Radiometric Dating -- Radioactive Decay
Inquiry Question: How do we know the age of the Earth?
Background Information
Read p. 220 and define:
RadioactivityNuclear DecayNuclear RadiationRead p. 225-227 and define:
Half-Life-
Every radioactive isotope has its own rate of decay. This rate is called and measured as a half life.
A half life of an isotope is defined as the amount of time it takes for half of the atoms in the
sample to decay. Half lives can be as short as milliseconds and as long as billions of years. For
example, if we examine 100 grams of lead-214, we will find a half life of 27 minutes. This means
after 27 minutes pass, we will have only 50 grams of lead-214 and the rest will have “decayed”
into a totally different element, in this case bismuth-214. Thus, by examining a substance
containing both lead-214 and bismuth-214, scientists can determine how old it is by the
percentage of each element present in the sample.
In this activity, each coin represents 1 gram of a radioactive element. A coin has a 50 percent
chance of showing heads, so in any round we would expect approximately half the coins to show
heads. This corresponds to the decay of half the atoms in a radioactive substance, and therefore
the coins are indeed a good model. Students will use the scientific method to test and observe
the pattern of the coin sample and extrapolate the “half life” of the coin sample from the obtained
data. The result should be a clear-cut operational concept of the half-life of radioactive
substances.
Materials:
 100 pennies
 cup
Notes:
1. Each penny represents 1 gram of a radioactive element.
2. Each “shake and dump,” or trial, represents 1 year.
3. Record your data in the Data Chart provided below.
Experimental Procedure:
1. Put the coins in the cup. Cover the cup with your hand and shake the coins.
2. Now spill the coins out on the table in front of you. Do not lose any!
3. Gather and count all the coins that are heads. Put them aside. In the Data Chart record
this number of coins in the column Coins Removed. Record the number of coins
remaining in the Coins Remaining column.
4. Now collect the coin that were tails and put them back in the cup. Repeat the procedure.
5. Repeat steps 2, 3 and 4 until you have run out of coins to put back in the cup.
Data Chart
Trial (years)
# of Coins Removed (grams)
# of Coins Remaining
(grams)
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
1. Now take the data you have collected and create a line graph showing what happened to
the size of your coin sample as you went through all of the steps.
A. X-axis will be Time (years), represented by the trial number
B. Y-axis will be Mass (grams), represented by the number of coins remaining
2. Review all of your data and answer the following questions:
Explain what each coin represents. In terms of exponential decay, what does a heads roll mean?
What does a tails roll mean?
Note how many times you had to toss the coins before they were all used up. Did you find a
pattern?
If you repeated this experiment again, do you think you would get the same or different results?
Why?
Explain why this experiment was useful in replicating a model of radioactive decay. Why were
the coins a good way to model half-lives? Would the model work as well if we used 1000 coins?
Explain how scientists can use half-lives to date rocks on Earth.