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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-23892 protons 92 protons 92 protons142 neutrons 143 neutrons 146 neutronsTrace 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.
U23892 Th
234
90 + He4
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 DecayElectron 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 ParticlesParticle Nuclear
EquationExample
Alpha= 2 protons and 2 neutrons
Nucleus 4He 226Ra 222Rn +4He
Beta =neutron converts to proton and a high energy electron
1n 1p + 0e 131I131Xe + 0e
Electron Capture= electron captured by nucleus
1p + 0e 1n 81Rb + 0e 81Kr
88 86 22
0 1 -1 53 54 -1
1 -1 037 -1 36
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Radioactive Decay Particles
Particle Nuclear Equation
Example
Positron=proton 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.
1 0 1
Remember a positron has the same mass as an electron, but the opposite charge
6 5 1
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Radioactive Decay Particles
Particle EffectAlpha 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)
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.
Half Life Simulation
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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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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?
Half Life Problems
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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 ReactionCollision 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 ReactionTremendous 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).
