Unit II: Radiometric Dating of the Solar System

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After Completing These Learning Activities You Should Be Able To:

Read This Material in Your Text:

Read These Notes: Below are some common parent /daughter pairs and their half-lives. Notice that the half-life varies from one radioactive material to another. Materials with a long half-life are useful in dating materials that are very ancient. They are most used to date the most ancient rocks and therefore are the ones used to date the Solar System. radioactive atoms such as Carbon-14 have short half-lives and therefore are used to date materials of more recent age. C-14 is commonly used in dating archeological sites, for example.

Parent Daughter Half-life Use
Carbon-14 Nitrogen-14 5,730 years Archeology
Potassium-40 Argon-40 12,500 million years Geology
Rubidium-87 Strontium-87 48,800 million years Archeology
Uranium-235 Lead-207 704 million years Geology
Uranium-238 Lead-206 4,470 million years Geology
Thorium-238 Lead-208 14,010 million years Geology

atom.gif - 0.2 K In order to calculate the age of a radioactive material, two things must be known:
  1. the abundance of either the parent or the daughter element.
  2. the half-life of the parent element

Knowing these two values one can determine the age of a material using the equation:

Age = # of half-lives x half-life

Example A: If a rock today which contains 25% Uranium 235 and 75% Lead 207, determine the age of the rock.

The parent atom in this instance is Uranium 235. Since it is assumed that originally there was 100% uranium and 0% lead, two half lives must have passed by. During the first half life the abundance of parent atoms was reduced from 100% to 50%. During the second half-life the remaining 50% parent was reduced to 25% Thus:

Age = 2 X 704 million = 1,408,000,000 years old (1.408 billion years old)

Complete These Self-check Questions: These questions and their answers are designed to help you determine how well you understand this objective and to provide additional instruction.

Homework Questions: Answer these homework questions after reading the on-line lecture and the readings in the text. Complete the self-check questions before attempting the homework. Follow the instructions for submitting the homework on this unit homework page.

  1. Use your knowledge of alpha decay and the periodic table to identify the daughter element that is produced when Polonium 213 (Po-213) alpha decays.
  2. Use your knowledge of alpha decay and the periodic table to identify the daughter element that is produced when Radon 222 (Rn-222) alpha decays.
  3. You find an archeological site. At the site are some wooden beams which you assume are as old as the site. The wooden beams contain 25% carbon-14 (C-14). You will need to use this chart of radioactive parents.
    1. How much nitrogen-14 would you expect to be present
    2. What is the age of the archeological site? (Show your work.)
  4. A rock containing 75% lead-208 is found. Determine the age of the rock. (Show your work) (You will need to use this chart of radioactive parents.
  5. You find a rock containing 12.5% potassium-40. Calculate the age of the rock. You will need to use this chart of radioactive parents.
  6. a. What is the age of the solar system?
    b. What i    s the age of the universe?
    c. Is the age of the material you calculated in the previous question a realistic age for something in our solar system? in our universe? Explain

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