Absolute Dating by the Use of Radioactive Isotopes

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Absolute Dating by the Use ofRadioactive
Isotopes
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What is Absolute Dating?

• Age of fossil or rock is given in years instead
  of relative terms like before and after, early
  and late.
• Does not mean it isnt without error
• Radiometric dating is the most common type of
  absolute dating.

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Atoms and Isotopes Lets review the basics
Chemical symbol for element

• The number of protons in an atom determines which
  element it is.
• If you change the of protons, the element
  changes the mass changes.
• If you change the of neutrons, the element
  stays the same, but the mass changes
• ISOTOPES - atoms of the same element that have
  different numbers of neutrons

Mass (protons neutrons)
Atomic (protons)
http//www.saskschools.ca/curr_content/chem30/imag
es/e_deuterium.jpg
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Why Are Some Isotopes Radioactive?

• Isotopes that have the right amount of neutrons
  are called stable. They always stay the same.
• Some isotopes have a few too many neutrons or
  not
• enough - This makes them unstable and
  radioactive.
• The nuclei of these radioactive atoms change or
  decay by giving off radiation in the form of
  particles or electromagnetic waves until the atom
  reaches a stable state.

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Radioactive Decay

• During radioactive decay, the number of protons
  in the atom changes, and one element transforms
  into another.
• Parent isotopes decay into daughter isotopes.
• Radioactive Decay is like popping popcorn.

• Each radioactive parent always decays to a
  specific daughter.
• There is no way to predict
  which atoms will decay first.
• Radioactive atoms decay at a specific rate.
• Once they decay, they can
  not change back.

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How Long Does Radioactive Decay Take?

• Half-Life - the time it takes for half of the
  radioactive or parent isotopes in a sample to
  decay to daughter isotopes.
• Each parent has a 50 chance of decaying during 1
  half-life.
• Measured in seconds, minutes, years, etc.
• Each isotope has its own unique half-life.
• From thousandths of a second to billions of years

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Starting the Stopwatch

• If you measure the ratio of parent to daughter
  isotopes, you can determine how many half-lives
  have passed.
• Find the half-life of the parent isotope
• of half-lives ? length of half-life age of
  sample
• Example 3 half-lives 1 half-life 200 years

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Atoms Dont Age the Way We Do
Start with 16 baby aliens
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Have 70 year half-lives
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4 half-lives 280 years
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Each atom has a 50 chance of decaying during a
half-life.
http//www.colorado.edu/physics/2000/isotopes/imag
es/age280_baby.jpg
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How to Choose Which Isotope to Use

• Estimate the age of your sample and choose an
  isotope with an appropriate range.
• First find out what minerals are in your sample.
  The minerals in your rock need to have the
  element you want to use for dating.
• Carbon-14 can only be used to date samples that
  were once living (organic)
• Ex Wood, bone, cloth, paper

K-40 feldspar mica Uranium zircon
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Lets Practice Absolute Dating