L6.3 PHET Radioactive Dating Lab

Tab 1: Half-Life

In this part, you observe how unstable Carbon-14 atoms decay into stable Nitrogen-14 atoms over time. Each half-life, half of the remaining Carbon-14 changes into Nitrogen-14.

Steps to Investigate:

1. 1. Click on the “Half-Life” tab.

2. 2. Select Carbon-14 as the isotope and start the simulation.

3. 3. Watch how the yellow parent atoms (Carbon-14) turn into blue daughter atoms (Nitrogen-14).

4. 4. Use the timer and graph to see how many half-lives pass and what percent of Carbon-14 is left.

1. Question 1: What does the simulation show happening to Carbon-14 atoms as time passes? Describe what you see.

Question 2: After one half-life, what fraction of Carbon-14 remains and what fraction has become Nitrogen-14?

Question 3: After three half-lives, about what percent of Carbon-14 is left?

Decay Rates

Here you compare the decay rates of Carbon-14 → Nitrogen-14 and Uranium-238 → Lead-206. Carbon-14 has a short half-life (good for dating young fossils), while Uranium-238 has a very long half-life (good for dating ancient rocks).

Steps to Investigate:

1. 1. Click on the “Decay Rates” tab.

2. 2. Compare the decay of Carbon-14 → Nitrogen-14 with Uranium-238 → Lead-206 by running both simulations.

3. 3. Observe the half-life times shown on the graph.

4. 4. Adjust the clock speed to compare short vs. long half-lives.

Question 4: Which isotope decays faster—​ (a)   or ​ (b)   ?

• Carbon-14
• Uranium-238

Question 5: Why is Carbon-14 better for dating fossils that are thousands of years old, but not millions?

Question 6: Why is Uranium-238 better for dating rocks that are billions of years old?

• Uranium-238 is used for dating fossils due to its short half-life.
• Uranium-238 is better for dating rocks that are billions of years old due to its long half-life.

Measurement

In this section, you measure how much of the parent isotope (Carbon-14 or Uranium-238) and daughter isotope (Nitrogen-14 or Lead-206) remain in a sample. From this, you calculate how many half-lives have passed and estimate the sample’s age.

Steps to Investigate:

1. 1. Click on the “Measurement” tab.

2. 2. Select either a fossil (Carbon-14/Nitrogen-14) or a rock (Uranium-238/Lead-206) sample.

3. 3. Measure the amount of parent isotope (Carbon-14 or Uranium-238) left in the sample.

4. 4. Use the ratio of parent to daughter isotopes to estimate how many half-lives have passed.

Question 7: If a fossil contains 25% of its original Carbon-14, how many half-lives have passed?

Question 8: What is the approximate age of a fossil with 12.5% of its Carbon-14 remaining?

• Approximately 5200 years

• Approximately 14740 years

• Approximately 22920 years

Question 9: If a rock contains mostly Lead-206 and very little Uranium-238, is it young or old? Explain.

Dating Game

Radioactive decay is the process by which unstable atomic nuclei lose energy by emitting radiation. In the PHET simulation "Radioactive Decay: Dating Game," you can explore how scientists use the predictable rates of decay of radioactive isotopes to determine the age of rocks, fossils, and other ancient objects. Each radioactive isotope decays at its own unique rate, called the "decay rate" or "half-life." By measuring the amounts of parent and daughter isotopes in a sample and knowing the decay rate, scientists can estimate how long the decay process has been happening—this is called radiometric dating! This is the challenge section. You apply what you’ve learned to identify the ages of unknown fossils and rocks. You must choose the correct isotope: Carbon-14/Nitrogen-14 for young fossils and Uranium-238/Lead-206 for ancient rocks.

Steps to Investigate:

• 1. Click on the “Dating Game” tab.

• 2. Choose a mystery fossil or rock sample.

• 3. Decide which isotope is best for dating:

  • Fossils → Carbon-14/Nitrogen-14

  • Rocks → Uranium-238/Lead-206

• 4. Measure the remaining parent isotope and calculate the age.

Questions 10: The process by which unstable atoms lose energy is called ​ (a)   .

• radioactive decay
• half-life
• decay rates