Teacher Notes



Teacher Notes
Publication No. 12625
Carbon Dating ActivitySuper Value GameMaterials Included In Kit
Picture tiles, laminated sheets, 10
Story tiles, laminated sheets, 10 Additional Materials Required
Glue stick
Paper, blank, 8" x 11", 4 sheets Scissors Tape Lab Hints
Correlation to Next Generation Science Standards (NGSS)^{†}Science & Engineering PracticesDeveloping and using modelsAsking questions and defining problems Planning and carrying out investigations Analyzing and interpreting data Using mathematics and computational thinking Engaging in argument from evidence Obtaining, evaluation, and communicating information Disciplinary Core IdeasMSESS1.C: The History of Planet EarthHSESS1.C: The History of Planet Earth Crosscutting ConceptsPatternsScale, proportion, and quantity Systems and system models Energy and matter Stability and change Performance ExpectationsMSPS12: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred. Answers to Questions
ReferencesWe are grateful to Bob Becker, Kirkwood High School, Kirkwood, MO, for providing us with the idea and instructions for this activity. This activity has been adapted from Nuclear Chemistry, Volume 18 in the Flinn ChemTopic™ Labs series; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2006). Recommended Products


Student Pages


Student PagesCarbon Dating ActivityIntroductionArchaeologists and geologists have been able to reconstruct some of the ancient history of the Earth by “dating” various artifacts. The predictable process of nuclear decay can be used to date objects made of wood or cloth based on the amount of radioactive carbon14 contained in the object. Concepts
BackgroundCosmic rays continually bombard the upper atmosphere of the Earth. The rays collide with gases producing neutrons, which in turn collide with nitrogen atoms to produce carbon14 and knock out a proton (Equation 1). {12625_Background_Equation_1}
Carbon14 is radioactive and decays to nitrogen14 by beta decay (Equation 2).
{12625_Background_Equation_2}
The halflife, t_{½}, of carbon14 is 5,730 years. The amount of carbon14 in the atmosphere remains relatively constant over time. This is because the rate at which carbon14 is produced is approximately equal to the rate at which it decays. During photosynthesis, plants take up carbon14, along with the much more abundant isotopes carbon12 and carbon13, in the form of carbon dioxide. The carbon14 is incorporated into starch molecules in plants. Plants are consumed by planteating animals, which are then consumed by carnivores. Over time, this will cause living organisms to achieve a steadystate ratio of carbon14 to carbon12, which remains constant until the organism dies. At the time of death, the level of carbon14 is approximately the same as the ratio of atmospheric carbon14. After an organism dies, the number of carbon14 atoms is depleted due to radio active decay, causing the carbon14 to carbon12 ratio to decrease. If the ratio of carbon14 to carbon12 is known for a similar living organism, the age of an artifact can be determined. This is done by measuring the amount of carbon14 in similar size samples of both the living organism and the artifact. As time passes, the amount of carbon14 in an artifact decreases according to the decay rate equation; {12625_Background_Equation_3}
Where
^{14}C_{o} = initial amount of carbon14 {12625_Background_Equation_4}
For carbon14 (t_{½} = 5730 yrs), the value of the rate constant k is equal to {12625_Background_Equation_7} Rearranging Equation 3 to solve for t gives
{12625_Background_Equation_5}
Since the radioactivity of carbon14 is directly related to the number of carbon14 atoms, the measured radioactivity (A) in counts per minute can be substituted into Equation 5. Equation 5 is then simplified to Equation 6:
{12625_Background_Equation_6}
By measuring the carbon14 activity in similar sample sizes of organic material for both the current organism and the artifact, the age of the artifact can be calculated. After approximately six halflifes have passed (6 x 5730, or 35,000 years), the activity of carbon14 is reduced to near background radiation levels. Unless very sophisticated instrumentation is used, this limits the dating of artifacts by carbon14 measurements to those less than 35,000 years old. To date objects of much greater age, a different naturally occurring radioactive isotope with a much longer halflife is used. One such isotope that is used for rocks and minerals is potassium40. Potassium40 decays by beta emission to argon40, with a halflife of 1.3 x 10^{9} years. Since argon is a noble gas not naturally found in rock formations, the only source of trapped argon in minerals would be the result of potassium40 decay. By determining the ratio of argon40 to potassium40, the age of a rock and everything deposited around it, may be determined. Experiment OverviewThe purpose of this activity is to solve a carbon14 dating puzzle by arranging a set of story tiles and picture tiles in a logical sequence to explain the process of carbon14 dating. Materials
Blank paper, 8" x 11", 4 sheets
Glue stick Picture tiles, laminated sheet Scissors Story tiles, laminated sheet Tape Procedure
Student Worksheet PDF 