Nuclear Chemistry

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Nuclear Chemistry

• Radiation and Radioactivity

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What do you think? T/F

• Radioactivity first appeared during WWII.
• Atoms cannot be changed from one element to
  another.
• Fission and fusion are the same thing.
• Radioactivity lasts forever.
• Exposure to radiation makes something
  radioactive.
• Nuclear power plants can explode like bombs.
• Radioactivity is man-made.
• When radioactive substances decay, they disappear.

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What is radiation?

• Electromagnetic Radiation
• Electric Fields
• Magnet Fields
• Fields oscillate and travel in waves

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Rules of Radiation

• An atom will release energy as electromagnetic
  radiation in order to become stable.
• A stable nucleus has at least as many or more
  neutrons as protons.
• Atoms with a mass 209 or greater are never
  stable.

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Decay doesnt mean disappear!

• Radioactive decay is the process by which the
  nucleus of an unstable atom loses energy by
  emitting ionizing radiation.
• The emission is spontaneous, in that the atom
  decays without any interaction with another
  particle from outside the atom

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The Spectrum
Strength

• Gamma Rays
• X-Rays
• Ultraviolet
• Visible Line Spectrum
• (ROYGBIV)
• Infared
• Microwaves
• Radio waves

Wavelength
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Fission or Fusion?

• Fission one atom splits into two
• Uranium-235 to Barium and Krypton
• Used in medical radiology
• Fusion two atoms join to form one new
• The Sun
• H H He energy

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Radiation

• There are three main types of radiation
• Alpha radiation
• Beta radiation
• Gamma radiation

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Alpha Decay

• Alpha (a) decay occurs is because the nucleus has
  too many protons which cause excessive repulsion.
  
• Alpha particles can be stopped by a thin sheet of
  paper
• In an attempt to reduce the repulsion, a Helium
  nucleus is emitted.
• 4
• 2

He
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Alpha Decay
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Beta Decay

• Beta (ß) decay occurs when the neutron to proton
  ratio is too great in the nucleus and causes
  instability.
• Beta particles can be blocked by a thin sheet of
  metal
• In basic beta decay, a neutron is turned into a
  proton and an electron. The electron is then
  emitted.
• 0
• -1

e
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Beta Decay
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Beta Example

• When Carbon-14 decays, one of the neutrons is
  converted into a proton, and an electron is
  emitted
• C ? N e

14 6
0 -1
14 7
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Positron

• There is also positron emission when the neutron
  to proton ratio is too small.
• A proton turns into a neutron and a positron and
  the positron is emitted.
• A positron is basically a positively charged
  electron.
• This is another form of Beta decay

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Positron Emission
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Electron Capture

• The final type of beta decay is known as electron
  capture and also occurs when the neutron to
  proton ratio in the nucleus is too small.
• The nucleus captures an electron which basically
  turns a proton into a neutron.

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Electron Capture
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Gamma Decay

• Gamma (?) decay occurs because the nucleus is at
  too high an energy.
• The nucleus falls down to a lower energy state
  and, in the process, emits a high energy photon
  known as a gamma particle.
• Gamma radiation can only be blocked with very
  thick metal, usually Lead

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Summary
Type of Radiation Particle Emitted Example
Alpha decay Helium 4 Alpha particles U ? Th He
Beta (-) decay Beta particle Neutron ?Proton Electron C ? N e
Positron Emission Positron Proton ?Neutron Positive electron Ne ? F e
Electron Capture Electron is absorbed Be e ? Li
Gamma Decay Gamma Ray ? e e ? 2?
238 92
234 90
4 2
14 14 0 6 7 -1
19 19 0 10 9 1
7 0 7 4 -1
3
0 0 -1 1
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Half Life

• The rate of radioactive decay is related to the
  energy change that accompanies the
  transformation, but it is not a direct
  relationship.
• The rate of radioactive emissions of a
  radioactive nuclide is directly proportional to
  the amount of radioactive material present.
• The rate of decay of a radioactive nuclide is
  measured by its half-life.

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• The half-life of a radioactive substance is the
  time it takes for half of an initial amount of
  the substance to decay.
• The half-live is independent of chemical
  activity, external pressure, and temperature.

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• Consider a 10 g sample of Au-198 (half-Life of
  2.69 days)
• After 0 half-life or 0 days 10 g are present.

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• After 1 half-life or 2.69 days, 5 g remains.

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• After 2 half-life or 5.38 days (2 x 2.69 days)
  2.5 g remains.

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Half-Life Formula

• T half life
• t total time elapsed
• Fraction remaining 1/2 (t/T)
• Number of half-life periods t / T
• ½ x ½ x ½ x ½ 1/16 with 4 half-lives

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Calculating Remaining Mass

• How much of a 100 ?g sample of Nitrogen-16 will
  remain after 28.8 seconds of decay? (Half life is
  7.2s, ?-decay)
• Fraction remaining 1 (t/T)
• 2 . . . So
• ½ (28.8/7.2) ½4 0.0625
• Then, 100?g x 0.0625 6.25 ?g left

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Calculating Half life periods

• How many half-lives are required for a
  radioisotope to decay to 1/32 of its initial
  value?
• Fraction remaining ½(t/T) and
• Number of periods t / T so
• 1/32 ½ t/T
• t/T 5

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• The half-life of Polonium-210 is 138.4 days. How
  many milligrams will be left after 415.2 days if
  you start with 2.0 mg?
• Step 1 How many half-life periods?
• 415.2 days / 138.4 days 3 periods
• Step 2 Determine the fraction left.
• ½ (t/T) (1/2)3 (1/8)
• Step 3 Using the fraction, determine the mass
  left.
• (1/8) x 2.0 mg 0.25 mg

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• After 4797 years, how much of a 0.250 g sample of
  Radium-226 is left? (half-life is 1599 years)
• 0.0312 G

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• The half-life of radium-224 is 3.66 days. What
  was the original mass of radium-224 if 0.0800 g
  remains after 7.32 days?
• 0.32 g