Radiometric Dating Activity

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This hands-on activity is a simulation of some of the radiometric dating techniques used by scientists to determine the age of a mineral or fossil. The activity uses the basic principle of radioactive half-life, and is a good follow-up lesson after the students have learned about half-life properties. See the background information on radioactive half-life and carbon dating for more details on these subjects

 

Objective:

Students will use half-life properties of isotopes to determine the age of different "rocks" and "fossils" made out of bags of beads. Through this simulation, they will gain an understanding of how scientists are able to use isotopes such as U-235 and Pb-207 to determine the age of ancient minerals.

 

National Science Education Standards :

Grades 5-8:

CONTENT STANDARD A (Science as Inquiry):

Mathematics is important in all aspects of scientific inquiry.

CONTENT STANDARD E (Science and Technology):

Science and technology are reciprocal. Science helps drive technology, as it addresses questions that demand more sophisticated instruments and provides principles for better instrumentation and technique. Technology is essential to science, because it provides instruments and techniques that enable observations of objects and phenomena that are otherwise unobservable due to factors such as quantity, distance, location, size, and speed. Technology also provides tools for investigations, inquiry, and analysis.

Grades 9-12:

CONTENT STANDARD A (Science as Inquiry) :

Mathematics is essential in scientific inquiry. Mathematical tools and models guide and improve the posing of questions, gathering data, constructing explanations and communicatingresults.

CONTENT STANDARD B (Physical Science):

Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.

The atom's nucleus is composed of protons and neutrons, which are much more massive than electrons. When an element has atoms that differ in the number of neutrons, these atoms are called different isotopes of the element.

Radioactive isotopes are unstable and undergo spontaneous nuclear reactions, emitting particles and/or wavelike radiation. The decay of any one nucleus cannot be predicted, but alarge group of identical nuclei decay at a predictable rate. This predictability can be used to estimate the age of materials that contain radioactive isotopes.

CONTENT STANDARD D (Earth and Space Science):

Geologic time can be estimated by observing rock sequences and using fossils to correlate the sequences at various locations. Current methods include using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.

 

Class Time:

45 minutes

 

Materials:

A worksheet with the following data table:

"Fossil" Number	Number of parent isotope atoms	Number of daughter isotope atoms	Number of half-lives	Age of "fossil"
1
2
3
4
5

 

Instructions:

Before class begins, prepare five bags filled with about 100 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 (1-5), 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:

1. Blue = Parent isotope Uranium 235 (15 beads), Red = Daughter isotope Lead 207 (45 beads), U-235 has a half-life of 704 million years

2. Blue = Parent isotope Uranium 235 (5 beads), Red = Daughter isotope Lead 207 (35 beads), U-235 has a half-life of 704 million years

3. Green = Parent isotope Uranium 238 (30 beads), Orange = Daughter isotope Lead 206 (10 beads), U-238 has a half-life of 4.5 billion years

4. Green = Parent isotope Uranium 238 (25 beads), Orange = Daughter isotpe Lead 206 (25 beads), U-238 has a half-life of 4.5 billion years

5. Yellow = Parent isotope Thorium 232 (45 beads), Purple = Daughter isotope Lead 208 (5 beads), Th-232 has a half-life of 14 billion years

When class begins, tell the students that in this activity they will use their knowledge of ratioactive decay and half-life properties to figure out the age of five different "fossils" at different stations around the room. 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:

# of parent isotopes / (# of parents isotopes + # of daughter isotopes) this is the initial # of parent isotopes

This ratio gives you the percentage of parent isotope atoms left after radioactive decay. 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 .25. Next, they will look at the graph of decay and see that when 25% of the parent isotope atoms are left, the isotope has gone through two half-lives. 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 (704 million years for fossil one):

# of half-lives x length of half-life = age of sample

If you are using the five example bags, the correct answers that the students should come up with are:

1. 2 half-lives: 1,408,000,000 years old (1.408 billion years)

2. 3 half-lives: 2,112,000,000 years old (2.112 billion years)

3. 1/2 half-life: 2.25 billion years old

4. 1 half-life: 4.5 billion years old

5. 1/5 half-life: 1.4 billion years old

 

Follow-up Questions:

The following questions can be used for assessment or for class discussion:

1. Rank the fossils from oldest to youngest.

2. Which two were very close in age?

3. 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?

4. Why do you think there were lots of beads of other colors in the bag besides the ones you were counting?

5. Do you think scientists can use more than one type of isotope to date the same rock or fossil?

6. 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 235 or Thorium 232? Why?

 

Futher Activities:

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:

http://www.ucmp.berkeley.edu/fosrec/McKinney.html

 

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