Chapter 21: Nuclear Chemistry

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Chapter 21 Nuclear Chemistry

• This unit looks at the nature of radiation,
• Types of radiation and decay products,
• Radiation Units and exposure precautions
• Nuclear fission and fusion reactions
• Applications of Nuclear Chemistry

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The Geiger Counter
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What does a Geiger Counter measure?

• Radiation that can cause atoms to lose or gain
  electrons and become ions.
• This type of radiation is called ionizing
  radiation.
• There are two causes of background radiation
• Outer Space
• Natural decay from isotopes in the earths
  crust/core

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Daily Background Counts
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Historical Perspectives

• 1895 Wilhelm Roentgen discovers X-rays and their
  effects.
• 1896 Henri Becquerel discovers radioactive
  Uranium.
• 1898 Pierre and Marie Curie discover two new
  elements, polonium and radium.
• 1905 Albert Einstein theory of relativity and
  mass defect.
• 1908 Hans Geiger creates an instrument to
  measure ionizing radiation.
• 1934 Enrico Fermi proposes transuranes
  elements beyond uranium.
• 1939 Lise Meitner , Otto Hahn and Fritz
  Stassman explain nuclear fission.

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Daily Background Counts
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Nuclear Composition

• Nucleons are any particles found in the nucleus,
  commonly they are protons and neutrons.
• We would expect, the total mass of the electrons,
  protons, and neutrons would be the mass of the
  atom, it is not, but rather it is a smaller
  value.
• Mass defect is the difference between the mass of
  an atom and the sum of the masses of its protons,
  neutrons and electrons.
• Einstein explained this loss of mass as the
  result of the nucleus formation. Energy is given
  off from the conversion of matter to energy
  (Emc2 ).
• This loss of mass from its conversion to energy
  provides nuclear stability.

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Nuclear Binding Energy

• The energy released when a nucleus is formed from
  nucleons is called the nuclear binding energy.
• This can be thought of the amount of energy to
  break a nucleus apart.
• The higher the nuclear binding energy of a
  nuclide. the greater the nuclide stability.
• The binding energy per nucleon is the binding
  energy of the nucleus divided by the number of
  nucleons(mass number) it contains.
• Elements with intermediate atomic masses (iron
  through lead) have the greatest binding energies
  (stability).

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Nuclear Stability

• The neutron/proton ratio can be used to predict
  nuclear stability.
• For elements with low atomic numbers (1-30) the
  nucleus is stable when there is a 11 ratio.
• For elements with a high atomic number (up to
  element 83), the nucleus is stable when the ratio
  is 1.51.
• Elements having an atomic number greater than 83
  are unstable or radioactive.
• Stable nuclei tend to have even numbers of
  nucleons in their nucleus.

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N/P Ratio
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Nuclear Shell Model

• Stable nuclei tend to have even numbers of
  nucleons in their nucleus. (protons, neutrons or
  total nucleons)
• The most stable atoms have 2, 8, 20, 28, 50, 82
  or 126 protons, neutrons, or total nucleons.
• The nuclear shell theory states that nucleons
  exists in different energy levels, or shells, in
  the nucleus. Completed nuclear energy levels are
  those with 2, 8, 20, 28, 50, 82 and 126.nucleons.
• These numbers are sometimes called the magic
  numbers for nuclear stability.

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Nuclear Reactions and terms

• 4 types of Nuclear Reactions
• Radioactive decay refers to the emission of an
  alpha particle, a beta particle, or gamma ray and
  the formation of a slightly lighter and more
  stable nucleus.
• Nuclear disintegration is when an unstable nuclei
  from nuclear bombardment emits a proton or
  neutron and becomes more stable.
• Fission refers to the process in which a very
  heavy nucleus splits to form two or more
  medium-mass nuclei.
• Fusion refers to the process in which lightweight
  nuclei combine to form heavier more stable
  nuclei.

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Other Nuclear Terms

• Transmutation is the change in the identity of a
  nucleus as a result of a change in the number of
  protons.
• Radioactive decay is spontaneous disintegration
  of a nucleus into slightly lighter and more
  stable nucleus, accompanied by the emission of
  particles, electromagnetic radiation or both.
• Radiation- the process of emitting or releasing
  waves of energy, such as light, x-rays, or other
  types of electromagnetic waves.
• Radioactivity is the property of some elements to
  spontaneously emit alpha or beta particles with
  gamma rays by the disintegration of the nuclei.

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Properties of Radioactive Nuclides

• They expose light sensitive emulsions.
  (Roentgen, 1895)
• They fluoresce or glow with certain compounds.
  (Curie, 1898)
• They produce charged or ionized gas particles.
  (Geiger, 1908)
• Exposure to radio-nuclides can cause harmful
  physiological effects leading to death.
• They undergo radioactive decay and have a
  half-life.

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Half-Life of a Radioisotope

• Half-life is the time it required for half the
  atoms of a radioactive nuclide to decay. It can
  be measured in seconds, minutes, days, or years.
• decay curve
• 8 mg 4 mg 2 mg 1 mg

initial
1 half-life
2
3
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Examples of Half-Life

• Isotope Half life
• C-15 2.4 sec
• Ra-224 3.6 days
• Ra-223 12 days
• I-125 60 days
• C-14 5700 years
• U-235 710 000 000 years

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Half-Life Problem

• Ra-223 has a half-life of 12 days. If today, you
  had 100 grams of this isotope, how much would
  remain after 36 days?

• How many half-life periods has it undergone in 36
  days?
• 36 days 3 half
  life periods 12 days/half-life

100 g 50g
25g 12.5 g
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Types of Radioactive Decay

• Alpha Emission
• Beta Emission
• Positron Emission
• Electron Capture
• Gamma Emission

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Decay Models
Graphic shows U-238 with an alpha and one beta
decays. Protactinium atomic number 91 is formed.
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Alpha and Beta Decay
Alpha decay
Beta Decay
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Alpha Emission

• consists of a Helium nucleus with no electrons.
• has 2 protons and 2 neutrons.
• has a 2 charge
• has an atomic mass of 4
• has a speed that is 1/10 the speed of light.
• can be stopped by a piece of paper, cloth, or
  skin.
• The symbol is the Greek letter alpha a particle
  or 4
  2 He

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Beta Emission

• is a stream of negatively charged electrons.
• has a very light mass of an electron
• has a -1 charge
• can be stopped by a piece of aluminum
• has a speed that is 90 of the speed of light.
• can ionize air and other particles.
• The symbol is the Greek letter, beta
• b- particle or 0
  -1 e

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Positron Emission

• is a stream of positively charged electrons.
• has a very light mass of an electron
• has a 1 charge (change this in notes)
• can be stopped by a piece of aluminum
• has a speed that is 90 of the speed of light.
• can ionize air and other particles.
• The symbol is the Greek letter, beta
• b particle or o
  1 e

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Electron Capture

• is a capture of an inner orbital electron by the
  nucleus.
• has a very light mass of an electron.
• has a -1 charge.
• results in a combination of an electron and a
  proton to form a neutron.
• The symbol on the reaction side of a nuclear
  reaction is o
  -1 e

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Gamma Emission

• is form of energy or electromagnetic radiation.
• has an extremely short wavelength.
• has no mass since it is energy.
• travel at the speed of light.
• can cause air and most materials to become
  ionized or charged.
• can only be stopped by using 2 to 4 inches of
  lead or many feet of concrete.
• does not change the identity of the radionuclide.
• The symbol is the Greek letter, gamma
• g

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(No Transcript)
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Example Nuclear Reactions

• 226 Ra 226 Ac
  ________
• 88 89
• 226 Pu 4 He
  ________
• 94 2
• 235 U 235 Pa
  ________
• 92 91

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Decay Series

• A decay series is a series of radioactive
  nuclides produced by successive radioactive decay
  until a stable nuclide is reached.
• The heaviest nuclide in a decay series is called
  the parent nuclide.
• The particles in a decay series that are produced
  from parent nuclides are called daughter
  nuclides.
• U-238 the parent nuclide decays to Pb-206, which
  is stable and non-radioactive.

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U-238 Decay Series
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Units of Radioactivity

• Roentgen the amount of gamma or x-rays required
  to produce one unit of electrical charge per
  cubic centimeter from ionization of air. (1
  roentgen 86 ergs per gram)
• REP (roentgen equivalent units) the amount of
  radiation to produce an harmful effect on living
  tissue.
• REM (roentgen equivalent man) the amount of
  radiation that produces the same biological
  damage in man resulting from the absorption of 1
  REP of radiation.

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Additional Units of Radioactivity

• Curie the number of nuclear disintegrations that
  occur in one second. Commonly used in medical
  laboratory diagnostic procedures. One cure is
  3.7 x 1010 nuclear disintegrations.
• RAD (radiation absorbed dose) similar to a REM,
  and is used in monitoring dosimeter measurements
  for X-ray personnel.
• REMS, and RADS are the two most common units for
  measuring radiation exposure.

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Exposure Limits

• Average citizen No more than 150 millirems per
  year. X-rays can cause exposures of 100 millirem
  per procedure.
• Radiation or Nuclear medicine workersNo more
  than 5 rems per year.
• Physiological effects Acute Radiation sickness
  100-400 rems LD-50 (lethal dose 50) 400
  rems LD-100 Death over 1000 rems
• Hyperlink to Radiation poisoning

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Protection from Radiation

• Three factors to protect radiation workers are
• S-Shielding the use of lead and or
  concrete in high radiation
  areas.
• T-Time limit the amount of
  time in high radiation areas.
• D-Distance the farther away from a
  high radiation area the lower the exposure.

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Shielding Graphic
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Nuclear Fission

• When a nucleus fissions, it splits into several
  smaller fragments or atoms.
• These fragments, or fission products, are about
  equal to half the original mass.
• Two or three neutrons can also be emitted.
• The sum of the masses of these fragments is less
  than the original mass. This 'missing' mass
  (about 0.1 percent of the original mass) has been
  converted into energy according to Einstein's
  equation.
• Fission can occur when a nucleus of a heavy atom
  captures a neutron, or it can happen
  spontaneously

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Fission Reactors

• The heat from a fission reactor is used to heat
  water to steam, which turns turbines to generate
  electricity.
• Fuels rods made of aluminum hold the Uranium-235
  or U-238 which is the most common nuclide used in
  fission reactors.
• Control rods made of neutron-absorbing steel are
  used to limit the number of free neutrons.
• Graphite(carbon) is used to slow down fast
  neutrons produced from fission.
• Control rods allow for a limited self-sustaining
  reaction.

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Oak Ridge Fission Reactor
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Production of Electricity
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Nuclear Fusion

• Nuclear energy can also be released by fusion of
  two light elements (elements with low atomic
  numbers).
• The power that fuels the sun and the stars is
  nuclear fusion.
• In a hydrogen bomb, two isotopes of hydrogen,
  deuterium and tritium are fused to form a nucleus
  of helium and a neutron.
• Unlike nuclear fission, there is no limit on the
  amount of the fusion that can occur.

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Applications of Nuclear Chemistry

• Radioactive Dating using C-14
• Treatment of Cancer (Phosphorous and Cobalt)
• NMR and CAT scans in Radiology
• Sterilization of foods
• Radioactive tracers (cardiology)
• Fission reactors for Electrical Power
• Medical Laboratory procedures
• Defensive and Offensive Weapons