Carbon-14 Dating Chemistry Lab
Name ___________________ Date _____________ Per __
Radioactive atoms change over time into other types of atoms in a natural process known as radioactive decay. The decay of radioactive atoms occurs in a predictable way. After a length of time has elapsed, exactly one-half of the original atoms will have changed, leaving the other half unchanged. The length of time it takes for half of the atoms to decay is called a half-life. Part One: C-14 Dating Simulation Complete the following activity to demonstrate the concept of half-life. 1. Put exactly 100 pennies into a paper cup. The 100 pennies represent carbon-14 atoms before decay. 2. Shake the cup and empty it onto the lab bench. For this activity, each shake of the cup (each run) represents one half-life. 3. Remove the pennies that are the tails up – these are the atoms that have “decayed.” 4. Record the number of remaining, undecayed, pennies in the table below under Run 1. 5. Repeat this procedure with several more runs until all pennies are gone or until there is only one left. 6. Graph each of the data points and connect them to form a smooth line graph. 7. Obtain the class average for each run and plot the data to make a second line on the graph. 8. Finally, calculate the theoretical half-life values for all runs and graph them as a third line. The theoretical values are based on the assumption that head side and tail side down are equally probable – for example, the theoretical value is 50 after the first run. 9. Label the three lines on the graph.
Number of Undecayed pennies Start Group value Class Average Theoretical value
100 100 100
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Run 8
Run 9
Run 10
Graph of Penny Decay
# of undecayed pennies
100
0 0
10
Number of Runs (half-lives)
1. Approximately what fraction of remaining pennies was removed after each run?
2. Which graph line – the individual or class average – most closely follows the theoretical line? Why?
3. How many runs (half-lives) long did it take for 33 pennies to decay? Why Carbon-14? One of the tools available to scientists who study ancient climates is called Carbon-14 dating (also known as carbon dating or radiocarbon dating). This method is used, within limits, to determine the ages of certain types of objects. Read pages 806 and 814-815 of your textbook to determine why carbon-14 is used to determine the age of an artifact. Part Two: Graphing the Decay of Carbon-14 Knowing that the half-life of carbon-14 is 5,730 years, it is easy to construct a decay curve like the one for the pennies. Complete the following table and make a line graph of the data on the chart provided. The result should be a smooth, curving line through all points.
Decay of Carbon-14 Years from present % of original 14C remaining
0 100
5,730
11,460
17,190
22,920
28,650
34,380
40,110
45,840
51,570
100
% of original 14C remaining
Decay of Carbon-14
0
10,000
20,000
30,000
40,000
50,000
Years from Present Applying the Concepts 1) Some radioisotopes have very long half-lives. Uranium has a half-life of 4.5 billion years. Explain when using Uranium would be necessary.
2) Radioisotopes with short half-lives are useful in nuclear medicine. Why?
3) Carbon-14 undergoes beta decay. Write the nuclear equation for this decay.
4) An unearthed wooden tool was found to have only 34% of the carbon-14 content of a sample of living wood. How old is the wooden tool?
5) Carbon dating was performed on a skull. It was determined that the person was alive around 12,000 B.C. What percent of the original carbon-14 was remaining in 2010?
6) There was a shirt for sale on eBay that is said to have been worn by King Tut. Being an excellent FHS student, you know that King Tut lived from 1341 BC to 1323 BC. The site offered proof of it authenticity by providing lab results showing that 67% of the carbon-14 remained. Is it possible this shirt was worn by King Tut? Explain your results.