mr. hollister holliday legacy high school chemistry radioactivity & nuclear chemistry
TRANSCRIPT
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Mr. Hollister Holliday
Legacy High School Chemistry
Radioactivity &
NuclearChemistry
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The Discovery of Radioactivity
• Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off x-rays.
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The Discovery of Radioactivity, Continued
• Bequerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays.Like x-rays.But not related to fluorescence.
• Bequerel determined that: All the minerals that produced these rays
contained uranium.The rays were produced even though the
mineral was not exposed to outside energy.• Energy apparently being
produced from nothing?
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Marie Curie• Marie Curie used an electroscope
to detect the radiation of uranic rays in samples.
• By carefully separating minerals into their components, she discovered new elements by detecting the radiation they emitted.Radium named for its green
phosphorescence. Polonium named for her homeland.
• Since the radiation was no longer just emitted from of uranium, she renamed it radioactivity.
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Electroscope++
+ +++
When charged, the metalfoils spread apart due to
like-charge repulsion.
When exposed to ionizing radiation,
the radiation knocks electrons
off theair molecules,
which jump onto the foils and
discharge them, causing them to
drop down.
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Properties of Radioactivity• Radioactive rays can ionize matter.
Cause uncharged matter to become charged.
Basis of Geiger counter and electroscope.
• Radioactive rays have high energy.
• Radioactive rays can penetrate matter.
• Radioactive rays cause phosphorescent chemicals to glow.Basis of scintillation counter.
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What Is Radioactivity?• Radioactivity = release of tiny,
high-energy particles from an atom.• Particles are ejected from the
nucleus.
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Types of Radioactive Rays• Rutherford discovered there were
three types of radioactivity:1. Alpha rays (𝝰):Have a charge of +2 c.u. and a mass
of 4 amu.What we now know to be helium
nucleus.2. Beta rays (β):Have a charge of -1 c.u. and
negligible mass.Electron-like.3. Gamma rays (𝜸):Form of light energy (not particle like
α and β).
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Rutherford’s Experiment
++++++++++++
--------------
a
gb
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Penetrating Ability of Radioactive Rays
ab g
0.01 mm 1 mm 100 mm
Pieces of Lead
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Facts About the Nucleus• Very small volume compared to
volume of the atom.• Essentially entire mass of atom.• Very dense.• Composed of protons and
neutrons that are tightly held together.Nucleons.
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Facts About the Nucleus, Continued
• Every atom of an element has the same number of protons; equal to the atomic number (Z).
• Atoms of the same elements can have different numbers of neutrons.Isotopes.Different atomic masses.
• Isotopes are identified by their mass number (A).Mass number = number of protons +
neutrons.
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Facts About the Nucleus, Continued
• The number of neutrons is calculated by subtracting the atomic number from the mass number.
• The nucleus of an isotope is called a nuclide.Less than 10% of the known nuclides
are non-radioactive, most are radionuclides.
• Each nuclide is identified by a symbol.Element − mass number.238Uranium
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Important Atomic SymbolsParticle Symbol Nuclear
symbolProton p+
Neutron n0
Electron e-
Alpha a
Beta , b b-
Positron , b b+
p H 11
11
n10
e01
He α 42
42
e β 01
01
e β 01
01
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Radioactivity• Radioactive nuclei spontaneously
decompose into smaller nuclei.Radioactive decay.We say that radioactive nuclei are unstable.
• The parent nuclide is the nucleus that is undergoing radioactive decay; the daughter nuclide are the new nuclei that are made.
• Decomposing involves the nuclide emitting a particle and/or energy.
• All nuclides with 84 or more protons are radioactive.
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Transmutation• Rutherford discovered that during the
radioactive process, atoms of one element are changed into atoms of a different element—transmutation.
• In order for one element to change into another, the number of protons in the nucleus must change.
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Chemical Processes vs. Nuclear Processes
• Chemical reactions involve changes in the electronic structure of the atom.Atoms gain, lose, or share electrons.No change in the nuclei occurs.
• Nuclear reactions involve changes in the structure of the nucleus.When the number of protons in the
nucleus changes, the atom becomes a different element.
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Nuclear Equations• We describe nuclear processes using
nuclear equations.• Use the symbol of the nuclide to
represent the nucleus. • Atomic numbers and mass numbers are
conserved.Use this fact to predict the daughter nuclide
if you know parent and emitted particle.
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Alpha Emission (Decay)• An ά particle contains 2 protons and 2
neutrons.Helium nucleus.
• Loss of an alpha particle means:Atomic number decreases by 2.Mass number decreases by 4.
Rn He Ra 21886
42
22288
He α 42
42
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ά Decay
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Beta Emission (Decay)• A β particle is like an electron.
Moving much faster.Produced from the nucleus.
• When an atom loses a β particle, its:Atomic number increases by 1.Mass number remains the same.
• In beta decay, a neutron changes into a proton.
Pa e Th 23491
01
23490
e β 01
01
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β Decay
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Gamma Emission• Gamma (𝜸) rays are high-energy
photons of light.• No loss of particles from the
nucleus.• No change in the composition of
the nucleus, however, the arrangement of the nucleons changes.Same atomic number and mass
number.• Generally occurs after the nucleus
undergoes some other type of decay and the remaining particles rearrange.
γ00
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Positron Emission (Decay)• Positron has a charge of 1+ c.u.
and negligible mass.Anti-electron.
• When an atom loses a positron from the nucleus, its:Mass number remains the same.Atomic number decreases by 1.
• Positrons appear to result from a proton changing into a neutron.
Ne e Na 2210
01
2211
e β 01
01
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β + Decay
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Nuclear Equations• In the nuclear equation, mass
numbers and atomic numbers are conserved.
• We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay.
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Practice—Write a Nuclear Equation for Each of the Following:
• Alpha emission from U-238.
• Beta emission from Ne-24.
• Positron emission from N-13.
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• Alpha emission from U-238.
• Beta emission from Ne-24.
• Positron emission from N-13.
Practice—Write a Nuclear Equation for Each of the Following, Continued:
Th He U 23490
42
23892
Na β Ne 2411
01-
2410
C β N 136
01
137
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Decay Series• In nature, often one radioactive
nuclide changes in another radioactive nuclide. Daughter nuclide is also radioactive.
• All of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay series.
• To determine the stable nuclide at the end of the series without writing it all out:
1. Count the number of a and b decays.2. From the mass nunmber, subtract 4 for each a
decay.3. From the atomic number, subtract 2 for each a
decay and add 1 for each b.
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U-238 Decay Series
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Practice—Write All the Steps in the U-238 Decay Series and Identify the
Stable Isotope at the End of the Series.
• , , , , , , , , , , , , , a b b a a a a b a b a b ba
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Practice—Write All the Steps in the U-238 Decay Series and Identify the
Stable Isotope at the End of the Series, Continued.
• , , , , , , , , , , , , , a b b a a a a b a b a b ba a ab b
a a b a
Po-214 Pb-210 Bi-210 Po-210 Pb-206a ab b
a
b
Ra-226 Rn-222 Po-218 At-218 Bi-214
U-238 Th-234 Pa-234 U-234 Th-230
Daughter GranddaughterGreat
granddaughterGreat great
granddaughter
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Practice—Determine the Stable Isotope at the End of the U-238
Decay Series.
• , , , , , , , , , , , , , a b b a a a a b a b a b ba
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Practice—Determine the Stable Isotope at the End of the U-238
Decay Series, Continued.• , , , , , , , , , , , , , a b b a a a a b a b a b b
a
238
92U
8 a 238 - 32
92 - 16?
6 b 206 - 0
76 + 6?
206
82Pb=
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Selected Types of Radioactive Decay
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Detecting Radioactivity• To detect when a phenomenon is
present, you need to identify what it does:
1. Radioactive rays can expose light-protected photographic film. Use photographic film to detect the
presence of radioactive rays — film badges.
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Detecting Radioactivity, Continued
2. Radioactive rays cause air to become ionized. An electroscope detects radiation by its
ability to penetrate the flask and ionize the air inside.
Geiger-Müller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays.
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Detecting Radioactivity, Continued
3. Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical. A scintillation counter is able to count the
number of flashes per minute.
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Natural Radioactivity• There are small amounts of
radioactive minerals in the air, ground, and water.
• It’s even in the food you eat!• The radiation you are exposed to from
natural sources is called background radiation.
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Radioactivity in Medicine
• An isotope scan Technetium-99 is often used as the radiation source for bone scans such as this one.
• Phosphorus-32 is used to image tumors because it is preferentially taken up by cancerous tissue.
• Iodine-131 is used to diagnose thyroid disorders.
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Radiotherapy for Cancer • Treatment involves exposing a malignant tumor to
gamma rays, typically from radioisotopes such as cobalt-60.
• The beam is moved in a circular pattern around the tumor to maximize exposure of the cancer cells while minimizing exposure of healthy tissues.
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Half-Life• Each radioactive isotope decays at a
unique rate.Some fast, some slow.Not all the atoms of an isotope change
simultaneously.Rate is a measure of how many of them
change in a given period of time.Measured in counts per minute, or grams
per time.• The length of time it takes for half of
the parent nuclides in a sample to undergo radioactive decay is called the half-life, t1/2.
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Half-Lives of Various Nuclides
Nuclide Half-life Type of decay
Th-232 1.4 x 1010 yr Alpha
U-238 4.5 x 109 yr Alpha
C-14 5730 yr Beta
Rn-220 55.6 sec Alpha
Th-219 1.05 x 10–6 sec Alpha
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Decay Series of Uranium-238
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How “Hot” Is It? • When we speak of a
sample being hot, we are referring to the number of decays we get per minute.
• For samples with equal numbers of radioactive atoms, the sample with the shorter half-life will be hotter.That is, more atoms will
change in a given period of time.
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Half-Life• Half of the radioactive atoms decay each
half-life.Radioactive decay
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10
Time (half-lives)
Pe
rce
nta
ge
of
ori
gin
al s
am
ple
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Decay of Au-198half-life = 2.7 days
0
10000
20000
30000
40000
50000
60000
0 2 4 6 8 10 12 14 16 18 20 22
Time (days)
Rad
ioac
tivity
(cpm
.)
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• All things that are alive or were once alive contain carbon.
• Three isotopes of carbon exist in nature, one of which, C-14, is radioactive.– C-14 radioactive with
half-life = 5730 years
• Atmospheric chemistry keeps producing C-14 at nearly the same rate it decays.
Radiocarbon Dating
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• While an organism is still living, C-14/C-12 is constant because the organism replenishes its supply of carbon.– CO2 in air is the ultimate source of
all C in an organism.
• Once the organism dies the C-14/C-12 ratio decreases.
• By measuring the C-14/C-12 ratio in a once-living artifact and comparing it to the C-14/C-12 ratio in a living organism, we can tell how long ago the organism was alive.
• The limit for this technique is 50,000 years old.– About 9 half-lives, after which
radioactivity from C-14 will be below the background radiation
Radiocarbon Dating
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Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There
be in 5.4 Weeks? (Rn-222 Half-Life Is 3.8 Days.)
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Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4
Weeks? ( Rn-222 Half-Life Is 3.8 Days.), Continued
Amount of Rn-222
Number of Half-lives
Time(days)
1024 g 0 0
512 g 1 3.8
256 g 2 7.6
128 g 3 11.4
64 g 4 15.2
32 g 5 19.0
5.4 weeks x 7 days/wk = 37.8 38 days
Amount of Rn-222
Number of Half-lives
Time(days)
16 g 6 22.8
8 g 7 26.6
4 g 8 30.4
2 g 9 34.2
1 g 10 38
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Practice — How Much of a Radioactive Isotope, Rn-224 (with Half-Life of 10 Minutes)
Did You Start with if, After One Hour if You Have 2 g?
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Practice—How Much of a Radioactive Isotope, Rn-222(with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g?, Continued
Amount of Rn-222
Number of half-lives
Time(min)
128 g 0 0
64 g 1 10
32 g 2 20
16 g 3 30
8 g 4 40
4 g 5 50
2 g 6 60
Fill in the “Number of half-lives” and “Time…” columns first, then work backwards up the “Amount…” column.
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Nonradioactive Nuclear Changes
• A few nuclei are so unstable, that if their nuclei are hit just right by a neutron, the large nucleus splits into two smaller nuclei. This is called fission.
• Small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together to make a larger nucleus. This is called fusion.
• Both fission and fusion release enormous amounts of energy.Fusion releases more energy per gram than
fission.
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Fission+ energy!!
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Fission Chain Reaction• A chain reaction occurs when a
reactant in the process is also a product of the process.In the fission process it is the neutrons.So you only need a small amount of
neutrons to start the chain.• Many of the neutrons produced in the
fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238.
• Minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass.
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Fission Chain Reaction, Continued
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Fissionable Material• Fissionable isotopes include U-235,
Pu-239, and Pu-240.• Natural uranium is less than 1% U-
235.The rest is mostly U-238.Not enough U-235 to sustain chain
reaction.• To produce fissionable uranium the
natural uranium must be enriched in U-235:To about 7% for “weapons grade.”To about 3% for “reactor grade.”
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Nuclear Power• Nuclear reactors use fission to
generate electricity.About 20% of U.S. electricity.The fission of U-235 produces heat.
• The heat boils water, turning it to steam.
• The steam turns a turbine, generating electricity.
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Nuclear Power Plants vs. Coal-Burning Power Plants
• Use about 50 kg of fuel to generate enough electricity for 1 million people.
• No air pollution.
• Use about 2 million kg of fuel to generate enough electricity for 1 million people.
• Produces NO2 and SOx that add to acid rain.
• Produces CO2 that adds to the greenhouse effect.
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Nuclear Power Plant
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Nuclear Power Plants—Core• The fissionable material is stored in
long tubes, called fuel rods, arranged in a matrix.Subcritical.
• Between the fuel rods are control rods made of neutron absorbing material.B and/or Cd.Neutrons needed to sustain the chain
reaction.• The rods are placed in a material to
slow down the ejected neutrons, called a moderator.Allows chain reaction to occur below
critical mass.
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PLWR
Core
Containmentbuilding
Turbine
Condenser
Coldwater
Boiler
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PLWR—Core
Coldwater
Fuelrods
Hotwater
Controlrods
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Problems with Nuclear Reactors
Chernobyl
Fukushima
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Nuclear Fusion• Fusion is the combining of light nuclei
to make a heavier one.• The sun uses the fusion of hydrogen
isotopes to make helium as a power source.
• Requires high input of energy to initiate the process.Because need to overcome repulsion of
positive nuclei. • Produces 10x the energy per gram as
fission.• No radioactive byproducts.• Unfortunately, the only currently
working application is the H-bomb.
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Fusion+ +
21H 3
1H 42He 1
0n
deuterium + tritium helium-4 + neutron