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11.3: Absolute Ages of Rocks

Difficulty Level: At Grade Created by: CK-12
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Lesson 11.3: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. The number of protons in atoms of the same element may vary.

_____ 2. Almost all carbon atoms are atoms of carbon-14.

_____ 3. When an atom of carbon-14 decays, it loses an electron.

_____ 4. Carbon-14 atoms decay to carbon-13 atoms.

_____ 5. The half-life of a radioactive isotope is constant.

_____ 6. A living thing takes in carbon-14 only while it is alive.

_____ 7. Carbon-14 dating can be used to determine the ages of rocks.

_____ 8. The half-life of carbon-14 is 5730 years.

_____ 9. All fossils can be dated with carbon-14 dating.

_____ 10. To date a rock that is as old as Earth, you could use potassium-40 dating.

Lesson 11.3: Critical Reading

Name___________________ Class______________ Date________

Read this passage based on the text and answer the questions that follow.

Carbon-14 Dating

The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks. This type of dating is called radiometric dating. The best-known method of radiometric dating is carbon-14 dating.

Here’s how carbon-14 dating works. While an organism is alive, it takes in unstable carbon-14 (along with stable carbon-12). As the carbon-14 in the organism’s body decays, it is replaced with more carbon-14. After the organism dies, it stops taking in carbon, including carbon-14. The carbon-14 that is already present in its body continues to decay. So the organism’s remains contain less and less carbon-14 as time goes on. We can estimate the amount of carbon-14 that has decayed. We also know how quickly carbon-14 decays. With this information, we can tell how much time has passed since the organism died.

Carbon-14 dating has some drawbacks. For one thing, carbon-14 has a relatively short half-life at just 5730 years. It decays quickly compared to some other unstable isotopes. As a result, carbon-14 dating can be used only for specimens that aren’t very old. Generally, specimens must be less than 50,000 years old for carbon-14 dating to be used. Another drawback of carbon-14 dating is that it can be used only for the remains of once-living things. It can’t be used to date rocks.


  1. Compare the levels of carbon-14 in an organism before and after it dies.
  2. How can the amount of carbon-14 left in an organism’s remains be used to determine when it died?
  3. What are two drawbacks of carbon-14 dating?

Lesson 11.3: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. Absolute ages are based on evidence from
    1. key beds.
    2. stratigraphy.
    3. index fossils.
    4. radiometric dating.
  2. Which of the following atomic particles may vary for atoms of a given element?
    1. protons
    2. neutrons
    3. electrons
    4. all of the above
  3. How many protons are found in each atom of carbon-14?
    1. 14
    2. 8
    3. 7
    4. 6
  4. If a carbon atom has 7 neutrons, it is the isotope named
    1. carbon-11.
    2. carbon-12.
    3. carbon-13.
    4. carbon-14.
  5. Plants use carbon dioxide for the process of
    1. respiration.
    2. germination.
    3. reproduction.
    4. photosynthesis.
  6. New atoms of carbon-14 form in the atmosphere because of
    1. pollution.
    2. cosmic rays.
    3. global warming.
    4. burning of fossil fuels.
  7. If you start with 1.00 g of carbon-14, the amount left after two half-lives will be
    1. 0 g.
    2. 0.25 g.
    3. 0.50 g.
    4. 0.75 g.

Lesson 11.3: Matching

Name___________________ Class______________ Date________

Match each definition with the correct term.


_____ 1. using radioactive decay to estimate the age of a fossil or rock

_____ 2. radioactive element with a relatively long half-life

_____ 3. rate of decay of a radioactive element

_____ 4. atom of an element with a different number of neutrons

_____ 5. stable isotope of carbon

_____ 6. radioactive element with a relatively short half-life

_____ 7. breakdown of unstable elements into stable elements


a. isotope

b. carbon-14

c. carbon-12

d. uranium-238

e. radioactive decay

f. half-life

g. radiometric dating

Lesson 11.3: Fill in the Blank

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

  1. The age of a rock or fossil in years is its __________ age.
  2. Two different isotopes of the same element differ in their number of __________.
  3. Isotopes are named for their number of neutrons plus __________.
  4. Atoms of carbon-14 form in the atmosphere from atoms of __________.
  5. Carbon-14 enters food chains when plants take in __________.
  6. All atoms of a given element contain the same number of __________.
  7. The time it takes for half of a given amount of an unstable isotope to break down is its __________.

Lesson 11.3: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Radioactive elements have a constant half-life. Why is this necessary for radiometric dating to work?

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