Pennies Half Life Lab

Pennies Half Life Lab

Background: Uranium-238 or U-238 is a radioactive isotope of the element uranium.  Uranium-238 decays to lead-206, which is a stable isotope of the element lead.  The half-life of uranium-238 is 4.5 billion years. So every 4.5 billion years, half of the uranium-238 atoms in a sample will decay to lead-206.  In other words, during any 4.5 billion year period, the probability that a particular uranium-238 atom will decay is ½.

            The absolute age of a rock can be found by analyzing the rock for uranium-238 and lead-206. Knowing the amounts of both of these isotopes enables scientists to calculate how long ago the rock formed. In the following experiment, you will use pennies to model radioactive decay.

Objectives:

1.      To model radioactive decay using pennies to represent uranium atoms. 

2.      To demonstrate the concept of half-life and how it is used in radiometric dating.

Pre-Lab Questions:

1. Define the term “half-life”.

2. What does it mean when we say that an atom has “decayed”?

3. If you start with 8,000 atoms how many will you have left after four half-lives? Show your work.

4. If you start with 1,920 atoms and now have 240 atoms left, how many half-lives have past? Show your work.

Hypothesis:

1. Calculate if you have ___ atoms, how many half-lives will it take until you have zero atoms left.

2. How many shakes do you think you will need to remove all your pennies from the box.

Procedure:

1. Place ____ pennies in your plastic cup.  Put your hand over the cup and shake it several times.

2. Shake the pennies out into your box until they are all flat on the box.

3. Remove all the pennies from the box that are _____ side are turned upward.

4. Count how many pennies you removed and how many pennies that you have left in the box. Record in the data table.

5.  Remove all the pennies from the box and place in the cup and shake out again.

6. Repeat the steps again until there are no pennies left in your container.

7. Repeat the steps again for a second trial until there are no pennies left again and record your data.

Shake number:		Number of pennies remaining:		Number of pennies removed:
Trial #1	Trial #2	Trial #1	Trial #2	Trial #1	Trial #2:
1	1
2	2
3	3
4	4
5	5
6	6
7	7

Analyzing your results:

1. Use the graph data example on the screen to graph your results.

2. Make sure to label your axes on your graph.

3. Graph trial #1 and trial #2 on your graph with two different colors. 

4. Draw a smooth line/curve to connect your points for each of your trials.

5. Write your results on the class data sheet at the front of the room.

Post-lab questions:

1.  Examine the half-life of uranium-238 graph on the screen and the graph you have made.  Does the lines/curves have the same shape?  Explain why or why not.

2. Assume that the pennies represent uranium-238 and lead-206 isotopes. Remember that uranium-238 is the parent isotope and decays to lead-206 the daughter isotope.  In this model, which isotope represents the “head” side of the penny? Which isotope represents the “tail” side of the penny?

3. If a “sample” of pennies had 75 heads and 25 tails showing, how many half-lives would have passed since the “sample” had formed and begun to decay?  (Hint: the original sample had 100 tails. Which each half-life, half of the pennies became heads.  How many half-lives does it take to get to 75 heads?)

4. Do you think the number of pennies (or atoms) you start with affect the outcome? Explain.

5. Describe the shape of the line/curve you made with your graph.  What does that shape tell you about the results?