Nuclear Chemistry
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Natural Radioactivity - spontaneous disintegration of the nucleus of an atom with the emission of particles and/or radiant energy
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Radioactivity - the nucleus of an atom starts to emit particles
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element has a large amount of neutrons compared to protons
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elements above 82 in atomic number (lead)
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Transmutation - when one element is changed to another element because of a change in the nucleus
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Types of Emanations (particle emissions and radiant energy released)
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Alpha Decay - alpha particles can be considered Helium nuclei
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Atoms which emit alpha particles are called alpha emitters
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consist of 2 protons and 2 neutrons
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atom affected has atomic # reduced by 2 and atomic mass reduced by 4
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Beta Decay - high speed electrons
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result of neutron disintegration
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a neutron disintegrates into a proton and electron
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the electron is emitted
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atomic number is increased by 1 and the atomic mass remains the same
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Gamma Radiation - high energy radiation
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not particles
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do not have a mass or charge
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reduces the energy content of the nucleus without affecting its charge or mass
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***See Table J for all symbols used in Nuclear Chemistry***
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Rules for Natural Radioactive Decay Reactions
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In natural disintegration of atoms, there is only one atom on the left and a total of three particles in the reaction.
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All numbers on the right must equal the numbers on the left
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Separating Emanations
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Magnetic or electric fields
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Alpha particles - deflected toward negative pole
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Beta particles - deflected toward positive pole
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Gamma rays - not affected
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Show drawing of the effects of magnetic fields on emanations
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Detecting Radioactivity
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Geiger counter (ionizes other atoms)
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causes fluorescent and photographic effects
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Half - Life - the time required for 1/2 of the nuclei of an isotope to disintegrate to a stable form.
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U 238 Pb 206=4.5X 109 years(series of alpha and beta emissions)
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C 14 N 14=5700 years(beta decay)
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effective up to 40,000 years old
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***See Table H in the reference tables for other radioisotopes***
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Artificial Radioactivity (Induced Radioactivity)
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Artificial Transmutation - the bombarding of elements with high energy particles such as protons, neutrons, and alpha particles to forma new radioactive isotope (radioisotopes)
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process can be done on smaller atoms
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ex.
ex.
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Rules for Artificial Radioactivity Reactions
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There are at least two reactants on the left side of the equation
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The sum of the atomic masses and atomic numbers on the left must equal the total number of atomic masses and numbers on the right
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Particle Accelerators - devices which give charged particles enough kinetic energy to penetrate the nucleus
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Cyclotron & Synchrotron - uses electromagnets to accelerate particles
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Linear accelerator - uses fields of electrical force in a linear tunnel to accelerate the particles
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Nuclear Energy – MAL only
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Mass Defect - energy changes due to the changes in binding energy
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Binding energy - the amount of energy released as nuclear particles merge in the nucleus when some of the mass is converted to energy
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Mass of 2 free protons= 1.6725 X 10-24gX2=3.3450 X 10-24g
Total mass of 2 free neutrons and protons=6.6946 X 10-24g
Actual mass ofHe=6.641236 X 10-24g
Mass deficiency (binding energy)=0.053364 X 10-24g(Use Einstein’s equation to determine the amount of energy released)
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Greater binding energy = more stable atom (takes more energy to separate)
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Fission - the splitting of heavier nuclei into lighter ones
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Only works on unstable elements with high atomic numbers
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Nucleus captures neutrons causing instability, then splitting into 2 stable elements
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energy is released (conversion of mass into energy)
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neutrons are released (can be used to cause further fission reactions -chain reaction)
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Atomic bomb - chain reaction is not controlled
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Nuclear Reactor - chain reaction is controlled by “control rods”
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Fission (Nuclear) Reactors
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Fuels - U 233, U 235, Pu 239 - fissionable materials
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Natural Uranium = 99.3% U-238, 0.7% U-235
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enriched uranium - has higher U-235 content
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Breeder Reactors - use U-238 Th-232 and Pu-239 (fuel)
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produces more fuel than consumed
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Moderators - materials that have the ability to slow down neutrons quickly without absorbing them
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water, heavy water (with deuterium), beryllium, graphite
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Control rods - controls number of neutrons available by absorbing them (regulates rate of reaction)
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boron, cadmium
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Coolants - keeps the temperatures from fission at reasonable levels in the reactor
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carries heat away to heat exchangers to turn turbines
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prevents meltdown in reactor core
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water, heavy water, air, helium, carbon dioxide, molten sodium or lithium
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Shielding
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Internal - steel lining to protect walls from radiation damage
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External - high density concrete
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acts as radiation containment vessel in case of an accident
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Radioactive Wastes - production of Strontium-90, Cesium-137 must be stored in special containers until they decay
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Low level radioactive wastes - diluted and released into environment
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radon-222, krypton-85, and nitrogen-16(all decay quickly
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Fusion Reaction - 2 light nuclei fuse into a heavier nucleus at high temperatures to form a more stable configuration with more binding energy per nucleon (causes release of mass as energy)
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isotopes produced are stable (not radioactive)
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Fuels - deuterium and tritium
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deuterium - contained in sea water (natural isotope of hydrogen)
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tritium - formed by neutron bombardment of lithium
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Problems
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High energy requirement - nuclei must have sufficient energy to overcome the charge (repulsion)
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Hydrogen must be used because of smallest possible charge
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Thermonuclear approach - use high temperatures to cause fusion (109C)
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Magnetic Bottle approach - use of magnetic fields to confine the reaction
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Uses of Radioisotopes
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Tracers in chemical reactions - isotopes act same as stable atom
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can be followed in a reaction
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ex. C-14, O-18 are used in biology to trace chemical pathways
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Radioactivity to cure and purify
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medical diagnosis, therapy, food preservation
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Age determination - half-life of various naturally occuring radioisotopes allow scientists to determine the age of artifacts and minerals