Introduction
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Christopher G. Hamaker, Illinois State University, Normal IL © 2008, Prentice Hall Chapter 18 Nuclear Chemistry INTRODUCTORY CHEMISTRY INTRODUCTORY CHEMISTRY Concepts & Connections Fifth Edition by Charles H. Corwin
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Introduction. As the Earth’s supply of fossil fuels is used up, nations are increasingly turning to nuclear power. Currently, approximately 20% of the world’s electricity needs are met using nuclear power. - PowerPoint PPT Presentation
Transcript of Introduction
Christopher G. Hamaker, Illinois State University, Normal IL© 2008, Prentice Hall
Chapter 18Nuclear Chemistry
INTRODUCTORY CHEMISTRYINTRODUCTORY CHEMISTRYConcepts & Connections
Fifth Edition by Charles H. Corwin
Chapter 18 2
• As the Earth’s supply of fossil fuels is used up, nations are increasingly turning to nuclear power.
• Currently, approximately 20% of the world’s electricity needs are met using nuclear power.
• However, disposal of radioactive waste and the threat of accidents are major concerns with the use of nuclear power.
• In this chapter, we will learn about nuclear chemistry.
Introduction
Chapter 18 3
• There are three types of radioactivity:
– alpha particles, beta particles, and gamma rays
• Alpha particles () are identical to helium nuclei, containing 2 protons and 2 neutrons.
• Beta particles () are identical to electrons.
• Gamma rays () are high-energy photons.
Natural Radioactivity
Chapter 18 4
• Alpha particles have a +2 charge, and beta particles have a –1 charge. Both are deflected by an electric field.
• Gamma rays are electromagnetic radiation and have no charge, so they are not deflected.
Charge of Radiation Types
Chapter 18 5
• Since alpha particles have the largest mass, they are the slowest-moving type of radiation.
• Gamma rays move at the speed of light since they are electromagnetic radiation.
Behavior of Radiation
Chapter 18 6
• Recall, we learned atomic notation in Chapter 5.
• A nuclide is the nucleus of a specific atom.
• The radioactive nuclide strontium-90 has 90 protons and neutrons. The atomic number is 38.
Sr9038
Atomic Notation
Chapter 18 7
• A nuclear reaction involves a high-energy change in an atomic nucleus.
• For example, a uranium-238 nucleus changes into a thorium-231 nucleus by releasing a helium-4 particle and a large amount of energy.
U23592
He42
Th23190
+
• In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products.
Nuclear Reactions
Chapter 18 8
1. The total of the atomic numbers (subscripts) on the left side of the equation must equal the sum of the atomic numbers on the right side.
2. The total of the atomic masses (superscripts) on the left side of the equation must equal the sum of the atomic masses on the right side.
3. After completing the equation by writing all the nuclear particles in an atomic notation, a coefficient may be necessary to balance the reaction.
Balancing Nuclear Reactions
Chapter 18 9
• Here is a listing of the common nuclear particles used to balance nuclear reactions.
Common Nuclear Particles
Chapter 18 10
• Radioactive nuclides can decay by giving off an alpha particle.
• Radium-226 decays by alpha emission.
Ra22688
He42
X A Z
+
• First, balance the number of protons: 88 = Z + 2, so Z = 86 (Rn).
• Second, balance the number of protons plus neutrons: 226 = A + 4, so A = 222.
Ra22688
He42
Rn 222 86
+
Alpha () Emission
Chapter 18 11
• Some radioactive nuclides decay by beta emission.
• Radium-228 loses a beta particle to yield actinium-228.
• Beta decay is essentially the decay of a neutron into a proton and an electron.
Ra22888
e 0-1
Ac 228 89
+
Beta (β ) Emission
Chapter 18 12
• Gamma rays often accompany other nuclear decay reactions.
• For example, uranium-233 decays by releasing both alpha particles and gamma rays.
• Note that a gamma ray has a mass and a charge of zero, so it has no net effect on the nuclear reaction.
U23392
He42
Th 229 90
+ 00
+
Gamma (γ ) Emission
Chapter 18 13
• A positron (+) has the mass of an electron, but a +1 charge.
• During positron emission, a proton decays into a neutron and a positron.
• Sodium-22 decays by positron emission to neon-22.
Na2211
e 0+1
+ Ne2210
Positron (β +) Emission
Chapter 18 14
• A few large, unstable nuclides decay by electron capture. A heavy, positively charged nucleus attracts an electron.
• The electron combines with a proton to produce a neutron.
• Lead-205 decays by electron capture.
Pb20582
e 0-1
Tl 205 81
+
Electron Capture
Chapter 18 15
• Some heavy nuclides must go through a series of decay steps to reach a nuclide that is stable.
• This stepwise disintegration of a radioactive nuclide until a stable nucleus is reached is called a radioactive decay series.
• For example, uranium-235 requires 11 decay steps until it reaches the stable nuclide lead-207.
Decay Series
Chapter 18 16
• Uranium-238 undergoes 14 decay steps before it ends as stable lead-206.
• The decay series for uranium-238 is shown here.
• The series includes 8 alpha-decays and 6 beta-decays.
Uranium-238 Decay Series
Chapter 18 17
• The term parent-daughter nuclides describes a parent nuclide decaying into a resulting daughter nucleus.
• For example, the first step in the decay series for U-238 is:
• U-238 is the parent nuclide and Th-234 is the daughter nuclide.
U23892
He42
Th 224 90
+
Parent and Daughter Nuclides
Chapter 18 18
• The number of nuclei that disintegrate in a given period of time is called the activity of the sample.
• A Geiger counter is used to count the activity of radioactive samples.
• Radiation ionizes gas in a tube, which allows electrical conduction
• This causes a clicking to be heard and the number of disintegrations to be counted.
Activity
Chapter 18 19
• The level of radioactivity for all radioactive samples decreases over time.
• Radioactive decay shows a systematic progression.
• If we start with a sample that has an activity of 1000 disintegrations per minute (dpm), the level will drop to 500 dpm after a given amount of time. After the same amount of time, the activity will drop to 250 dpm.
• The amount of time for the activity to decrease by half is the half-life, t½.
Half-Life Concept
Chapter 18 20
• After each half-life, the activity of a radioactive sample drops to half its previous level.
• A decay curve shows the activity of a radioactive sample over time.
Half-Life
Chapter 18 21
• A sample of plutonium-239 waste from a nuclear reactor has an activity of 20,000 dpm. How many years will it take for the activity to decrease to 625 dpm?
• The half-live for Pu-239 is 24,000 years.
• It takes 5 half-lives for the activity to drop to 625 dpm.
5 t½ × = 120,000 y1 t½
24,000 y
Radioactive Waste
Chapter 18 22
• Iodine-131 is used to measure the activity of the thyroid gland. If 88 mg of I-131 are ingested, how much remains after 24 days (t½ = 8 days).
• First, find out how many half-lives have passed:
24 days × = 3t½
1 t½
8 days
• Next, calculate how much I-131 is left:
88 mg I-131 × = 11 mg I-13112
12
12× ×
Half-Life Calculation
Chapter 18 23
• A nuclide that is unstable is called a radionuclide.
• Carbon-14 decays by beta emission with a half-life of 5730 years.
C14 6
e 0-1
+ N14 7
• The amount of carbon-14 in living organisms stays constant with an activity of about 15.3 dpm. After the plant or animal dies, the amount of C-14 decreases.
Radiocarbon Dating
Chapter 18 24
• The age of objects can therefore be determined by measuring the C-14 activity. This is called radiocarbon dating.
• The method is considered reliable for items up to 50,000 years old.
Radiocarbon Dating, continued
Chapter 18 25
• Uranium-238 decays in 14 steps to lead-206. The half-life for the process is 4.5 billion years.
• The age of samples can be determined by measuring the U-238/Pb-206 ratio.
• A ratio of 1:1 corresponds to an age of about 4.5 billion years.
e 0-1
+ 6 U23892
He42
Pb 206 82
+ 8
Uranium-Lead Dating
Chapter 18 26
Agricultural Use of Radionuclides• Cobalt-60 emits gamma rays when it decays and is
often used in agriculture.
• Gamma radiation is used to sterilize male insects instead of killing them with pesticides.
• Gamma-irradiation of food is used to kill microorganisms:– irradiation of pork to kill the
parasite that causes trichinosis– irradiation of fruits and
vegetables to increase shelf life
Chapter 18 27
Critical Thinking: Nuclear Medicine• The term nuclear medicine refers to the use of
radionuclides for medical purposes.
• Iodine-131 is used to measure thyroid activity.
• The gas xenon-133 is used to diagnose respiratory problems.
• Iron-59 is used to diagnose anemia.
• Breast cancer can be treated using the isotope iridium-159.
Chapter 18 28
• A nuclide can be converted into another element by bombarding it with an atomic particle.
• This process is called transmutation.
• The elements beyond uranium on the periodic table do not occur naturally and have been made by transmutation.
• For example, rutherfordium can be prepared from californium:
n 1 0
+ 4 Cf24998
12C6
Rf 257 104
+
Artificial Radioactivity
Chapter 18 29
• Nuclear fission is the process where a heavy nucleus splits into lighter nuclei.
• Some nuclides are so unstable, they undergo spontaneous nuclear fission.
• A few nuclides can be induced to undergo nuclear fission by a slow-moving neutron.
n 1 0
+ 4 Cf25298
Ba 142 56
+ Mo 106 42
n 1 0
+ 3 U23592
Ba 141 56
+ Kr 92 36
+ n 1 0
+ energy
Nuclear Fission
Chapter 18 30
• Notice that one neutron produces three neutrons. These neutrons can induce additional fission reactions and produce additional neutrons.
• If the process becomes self-sustaining, it is a chain reaction.
Nuclear Chain Reaction
Chapter 18 31
• The mass of material required for a chain reaction is the critical mass.
Nuclear Chain Reaction
Chapter 18 32
Critical Thinking: Nuclear Power Plant• Nuclear energy is an attractive source because of
its potential:– 1 gram of U-235 produces about 12 million times more
energy than 1 gram of gasoline
• A nuclear power plant produces energy from a nuclear chain reaction.
• Fuel rods are separated by control rods to regulate the rate of fission.
• A liquid coolant is circulated to absorb heat.
Chapter 18 33
• Nuclear fusion is the combining of two lighter nuclei into a heavier nucleus.
• It is more difficult to start a fusion reaction than a fission reaction, but it releases more energy.
• Nuclear fusion is a cleaner process than fission because very little radioactive waste is produced.
• The Sun is a giant fusion reactor, operating at temperatures of millions of degrees Celsius.
Nuclear Fusion
Chapter 18 34
• The Sun is about 73% hydrogen, 26% helium, and 1% all other elements.
• Three common fusion reactions that occur in the Sun are:
+ H 1 1
+ energy e 0+1
H 1 1
H 2 1
+
+ H 2 1
+ energy H 1 1
He 3 2
+ He 3 2
+ energy e 0+1
H 1 1
He 4 2
+
Fusion in the Sun (and Other Stars)
Chapter 18 35
• There are two fusion reactions being investigated for use in commercial power generation.
• The first uses deuterium (H-2) as a fuel:
• The second involves deuterium and tritium (H-3) as fuels:
+ H 2 1
+ energy H 2 1
He 4 2
+ H 3 1
+ energy n 1 0
H 2 1
He 4 2
+
Fusion Energy
Chapter 18 36
• There are three types of natural radiation:
– alpha particles
– beta particles
– gamma rays
• Gamma rays are electromagnetic radiation.
• Alpha particles are helium nuclei, and beta particles are electrons.
Chapter Summary
Chapter 18 37
• Radioactive nuclides decay by 4 processes:– alpha emission– beta emission– positron emission– electron capture
• The parent nuclide decays to yield the daughter nuclide.
• If a nuclide decays through the emission of radiation in more than one step, the overall process is called a radioactive decay series.
Chapter Summary, continued
Chapter 18 38
• The time required for 50% of the radioactive nuclei in a sample to decay is constant and is called the half-life. After each half-life, only 50% of the radioactive nuclei remain.
• Artificial nuclides are produced by transmutation.
• The splitting of a heavy nucleus into two lighter nuclei is nuclear fission.
• The combining of two lighter nuclei into one nucleus is nuclear fusion.
Chapter Summary, continued
