Name: ______Date: ______Period: ______
Half-Life Lab
Discussion: Carbon-14 is a radioactive element that breaks down, or decays, into nitrogen. The time it takes for one-half of the carbon-14 to break down into nitrogen is called its half-life. The carbon-14 in an organism begins to decay into nitrogen when the organism dies. For this reason, the half-life of carbon-14 can be used to find the absolute age of a fossil by comparing the amount of carbon-14 with the amount of nitrogen in the fossil. A disadvantage to using carbon-14 dating is that it can only be used on fossils younger than 60,000 years. Radioisotopes like Uranium and Potassium must be used on fossils older than 60,000 years.
Purpose: In this activity you will simulate radioactive decay using pennies. The pennies represent atoms of a radioactive element. They will be used to discover the relationship between the passage of time and the number of atoms that decay. Each trial represents one half-life. Heads-up coins represent un-decayed atoms. Tails-up coins represent atoms that have undergone radioactive decay.
Procedure:
1. Place 80 pennies HEADS-UP in a box. These pennies represent the starting composition.
2. Close the box and shake vigorously (making sure the top of the box doesn’t open).
3. Open the box. Remove all the TAILS-UP coins, count them and record the number in your table.
4. Count and record the number of HEADS-UP coins that remain (half-life # 1)
5. Repeat steps 2, 3 and 4 three more times. These trials represent half-lives #2, 3, and 4. You must
record the number of TAILS-UP coins produced and the number of HEADS-UP coins for each trial
(half-life).
6. Pool the class data and determine the number of decayed and un-decayed atoms for each half-life.
Record in your data table.
7. Prepare 2 graphs. 1) Graph showing your results. 2) Graph showing the pooled class results.
a. X-axis = number of half lives (start at 0 and number 1-4)
b. Y axis = number of un-decayed atoms (heads-up coins) that remain for each half-life
c. Label your axes and include a title for each graph.
Data Tables:
YOUR RESULTS / POOLED CLASS RESULTS# of half-lives / # of decayed atoms / # of non-decayed atoms / # of half-lives / # of decayed atoms / # of non-decayed atoms
0 / 0 / 80 / 0 / 0
1 / 1
2 / 2
3 / 3
4 / 4
Analysis Questions:
1. What did the pennies represent in this activity? ______
2. What did the heads-up pennies represent? ______
3. What did the tails-up pennies represent? ______
4. What did shaking the box represent? ______
5. Are your graphs straight or curved? ______Why do you think that is?
______
6. Which set of data, your own or pooled class provides the better demonstration of the concept of
half-life? ______Why? ______
______
7. In this simulation is there any way to predict when a particular penny (atom) will decay? ______
8. Which of the following fossils do you think is older: a fossil containing 25% carbon-14 and 75% nitrogen OR a fossil containing 50% carbon-14 and 50% nitrogen? How do you know?
______
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9. Do you think carbon-14 dating could be used to find the age of a dinosaur fossil? Why or why not?
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10. If a half-life is equal to 20 minutes,
a. How many un-decayed atoms (from the lab) remained after 40 minutes? ______
b. How many un-decayed atoms remained after one hour? ______
c. How many un-decayed atoms remained after 30 minutes? ______
d. How many un-decayed atoms would remain after 1 hour and 20 minutes? ______