Name: __________________________Date: ______________
Radiometric dating homework
For this homework activity, you will need access to the internet. Go to the
following website:
http://phet.colorado.edu/
Find the “Radioactive dating game” by clicking on “Play with Sims”  “Earth
Science”  “Radioactive Dating Game”
You should be prompted to either “Download” or “Run Now.” Choose either
one. (If the simulation will not run, it is most likely because your computer does
not have the most recent of JAVA.)
Play around with the Simulation. After you’ve played around, complete each of
the following parts.
Part I - The atomic nature of radioactive decay.
Directions: Click on the “Half Life” tab.
a. Place 20 atoms of C-14 onto the screen. Let the simulation run. Describe
what happens.
b. Press the “Reset the nuclei” button. Do it again. Observe that the atoms
do not decay in the exact same way each time. Why?
c. If you increase the number of atoms, say to 100 or 1,000 or 1 billion, what
would happen to the way in which the sample decays? Would the pattern
of decay become more or less consistent?
Name: __________________________Date: ______________
d. Compare the simulation between the decay of C-14 to N-14 vs. U-238 to
Lead-206? How are the decays different? (Color of the circles should not
be your entire answer!)
e. In your own words, describe how a large number of radioactive atoms
would change over time.
Part 2 - Using radiometric dating to determine the age of samples.
Directions. Select the tab titled, “Measurement” from the top of the simulation.
Play around.
a. When you plant a tree, you should notice that the %Carbon-14
concentration in the tree does not change while the tree is alive. Why do
you think this is so?
b. How “old” is the tree when the “% of 14C” reaches 50%?
c. Compare Uranium-238 decay to Carbon-14 decay. To do so, complete the
following:
 Plant a tree and observe the decay of both Carbon-14 and Uranium-238.
 Erupt the volcano and observe the decay of both Carbon-14 and
Uranium-238.
For the rock, when the “%Uranium-238 remaining” reaches 50%, it is not
the same age as the tree when its “%Carbon-14 remaining” reaches 50%.
Why? Explain your thinking.
Name: __________________________Date: ______________
Part 3 - Dating game.
Directions: Select the tab titled, “Dating Game.” Follow the directions to fill out
the table, then answer the questions at the end.
i. Click on the tab for Dating Game.
ii. Start measurements with living and dead objects on or just under the
surface of the Earth. With these objects use Carbon-14 for measurements.
iii. Guess the age of the object, and record on the data table.
iv. Move the double-sided arrow on the graph to match the % determined
from the probe. Once you’ve correctly guessed the age, record in the
“Measured age” on the table.
v. Ancient fossils will have no Carbon-14. You must use the ages of rocks
(from Uranium-238) in the same rock layer as the fossils to determine the
fossil’s age.
vi. Switch to Uranium-238. Now make guess and measure the ages of the
rocks. Record the data in the table.
Name: __________________________Date: ______________
Object
Carbon-14
Uranium238
% of
Original
Guessed
Age
Measured
Age
Animal
Skull
Living tree
House
Dead Tree
Bone
Wooden
Cup
1st Human
Skull
2nd Human
Skull
Fish Bones
Fish Fossil 1
Rock 1
Dinosaur
Skull
Rock 2
Trilobite
Rock 3
Rock 4
Rock 5
Answer in complete sentences.
a. Why can the “age” of both the house and dead tree be determined with
Carbon-14 dating, yet the living trees cannot?
b. Which object did you find the most challenging to find the age? Why?
c. Do the ages of the objects in the simulation match your understanding of
aging objects through relative dating (principles of stratification)?
Name: __________________________Date: ______________
d. Carbon dating isn’t a valid technique for dating fossils beyond a certain
age. It also is not useful for dating inorganic items (such as rocks). How
did you overcome its limitations?
e. What are two limits of using Uranium -238 in dating the age of objects?
f. How might scientists overcome to limitations of using Uranium-238 to
date the age of objects?
Part 4 – Application of radiometric dating learning.
a. Uranium-235 has a “half-life” of approximately 700 million years,
decaying into Lead (Pb-207). Use an example of a rock that originally
formed with 20,000 U-235 atoms to fill out the following table:
“Age of rock”
0 years
Number of U-235
atoms
20,000
Number of Pb207 atoms
0
% Uranium
remaining
0%
700 million years
1,400 million
years
2,100 million
years
2,800 million
years
b. Dr. Sue Pursmart, a scientist is studying meteorites. She wants to know
how old three different meteorites are. She has reason to believe that all
three meteorites started with only U-235 atoms (no Pb-205 atoms). She
counts the number of U-235 and Pb-206 atoms in it:
Rock I. 650,000 U-235 atoms; 100,000 Pb-207 atoms
Rock II. 20,000 U-235 atoms; 60,000 Pb-207 atoms
Rock III. 20,000 U-235 atoms; 20,000 Pb-207 atoms
i.
Determine the % Uranium remaining in each of the rock samples
above. Show your work.
Name: __________________________Date: ______________
ii.
iii.
Rank the age of each of the rocks (from youngest to oldest). Justify
your thinking.
Estimate the actual age of rocks II and III. Justify your responses.
c. Another radioactive decay used for dating organic matter, is the decay of
Carbon, from C-14 to N-14. The half-life of this decay is 5,730 years. Dr.
Sue’s colleague, Dr. Chuck Wagan, is more interested in fossils. He found
a fossil of an ancient cow with the following number of C-14 and N-14
atoms:
30,000 C-14 atoms; and 90,000 N-14 atoms.
How old is this fossil if it started with 100% C-14 atoms? Justify your
response.
Part 5 – Possible extension questions.
a. When the remains of a tree or rock are aged with radioactive dating, age
zero is defined by the formation of the object (the death of the organism or
formation of the rock (i.e. cooling of igneous rock). Do you think
radioactive dating would be useful to determine the age of sedimentary
layers of rock? Explain your thoughts.
b. In some circles, radiometric dating is argued to be an unreliable method
for determining the age of objects. Research the internet to determine one
reason why some people do not trust radiometric dating, as well as a
counter-point by a scientist. State the sources of all of your research. Two
possible resources could be :
i. http://www.answersingenesis.org/articles/nab/does-radiometricdating-prove (arguments against trusting radiometric dating)
ii. http://www.asa3.org/ASA/resources/wiens.html (counter-arguments
for trusting radiometric dating).
Name: __________________________Date: ______________