+
Uuu
+
83
28
111
0
Half-Life
• Half-life - The time during which one-half of
a radioactive sample decays
– Ranges from fraction of a second to billions of
years.
– You can’t hurry half-life.
Carbon-14 dating
•
14C
atoms get incorporated into living
things through breathing and eating.
• Ratio of 14C to12C in a living organism is
equal to that in the atmosphere
• When an organism dies, the ratio changes
as 14C radioactively decays. The amount
of 14C starts to decrease over time.
Carbon-14 dating
• By measuring the amount of 14C in traces of onceliving organisms, one can determine how long ago
it died.
– E.g., a 5730 years after death, only half of the 14C
remains.
• Reasonable to up to 50,000 years.
• There is a 15% margin of error
• Used in mummies, the Dead Sea Scrolls, Shroud of Turin
Half-Life
Isotope
Uranium-238
Half-life
4.51x109 years
Lead-210
20.4 years
Polonium-214
1.6x10-4 seconds
The polonium-214 will decay much sooner than the uranium. The uranium will be
radioactive pretty much until the earth is destroyed when our sun goes out in 10
billion years.
Half-life: Example 1
Carbon-14 has a half-life of 5730 years and
is used to date artifacts. How much of a
26 g sample will exist after 3 half-lives?
How long is that?
Half-life: Example 1
# of half-lives # of Years
passed
0
0
1
5730
2
3
Amount of Carbon14 remaining
26 grams
Half-life: Example 2
Tritium undergoes beta decay and has a half
life of 12.33 years. How much of a 3.0 g
sample of tritium remains after 2 half-lives?
Solution to Problem
# of halflives
0
1
2
# of Years
passed
0
Amount of tritium
remaining
Half-life: Example 3
Radon-226 has a half-life of 1600 years? How
much of a 30 gram sample remains after
6400 years?
Solution to Problem
# of halflives
0
1
2
3
4
5
# of Years
passed
0
Amount of radon
remaining
Half-life: Example 4
Cesium-137 has a half-life of 30 years. If you
start with a 200 gram sample, and you now
have 25 grams left, how much time has
passed?
Solution to Problem
# of halflives
0
1
2
3
4
5
# of Years
passed
0
Amount of Cesium
remaining
Half-life: Example 5
Calcium-45 has a half-life of 160 days. If you
start with a 500 gram sample, and you now
have 31.25 grams left, how much time has
passed?
Solution to Problem
# of halflives
0
1
2
# of Years
passed
0
Amount of
Calcium remaining
Rate Law
First order rate law
Rate = kN (N is the initial concentration)
Rate =
-DN
=
dN = -kN
Dt
dt
dN = -kN
dt
dN = -kdt
N
∫dN = ∫-kdt
N
∫dN = -k∫dt
N
lnNt = -kt
N0
(Integrate left from N0 to Nt
and time from 0 to t)
Calculating k or the half-life
lnNt = -kt
N0
ln1 = -kt½
2
k = 0.693
t½
Rate Law: Ex 1
Uranium-238 has a half-life of 4.5 X 109 yr. If
1.000 mg of a 1.257 mg sample of uranium238 remains, how old is the sample?
k = 0.693
t½
k = 0.693
4.5 X 109 yr
=
1.5 x10-10 yr
lnNt = -kt
N0
ln 1.000 = -(1.5 x10-10 yr)t
1.257
t = 1.7 X 109yr
Rate Law: Ex 2
A wooden object is found to have a carbon-14
activity of 11.6 disintegrations per second.
Fresh wood has 15.2 disintegrations per
second. If the half-life of 14C is 5715 yr, how
old is the object?
Rate Law: Ex 2
A wooden object is found to have a carbon-14
activity of 11.6 disintegrations per second.
Fresh wood has 15.2 disintegrations per
second. If the half-life of 14C is 5715 yr, how
old is the object?
ANS: 2230 yr
Rate Law: Ex 3
After 2.00 yr, 0.953 g of a 1.000 g sample of
strontium-90 remains. How much remains
after 5.00 years?
lnNt = -kt
N0
ln0.953 = -k(2.00 yr)
1.000
k = 0.0241 yr-1
lnNt = -kt
N0
ln x
= (0.0241 yr-1)(5.00 yr)
1.000
ln x
= -0.120
1.000
x = e-0.120
x =0.887 g
Ex 4
A sample for medical imaging contains 18 F
(1/2 life = 110 minutes). What percentage of
the original sample remains after 300
minutes?
ANS: 15.1%
E = mc2
• Energy changes in chemical reactions
– Exothermic – gives off energy, products mass
less than reactants
– Endothermic – absorbs energy, products mass
more than reactants
– THESE MASS CHANGES ARE WAY TOO
SMALL TO MEASURE
• Energy Changes in nuclear decay
– Mass loss from nuclei
– Energy always released
– This energy is additional kinetic energy given to
the products (products move faster than
reactants)
c = 3.00 X 108 m/s
E = mc2: Ex1

92U
238.0003 amu
238.0003 amu
238
234
4 He
Th
+
90
2
233.9942 amu 4.0015amu
237.9957 amu
Dm = -0.0046 g/mol = -4.6 X 10-6 kg/mol
E = mc2
E = (4.6 X 10-6 kg/mol)(3.00X108 m/s)2
E = 4.1 X 1011 J/mol
(can power a 60-W light bulb for 217 years)
E = mc2: Ex 2
Calculate the energy released from the
following decay.
60 Co
0 e +
60 Ni

27
-1
28
60 Co
27
0 e
-1
60 Ni
28
59.933819 amu
0.00054858 amu
59.930788 amu
ANS: 2.724 X 1011 J/mol
E = mc2: Ex 3
The following decay produces 2.87 X 1011
J/mol of 116C. What is the mass change in
this decay?
11
6C

11
5B +
ANS: -3.19 X 10-3 g/mol
0
1e
Binding Energy
• The mass of nuclei are ALWAYS less than
the masses of individual protons and
neutrons (nucleons).
• Mass defect
• Nuclear Binding Energy – energy needed
to separate nucleus into p & n
– The larger the binding energy, the more stable
the isotope
– Iron-56 has the highest binding energy
– Stars only make up to Iron-56 (unless
supernova)
The Four Forces
Force
Range
Description
Strong Nuclear Force
Short
Range
(nucleus)
Strongest, holds nucleus together
(gluons)
Electromagnetic
Infinite
Range
Between positive and negative
charges (virtual photons)
Weak Nuclear Force
Short
Range
(nucleus)
Involved in some nuclear decay
(quark to quark transmutations, J
particle)
Gravity
Infinite
Range
Weakest, between any object with
mass, even dark matter (gravitons)
Strong Nuclear Force
• Strong Nuclear Force
– Short-range force – operates only within nuclear
distances
– Force between p and n that overcomes protonto-proton repulsion
Binding Energy: Ex 1
Calculate the binding energy for a helium-4
nucleus given the following information:
4
2He
proton
neutron
4.00150 amu
1.00728 amu
1.00866 amu
Mass of individual nucleons
protons 2(1.00728 amu)
neutrons 2(1.00866 amu)
total
2.01456 amu
2.01732 amu
4.03188 amu
Mass defect
4.03188 amu
-4.00150 amu
0.03038 amu
Mass defect = 0.03038 g/mol
0.03038 g 1 kg
1 mol
1mol
1000 g 6.022X1023 atoms
= 5.045 X 10-29 kg/atom
E=mc2
E = (5.045 X 10-29 kg/atom)(3.00 X 108 m/s)2
E = 4.534 X10-12 J/atom
or
E = 4.534 X 10-12J/ 4 nucleons
E = 1.13X10-12 J/nucleon
Binding Energy: Ex 2
Calculate the binding energy for an iron-56
nucleus given the following information:
56
26Fe
proton
neutron
55.92068 amu
1.00728 amu
1.00866 amu
ANS: 1.41 X 10-12 J/nucleon
Fission: Chain Reaction
• Must absorb some of those neutrons or
fission continues unchecked (explosion?)
Turbine
Moderator (water)
Control Rods
Uranium Fuel Rods
Steam
Nuclear Fission Power
• Uses 235U
• First commercial nuclear power - 1957 at
Shippingport, PA
• People living near a nuclear power plant =
1/10 radiation of a coast-to-coast jet plane
trip (cosmic radiation).
• Three-Mile Island (1979) - partial
meltdown due. No fatalities, no serious
release of radiation.
• Chernobyl, Ukraine (1986) – full meltdown.
31 deaths, 260,000 exposed to high levels
of radiation.
Nuclear Fission: Bombs
• Nuclear bombs (uranium or plutonium)
• Critical Mass – minimum mass required for
a chain reaction
– Subcritical mass
– Supercritical mass
Fusion
• Fusion: Combining 2 nuclei of lighter element
• Thermonuclear fusion occurs at high
temperatures like in the sun (3 to 40 million
K).
– 657 million tons of hydrogen is fused to 653
million tons of helium each second
– Energy released = sunlight
• Not yet feasible for commercial reactors
Sources of Exposure to Radiation
Natural Exposure (~80%)
1. The atmosphere (Radon and carbon-14)
2. Particles that come from outer space
3. Rocks, soil and bricks (Uranium and
Thorium)
4. Foods (carbon-14)
Technological Sources (~20%)
1. Nuclear weapons testing
2. High-altitude plane flights
3. X-rays (even though they are not alpha,
beta or gamma)
4. Fossil fuel and nuclear electrical generation
5. Disturbances in rocks from mining, building
6. Smoking (VERY high levels)
Measuring Exposure to Radiation
1. Units
rad – total exposure
rem – [roentgen equivalent man] – total
damaging exposure
millirem (mrem) – 1/1000th of a rem
2. mrem is the unit used to measure possible
damage to human tissue.
3. U.S. Average = 360 mrem/year
Ionizing Radiation
• UV light and X-rays
 a, b and g from nuclear
decay
• Produces “free
radicals”
• Affects bone marrow,
blood, lymph nodes
Danger of Radon
1. Radon-222 gas passes in and out of the lungs.
2. Produced by decay of radium-226 from rocks,
soil, and building materials.
3. Radon has a half-life of 3.825 days and decays
into solid polonium-218.
4. Polonium-218 emits alpha particles which can
damage lung tissue.
222
86Rn 
218
218
84Po 
214
84Po +
4
82Pb +
4
2He
2He