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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. Nuclear fuel (235U and 239Pu)
2. Nuclear Weapons
3. Irradiated Food
4. Smoke Alarms (Amercium-241)
5. Cancer treatment (Cobalt-60)
6. Medical Tracers
Types of Nuclear Radiation
Alpha particle
(42He)
Helium nucleus
Beta particle
(0-1e)
fast-moving electron.
Gamma rays
high energy form of electromagnetic radiation
2 p+
2 n
e-
Light
Radio Radar Micro IR Visible Light
UV X-rays
Gamma
The Electromagnetic Spectrum
Safe radiation (non-ionizing) Dangerous (ionizing)
Produced by nuclear decay
What Stops Radiation
Paper Al Foil
Wood
Lead.
Iron,
ConcreteAlpha ()
Beta ()
Gamma ()
Decay Equations
Alpha Decay238
92U 42He + 234
90Th
Beta Decay 234
90Th 0-1e + 234
91Pa
Decay Equations
Gamma Decay
Occurs with alpha and beta decay
No change in atomic mass (gamma radiation has no mass 0
0)
Decay: Ex 1
What product is formed when radium-226 undergoes alpha decay?
22688Ra 4
2He +
Decay: Ex 2
What element undergoes alpha decay to form lead-208?
42He + 208
82Pb
Decay: Ex 3
What isotope is produced when thorium-231 beta decays?
23190Th 0
-1e +
Positron Emission– Same mass an electron, but opposite charge– Form of anti-matter
01e
Electron Capture– Nucleus captures a core electron– electron is added rather than lost
Common Particles
Particle Symbol
Alpha 42He
Beta 0-1e
Positron 01e
Electron 0-1e
Proton 11H or 1
1p
Neutron 10n
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 man-made and radioactive
82
Pb
207.2At least one
radioactive isotopeAll 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
• Atomic # >84– Alpha Decay
• Above belt– Too many neutrons– Beta emission
• Below belt– Too few neutrons– electron capture or positron emission
• Most heavy isotopes (above 84) decay by alpha emission
• Slide down to lead-206
Decay Modes: Ex 1
Predict the decay mode for carbon-14
8n : 6p Too many n’s, prefers 1:1
146C
Decay Modes: Ex 2
Predict the decay mode for xenon-118
64n : 54p =1.2 Too few n’s (check graph)
11854Xe
Or118
54Xe
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
147N + 4
2He 178O + 1
1H14
7N(p) 178O
• 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 : 27
13Al(n, ) 2411Na
Transmutation: Ex 2
Write the shorthand notation for:
168O + 1
1H 137N + 4
2He
Transmutation: Neutrons
• Neutrons produced from radioactive decay
• Cobalt-60 is used in radiation therapy
5826Fe + 1
0n 5926Fe
5926Fe 59
27Co + 0-1e
5927Co + 1
0n 6027Co
Transmutation: Transuranium Elements
23892U + 1
0n 23992U 239
93Np + 0-1e
23994Pu + 4
2He 24296Cm + 1
0n
20983Bi + 64
28Ni 272111Rg + 1
0n
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.
Half-Life
Isotope Half-life
Uranium-238 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.
Carbon-14 dating
• Upon death, 14C radioactively decays. (half-life = 5730 y)
• Reasonable to up to 50,000 years.
• 15% margin of error
• Mummies, the Dead Sea Scrolls, Shroud of Turin
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 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?
Half-life: Example 3
Radon-226 has a half-life of 1600 years? How much of a 30 gram sample remains after 6400 years?
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?
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?
Rate Law
First order rate law
Rate = kN (N is the initial concentration)Rate = -N = dN = -kN
t dtdN = -kNdtdN = -kdt N
∫dN = ∫-kdt
N
∫dN = -k∫dt (Integrate left from N0 to Nt
N and time from 0 to t)
lnNt = -kt or Nt = Noe-kt
N0
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 uranium-238 remains, how old is the sample?
k = 0.693 t½
k = 0.693 = 1.5 x10-10 yr-1
4.5 X 109 yr
lnNt = -kt
N0
ln 1.000 = -(1.5 x10-10 yr)t
1.257
t = 1.5 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 5730 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 5730 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?
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
23892U 234
90Th + 42He
238.0003 amu 233.9942 amu 4.0015amu
238.0003 amu 237.9957 amu
m = -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.
6027Co 0
-1e + 6028Ni
6027Co 59.933819 amu
0-1e 0.00054858 amu
6028Ni 59.930788 amu
ANS: 2.23 X 1011 J/mol
E = mc2: Ex 3
The following decay produces 2.87 X 1011 J/mol of 11
6C. What is the mass change in this decay?
116C 11
5B + 01 e
ANS: -3.19 X 10-3 g/mol
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 ForcesForce 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 and fusion(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 proton-
to-proton repulsion
Binding Energy: Ex 1
Calculate the binding energy for a helium-4 nucleus given the following information:
42He 4.00150 amu
proton 1.00728 amu
neutron 1.00866 amu
Mass of individual nucleons
protons 2(1.00728 amu) 2.01456 amu
neutrons 2(1.00866 amu) 2.01732 amu
total 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:
5626Fe 55.92068 amu
proton 1.00728 amu
neutron 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?)
Uranium Fuel Rods
Control Rods
Moderator (water)
Turbine
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– Critical mass (1 kg)
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-raysand 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.
22286Rn 218
84Po + 42He
21884Po 214
82Pb + 42He
12.a) 19179Au + 0
-1e 19178Pt
b) 20179Au 201
80Hg + 0-1e
c) 19879Au 198
80Hg + 0-1e
d) 18879Au 188
78Pt + 01e
14. a) 2411Na 24
12Mg + 0-1e
b) 18880Hg 188
79Au +01e
c) 12253I 122
54Xe + 0-1e
d) 24294Pu 238
92U + 42He
18.a) Positron emission, electron capture
b) Beta
c) Beta
d) Positron emission, electron capture
20.a) Even, even – more abundant
b) odd, even – more abundant
c) even, even – more abundant
d) even, even – more abundant
28.a) 3215P b) 7
3Li c) 18775Re
d) 9943Tc e) 99
38Sr
34. 2.6 min
36.85 d
40. 3520 y
46. 1.6143 X 1013 J/mol
48.a) 1.20 X 10-12 J/nucleon
b) 1.40 X 10-12 J/nucleon
c) 1.35 X 10-12 J/nucleon
50.a) -1.697 X 1012 J/mol
b) -3.13 X 1011 J/mol c) -1.773 X 1012 J/mol
