Nuclear%20Chemistry

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Nuclear Chemistry

• Chapter 25

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Learning Objectives

• Students will understand
• The criteria for nuclear stability and the
  processes by which unstable nuclides decay
• Issues of safety and health with respect to
  radioactivity
• Uses of radioactive isotopes in science and
  medicine

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Learning Objectives

• Students will be able to
• Write equations for the decay of radioactive
  elements and the synthesis of new isotopes
• Carry out calculations based on equations for the
  first-order rate of decomposition of unstable
  isotopes

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25.1 Natural Radioactivity

• occurs when a nucleus undergoes decomposition to
  form a different nucleus and additional particles
• when a radioactive particle is stopped by a
  material, or when radiation is absorbed, the
  energy associated with the radiation is
  transferred to the material
• damage caused by radiation is related to the
  energy absorbed

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Alpha Radiation

• An alpha particle, a particle, is a helium
  nucleus 4He
• Least penetrating, can be stopped by several
  sheets of ordinary paper or clothing
• Heavy radioactive nuclides, isotopes, undergo
  decay to form alpha particles.

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Practice Problem

• Write a balanced equation for the decay of
  uranium-235 by a-particle production.

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Beta Radiation

• A beta particle, ß particle, is an electron
  represented in a nuclear reaction by the symbol
  0-1e
• The beta particle has a mass number 0. Its mass
  is very small
• Can penetrate several millimeters of living bone
  or tissue, but about 0.5 cm of lead will stop the
  particles

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Gamma-Ray Production

• A gamma ray, ?, is a high-energy photon often
  produced during nuclear decay.
• Gamma rays are unaffected by electric and
  magnetic fields.
• The most penetrating and can pass completely
  through the human body, thick layers of lead or
  concrete are required to shield the body
• A nucleus with excess energy can relax to its
  ground state by emitting a gamma ray
• 235U ? 4He 231Th 0?

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25.2 Nuclear Reactions and Radioactive Decay

• Nuclear reactions occur when an isotope of one
  element changes into an isotope of a different
  element.

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Practice Problem

• Six aand four ßparticles are emitted in the
  thorium-232 radioactive decay series before a
  stable isotope is reached. The first three steps
  are a, ß, ßemission. What is the final product
  in this series?

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Other Types of Decay

• A positron is an emitted particle which has the
  same mass as an electron but opposite charge.
  (antimatter to an electron)
• results in a decrease in the atomic number
• Electron capture occurs when a nucleus captures
  an inner orbital electron.
• 7Be 0e ? 7Li

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Practice Problems

• Write a balanced equation for the decay of
  carbon-14 by ß-particle production.
• Write a balanced equation for the decay of
  bismuth-205 by positron production.

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Name Symbols Charge Mass (g/particle)
Alpha 4He, 4a 2 6.65 x 10-24
Beta 0-1e, 0-1ß -1 9.11 x 10-28
Gamma ? 0 0
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25.4 Rates of Nuclear Decay

• Half Life is the time required for half of the
  sample to decay to products (independent of the
  amount of sample for first-order decomposition)
• t1/2 0.693/k

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Practice Problem

• Tritium (3H), has a half-life of 12.3 years.
• Starting with 1.5 mg of this isotope, what mass
  (mg) remains after 49.2 years?
• How long will it take for a sample of tritium to
  decay to one eighth of its activity?
• Estimate the length of time necessary for the
  sample to decay to 1 of its original activity.

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Practice Problem

• A sample of Ca3(PO4)2 containing phosphorus-32
  has an activity of 3.35 x 103 dpm. Exactly 2
  days later, the activity if 3.18 x 103 dpm.
  Calculate the half-life of phosphorus-32.

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Homework

• After reading sections 25.1 25.2 25.4 you
  should be able to do the following
• P. 1007a-b (1-15 odd, 29, 31)