NUCLEAR CHEMISTRY

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NUCLEAR CHEMISTRY
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Radioactivity

• Radioisotopes
• A isotope that has an unstable nucleus and
  undergoes radioactive decay
• Always accompanied by the emission of large
  amounts of energy
• Nuclear reactions NOT Affected by
• Changes in temperature, pressure or the presence
  of catalyst
• Unaffected by compounds in which the unstable
  isotopes are composed
• Can not be slowed down, speed up or turned off

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Radioactivity

• Antoine Henri Becquerel (1852-1908)
• 1896- while studying uranium salts, observed that
  they fogged up photographic film plates
• Marie Curie (1867-1934) Pierre Curie
  (1859-1906)
• Demonstrated that fogging was caused by rays
  emitted by uranium atoms in the ore
• Radioactivity-process by which materials give off
  such rays
• Radiation-penetrating rays and particles emitted
  by a radioactive source

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Radioactivity

• Radioisotopes have unstable nuclei
• Stability dependent on proportion of neutrons to
  protons and overall size
• Radioactive Decay The spontaneous disintegration
  of a nucleus into a slightly lighter and more
  stable nucleus, accompanied by emission of
  particles, electromagnetic radiation, or both

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Types of Radioactive Decay

• Alpha Radiation
• Helium nuclei that have been emitted from a
  radioactive source
• 1. Alpha particle (a) is a helium nucleus,
  contains 2 proton and 2 neutron so it has a 2
  charge.
• 2. Loss of an alpha particle reduces atomic by
  2 and atomic mass by 4

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• Beta Radiation
• fast moving electrons formed by the decomposition
  of a neutron in an atom
• Beta particle (ß) is an electron emitted from the
  nucleus during nuclear decay
• Beta particles are emitted when a neutron is
  converted into a proton and an electron, atomic
  increases by 1

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• Gamma Emission
• 1. Gamma rays (?) are high-energy electromagnetic
  waves emitted from a nucleus as it changes from
  an excited state to a ground energy state
• 2. Gamma emission usually follows other types of
  decay that leave the nucleus in an excited state

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Nuclear Radiation

• A. Penetrating Ability
• 1. Alpha Particles
• a. Least penetrating ability due to large mass
  and charge
• b. Travel only a few centimeters through air
• c. Cannot penetrate skin
• d. Can cause harm through ingestion or inhalation
• 2. Beta Particles
• a. Travel at speeds close to the speed of light
• b. Penetrating ability about 100 times greater
  than that of alpha particles.
• c. They have a range of a few meters in air.
• 3. Gamma rays
• a. Greatest penetrating ability
• b. Protection requires shielding with thick
  layers of lead, cement, or both

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• B. Penetrating ability of radiation
• C. Radioactive Elements
• 1. All isotopes of all man-made elements are
  radioactive
• 2. Some naturally isotopes are radioactive
• a. All isotopes of all elements beyond bismuth
  (atomic 83) are radioactive

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Half-Life

• Half-Life (t1/2)
• The time required for half the atoms of a
  radioactive nuclei to decay
• a. More stable nuclei decay slowly
• b. Less stable nuclei decay rapidly

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Half-Life Equations
Total mass of decay number of half-lives x
number of years
Half - life
Fraction of sample remaining final mass of
sample
Initial mass of sample

1. If 100.0g of carbon-14 decays until only 25.0 g
   of carbon is left after 11460 yrs, what is the
   half-life of carbon-14?
2. Thallim-208 has a half-life of 3.053 min. How
   long will it take for 120.0g to decay to 7.50 g?
3. Gold-198 has a half-life of 2.7 days. How much of
   a 96 g sample of gold -198 will be left after 8.1
   days?

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Radioactive Decay Modeling

• Model the hypothetical nuclei of Mysterium (My)
  and its decay
• Using toothpicks connect 1 large marshmallow 1
  small marshmallow (large proton small
  electron) (proton electron Neutron)
• Form 6 Neutrons in total
• Combine 6 Neutron 4 protons in one pile
  Illustrate on poster and write initial state A
  My
• Illustrate alpha emission with equation and
  marshmallows (reduce by 2 protons 2 neutrons) A
  My ? A X 4 He
• Illustrate beta emission using new isotope in
  step 4 with equation and marshmallows A Y 0
  ß
• Label the following decay sequence as a or ß
  emission
• 238 U ? 234 Th ? 234 Pa ? 230 Th ? 226 Ra ? 222
  Rn

Z
Z
Z
Z
-1
92
90
91
92
88
86
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Neutron-Proton Ratios

• Any element with more than one proton (i.e.,
  anything but hydrogen) will have repulsions
  between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus
  from flying apart.
• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an
  important factor.

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Stable Nuclei
The shaded region in the figure shows what
nuclides would be stable, the so-called band of
stability.
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Stable Nuclei
Nuclei above this belt have too many
neutrons. They tend to decay by emitting beta
particles.
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Stable Nuclei
Nuclei below the belt have too many protons. They
tend to become more stable by positron (mass of
an electron but a positive charge) emission or
electron capture.
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Transmutation Reactions

• Transmutation the conversion of an atom of one
  element to an atom of another element
• High energy particles bombard the nucleus of an
  atom
• Naturally Occurring
• Nitrogen-14 ? Carbon-14 (upper atmosphere)
• Flourine-18 ? Oxygen-17 and one proton
• Transuranium elements elements in the periodic
  table with an atomic gt 92, all undergo
  transmutation
• None occure in nature
• All radioactive

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Nuclear Fission

• A very heavy nucleus splits into more stable
  nuclei of intermediate mass
• Occurs by chain reaction
• The mass of the products is less than the mass of
  the reactants. Missing mass is converted to
  energy

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Nuclear Chain Reaction

• In a chain reaction, some of the neutrons
  produced react with other fissionable atoms,
  producing more neutrons, which react with still
  more fissionable atoms

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Nuclear Reactors

• Use controlled fission to produce useful energy
• Energy genreated in form of heat
• Coolant fluid, removes heat from the core
• Heat generates steam which drives a turbine that
  generates electricity
• Two step process
• Neutron Moderation reduces speed of neutrons to
  continue chain reaction (water carbon)
• Neutron absorption decreases the of slow
  moving neutrons, prevents chain reaction from
  going too fast

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Nuclear Waste
300 fuel rods assembly 100 assemblies reactor
core Spent fuel rods are high-level nuclear waste
(highly radioactive) All nuclear power plants
have holding tanks for spent rods 12 meter deep
and filled with water cools rods and acts as a
radiation shield Spend a decade or more in
tank Limited time for plant operation due to
contamination
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Nuclear Fusion

• Nuclear Fusion
• Light-mass nuclei combine to form a heavier,
  more stable nucleus

Fusion Reactions 1. More energetic than fission
rxns 2. Occur only at high temperature in
excess of 40 000 000 C 3. Source of energy of
the hydrogen bomb 4. Potential energy source that
is inexpensive and readily available and not
radioactive Problems achieving high
temperatures, uncontrolled, plasma state of
products
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Detecting Radiation
Ionizing Radiation radiation with enough energy
to knock electrons off atoms of bombarded
substances to form ions Monitoring
Devices Geiger Counter a gas-filled metal tube
used to detect radiation ionizing radiation
penetrates the thin window at one end ionizing
the gas which conducts electricity making audible
clicks Scintillation counter A device that uses
a specially coated phosphor surface to detect
radiation ionizing radiation striking the
phosphor plate produces bright flashes of light
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Uses for Radiation
Nuclear Medicine
Thyroid imaging using Tc-99m
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Food Irradiation

• Food can be irradiated with g rays from 60Co or
  137Cs.
• Irradiated milk has a shelf life of 3 mo. without
  refrigeration.
• USDA has approved irradiation of meats and eggs.

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Chernobyl Disaster
April 26, 1986 Reactor 4 in a Nuclear Power
plant in Ukraine exploded. Further explosions and
resulting fire sent a plume of highly radioactive
fallout into the atmosphere and over an extensive
geographical location
http//www.youtube.com/watch?vuoEgkGNO-sQfeature
related http//ngm.nationalgeographic.com/2006/0
4/inside-chernobyl/audio-interactive