Earth Science

Standard II, Objective 1

Title: Radioactive Dating Game

Description: Students will use an interactive website to investigate radioactive decay.

Time Needed: 50 minutes

Materials: single projector and computer if done as a demonstration, computer lab for pairs of students if done independently. Student sheet (see below)

Procedures:

  1. Check your computers to make sure they can run Java software. You might want to work through this first yourself. It is not self-explanatory.
  2. Ask students to work in pairs.
  3. If you have a Geiger counter, demonstrate it’s use and allow students to hear the background radiation we are surrounded by.
  4. Read the introduction and directions with students.
  5. Direct students to:

This site takes some concentration to understand. Students must follow the directions and go step by step to be successful.

Answers to questions:

Half Life Tab

1. Did half the atoms decay before the half-life line? Not always but if you do enough atoms it becomes closer to 50/50.

2. Add 10 more atoms. Now, are half the atoms decayed before the half-life? Not always but if you do enough atoms it becomes closer to 50/50.

3. What is the a big difference in the decay of uranium? It takes uranium a lot longer to decay.

  1. No matter time frame what you choose, what is always true of half-life? Half the atoms have decayed.

Decay Rates Tab

  1. Does the amount of atoms that an object has at the beginning matter? No
  1. What do we call the point where the number of original atoms and the decayed atoms are equal in number? Half life

Measurement Tab

  1. What happens to the amount of C-14 when the tree dies? It starts to go down.
  1. If you found a piece of preserved wood, and 65% of the atoms were C-14, how old is it? About 5,000 years.
  1. Why is the C-14 line not forming on the graph? There is no carbon in the rock.
  1. Why is a line forming on the graph now? The rock contains uranium.

Dating Game Tab

  1. If “0.0%” comes up in the black box, what does that mean? That element is not found in the item.
  1. Practice estimating the age of several items. Average the age of the items in a layer to find the age of that layer. Fill in the chart below for each sedimentary layer:

Color of Layer / Things in the Layer / Approximate age of the layer.
Dark gray / Bone, cup, skull / 1000-2220 (+/- 500 yrs)
blue / Skull, fish skeleton / 15,000-16,000
pink / Fish, sea shell, alligator skull / 150,000,000 (only one item will date
Yellow green / Sea shells / 244,830,000 (only one item will date
Light gray / Sea shells / 434 MY 717 MY 1.25 BY
  1. The custom setting indicate that there are other radioactive atoms that can be used to date living things and rocks. If you are really good at this, you can figure out the custom setting. How old is the trilobyte? 309-310 MY

Student SheetName______

Period _____

Title: Radioactive Dating Game

Introduction: People have long wondered how old Earth is. Until the late 1800’s there was not an accurate way to find out. As scientific understanding of radioactive decay took place, a new picture emerged. It seems that radioactive atoms such as carbon-14 and uranium-238 (the numbers refer to the weight of the atom) give off energy and particles in a predictable (but random) time frame. For example, C-14 atoms change to N-14 atoms over a span of about 50,000 years. C-14 is absorbed by living things in a certain ratio when they are alive and then change to N-14 after the organism is dead. The ratio of radioactive C-14 to N-14 provides a good estimate for the age of the organism. Uranium has a much longer half-life (the time it takes for half the atoms to decay to Pb-206) and can be used to date much older things, as long as they contain uranium. In this activity, you will use a computer simulation to understand radioactive decay.

Procedures:

Open the PhET website at: Type “radioactive dating game” into the search bar. Click on “Run Now” to enter the game.

Half Life Tab

Click on “Add 10” and watch as the C-14 changes to N-14. Look at the chart on the top to see where the half-life is. Fill in the data table for both C-14 and U-238.

1.

Number of atoms decayed before the half-life line

Carbon - 14 / Uranium - 238
After 10 atoms
After 20 atoms
After 30 atoms
After 40 atoms
After 50 atoms

2. How many atoms are expected to decay before the half-life is over?

3. Why is the number not always perfect?

4. What is the big difference in the decay of uranium compared to carbon?

Choose “custom” and move the “half-life” bar on the chart and choose a decay time frame.

  1. No matter time frame what you chose, what is always true of half-life?
  1. Decay Rates Tab

Click on the bar on the bucket and drag it to the right to add atoms. Add several different amounts and watch where the half-life on the graph is (where the red and blue lines cross)

  1. Does the amount of atoms that an object has at the beginning matter?
  1. What do we call the time where the number of original atoms and the decayed atoms are equal in number?

Measurement Tab

Click on “plant tree” and push the pause button after the 3rd half –life.

  1. What happens to the amount of C-14 when the tree dies?
  1. If you found a piece of preserved wood, and 65% of the atoms were C-14, how old is it?

Click on “rock”.

  1. Why is the C-14 line not forming on the graph?

Click on “uranium-238”, “volcano” and “cool rock”. Notice the change on the time scale.

  1. Why is a line forming on the graph now?

Dating Game Tab

To estimate the age of the items found in the layers of sedimentary rock, point the microphone at it and notice the number in the black box. Move the arrow on the graph to that number. (Look in the smaller yellow box above the arrow) Use the graph to estimate the age.

  1. If “0.0%” comes up in the black box, what does that mean?
  1. Practice estimating the age of several items. Average the age of the items in a layer to find the age of that layer. Fill in the chart below for each sedimentary layer:

Color of Layer / Things in the Layer / Approximate age of the layer.
Dark gray / Bone, cup, skull
blue / Skull, fish skeleton
pink / Fish, sea shell, alligator skull
Yellow green / Sea shells
Light gray / Sea shells

13. The custom setting indicates that there are other radioactive atoms that can be used to date living things and rocks. If you are really good at this, you can figure out the custom setting. How old is the trilobyte?