## Radiometric dating of fossils worksheet

Before class begins, prepare five bags filled with about beads each. For each bag, count a specific number of "parent isotope" beads of one color and "daughter isotope" beads of another color.

Once you have a set of parent and daughter isotope beads in the bag, fill up the bag with a mixture of all the other colors. Next, label each bag with a number , put it at a separate station around the room, and make a sign that identifies the parent isotope type and color, daughter isotope type and color, and half-life. For instance, your five bags might be set-up something like: The bag itself represents the fossil and the beads inside represent some of the millions of atoms that make it up.

As scientists, their job is to count the number of parent and daughter isotope atoms in each bag, and from this data to determine how many half-lives the isotope has gone through and therefore the age of the rock. Have the students rotate in groups from station to station until they have figured out the age of all five fossils.

For younger students who may not have the math background, the easiest way for them to calculate the number of half-lives is to take: Instead of using exponents and natural logs, the students can just use a graph of predicted decay rates to determine the number of half-lives the isotope has gone through based on this percentage see graph. For instance, in fossil one, the students will take 15 divided by 60 and come up with the percentage. In this way, they get practice reading graphs and using them to understand and interpret data.

A good idea is to have the graph printed on the worksheet with the data table so that the students can have it right in front of them. Finally, to figure out the age of the fossil, they will take the number of half-lives, two in this case, and multiply it by the length of the half-life million years for fossil one: Rank the fossils from oldest to youngest.

Which two were very close in age? In this activity, which "fossil" came from the time just after the formation of the earth? Do you think any real fossils could come from that time? Why do you think there were lots of beads of other colors in the bag besides the ones you were counting?

Do you think scientists can use more than one type of isotope to date the same rock or fossil? If you wanted to date a sample that you estimated to be about 1 million years old, which isotope would you use to date it, Uranium or Thorium ?

For more great activities on half-life and radiometric dating, see the lesson plan entitled Determining Age of Rocks and Fossils by Frank K. McKinney on the webpage: