AP Chemistry Podcast 1.3 Nuclear Chemistry

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AP Chemistry Podcast 1.3Nuclear Chemistry
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Nuclear Chemistry

• Nuclear reactions involve changes that originate
  in the nucleus of the atom.
• Chemical changes involve changes in the electron
  cloud.
• Uses
• 60Co- gamma ray emitter- ionizing radiation for
  treatment of cancerous tumors.
• 201Thallium stress test of heart muscle
• Radiocarbon dating? 14C ½ life 5730 years
• Nuclear power 20 of US electricity production

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Radioactivity

• Recall that all atoms of the same element have
  the same number of protons. The number of
  neutrons in the atoms nucleus, however, may be
  different from one atom to the next Isotopes.
• Uranium- 234 Uranium-235 Uranium-238
• 92 protons 92 protons 92 protons
• 142 neutrons 143 neutrons 146 neutrons
• Trace 0.7 99.3
• Different isotopes have different abundances
• Different isotopes have different stabilities

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Patterns of Nuclear Stability
As the atomic number increases, the neutron to
proton ratio of the stable nuclei increases. The
stable nuclei are located in the shaded area of
the graph known as the belt of stability. The
majority of radioactive nuclei occur outside this
belt.
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Nuclear Equations

• Radionuclides are unstable nuclei that emit
  particles and electromagnetic radiation to
  transform into a stable nucleus.

238
234
U
4
Th

He
92
90
2
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Nuclear Equations

• Mass numbers and atomic numbers must be balanced
  in all nuclear equations.

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What product is formed when thorium-232 undergoes
alpha decay?
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Types of Radioactive Decay
Alpha decay- nucleus emits 2 protons and 2
neutrons (He nucleus)
Beta decay- a neutron in the nucleus decays into
a proton and an electron, the electron is emitted
Gamma- high energy, short wavelength
electromagnetic radiation- accompanies other
radioactive emissions.
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Types of Radioactive Decay

• Electron Capture- capture by the nucleus of an
  electron from the electron cloud surrounding the
  nucleus.
• Positron- particle with the same mass as an
  electron, but an opposite charge? collides with
  an electron and produces gamma radiation.

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Penetrating Power of Radioactive Decay
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Radioactive Decay Particles
Particle Nuclear Equation Example
Alpha 2 protons and 2 neutrons Nucleus ? 4He 226Ra ?222Rn 4He
Beta neutron converts to proton and a high energy electron 1n ? 1p 0e 131I?131Xe 0e
Electron Capture electron captured by nucleus 1p 0e ?1n 81Rb 0e ? 81Kr
88
86
2
2
-1
53
54
-1
1
0
37
36
-1
1
-1
0
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Radioactive Decay Particles
Particle Nuclear Equation Example
Positronproton converted to a neutron and an electron 1p ? 1n 0e 11C ?11B 0e
Gamma electromagnetic radiation Not shown in equations, but almost always accompanies other decay.
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5
1
1
0
1
Remember a positron has the same mass as an
electron, but the opposite charge
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Radioactive Decay Particles
Particle Effect
Alpha Decrease atomic mass by ___ and atomic number by _____.
Beta Atomic number _______________.
Electron Capture Atomic number _______________.
Positron Atomic number _______________.
Gamma
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Half Life- the time required for half of any
given quantity of a substance to react / decay.
(independent of initial quantity of atoms)
Half Life Simulation
Number of Th-232 atoms in a sample initially
containing 1 million atoms as a function of time.
Th-232 has a half-life of 14 billion years.
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Half Life Problems

•  Example
• An isotope of cesium (cesium-137) has a half-life
  of 30 years. If 1.0 mg of cesium-137
  disintegrates over a period of 90 years, how many
  mg of cesium-137 would remain?
•  

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Half Life Problems
1. A 2.5 gram sample of an isotope of
strontium-90 was formed in a 1960 explosion of an
atomic bomb at Johnson Island in the Pacific Test
Site. The half-life of strontium-90 is 28 years.
In what year will only 0.625 grams of this
strontium-90 remain? 2. Actinium-226 has a
half-life of 29 hours. If 100 mg of actinium-226
disintegrates over a period of 58 hours, how many
mg of actinium-226 will remain? 3.
Thallium-201 has a half-life of 73 hours. If 4.0
mg of thallium-201 disintegrates over a period of
6.0 days and 2 hours, how many mg of thallium-201
will remain? 4. Sodium-25 was to be used in an
experiment, but it took 3.0 minutes to get the
sodium from the reactor to the laboratory. If
5.0 mg of sodium-25 was removed from the reactor,
how many mg of sodium-25 were placed in the
reaction vessel 3.0 minutes later if the
half-life of sodium-25 is 60 seconds? 5.
Selenium-83 has a half-life of 25.0 minutes. How
many minutes would it take for a 10.0 mg sample
to decay and have only 1.25 mg of it remain?
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Uranium-238 an example of an unstable nucleus
decaying to form other unstable nuclei
Uranium-238 is radioactive, undergoing alpha
decay. But, the daughter nuclide is also
radioactive, undergoing beta decay, to produce
yet another radioactive nuclide, which decays.
The atom goes through a rather involved sequence
of radioactive decays (both alpha and beta),
until a stable isotope (lead-206) is reached.
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Fission Reaction
Collision of a neutron with a U-235 nucleus can
cause the nucleus to split, creating two smaller
nuclides and three free neutrons. The three
neutrons may travel outward from the fission,
colliding with nearby U-235 nuclei, causing them
to split as well. Each split (fission) is
accompanied by a large quantity of energy.
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Fission Chain Reaction
Collision of a neutron with a U-235 nucleus can
cause the nucleus to split, creating two smaller
nuclides and three free neutrons. The three
neutrons may travel outward from the fission,
colliding with nearby U-235 nuclei, causing them
to split as well. Each split (fission) is
accompanied by a large quantity of energy. If
sufficient neutrons are present, we may achieve a
chain reaction. If only one neutron were produced
with each fission, no chain reaction would occur,
because some neutrons would be lost through the
surface of the uranium sample.
Mousetrap Chain Reaction
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Fission Reaction
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Fusion Reaction
Tremendous energy needed to overcome the
repulsion between nuclei. Heat required for this
reaction is on the order of 40,000,000 K. The
energy from an atomic bomb could generate this
heat (hydrogen or thermonuclear weapon).