Unit II: Radiometric Dating of the Solar System

Read Seeds: Section 5.2

Lecture: 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.

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:

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:

Complete these Self-check Questions:

18. Why is carbon-14 more appropriate then uranium-235 for dating archeological sites? (ans.)
19. You find a rock which contains 75% uranium-235 (U-235) and 25% lead-207 (Pb-207). Refer to the table above and answer the following questions. (ans.)
    1. Which of the two elements is the parent and which is the daughter?
    2. In order for radiometric dating to be accurate, how much lead-207 must we assume was present when the rock first formed?
    3. How much uranium-235 must we assume was present when the rock was first formed?
    4. Determine the age of the rock.
20. Refer to the list of half lives. What is the half-life of potassium-40? (ans.)
21. Refer to the list of half lives. What is the daughter element produced by carbon-14? (ans.)
22. You determine that there is 12.5% strontium-87 in a substance. What is the parent element? How much parent is there? How many half-lives does that represent?(ans.)

Homework Questions: (To submit your answers to the homework questions, first copy the questions from this page and paste them into the homework form. Insert your answers below the questions. Fill in the remaining form elements and submit. Your homework will be e-mailed to me. I will return your graded homework to you in the private e-mail of the comm center).

17. 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. Radon 222 (Rn-222) alpha decays. ( 2 points)
18. The age of the solar system is thought to be 4.6 billion years old. However, the oldest Earth rocks are 3.8 billion years old. Explain this discrepancy. What solar system objects provide ages closer to 4.6 billion years? Why? (1 point)
19. 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 37.5% carbon-14 (C-14). You will need to use this chart of radioactive parents. (2 points)
    1. How much nitrogen-14 would you expect to be present
    2. What is the age of the archeological site? (Show your work.)
20. A rockcontaining 12.5% lead-208 is found. Determine the age of the rock. (Show your work) (You will need to use this chart of radioactive parents. (1 point)
21. 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 87.5% nitrogen-14. Calculate the age of the site. You will need to use this chart of radioactive parents. (1 point)

View These Optional Related Web Sites: Visiting these sites are optional, not required.

• Radiometric Dating
• Radiation and Life
• Radioactivity