Lab: Determining Absolute Age
Objective: Develop a quantitative model showing radioactive decay of a sample
Background: Radioactive elements emit particles and energy, and by doing so, they form new elements or isotopes. An isotope is the same element with a different atomic mass. Radioactive elements function as natural clocks because the rate at which they decay is measurable and not affected by external factors. U-238 (uranium-238) is a radioactive element that will decay until a stable, or non-radioactive form of the element forms, which in this case is lead-206. U238 is the radioactive mother element and Lead-206 is the stable daughter.
The time required for half of any given amount of U-238 to decay into Pb-206 is 4.5 billion years. Therefore we say that the half-life of U-238 is 4.5 billion years.
Prelab Questions:
1. What is a half-life?
2. What is the difference between a mother and a daughter element?
Procedure:
1. Shake the bag and pour the contents onto the flat desk top.
2. Locate all the chips that landed with the red side up. Replace these chips with green/yellow chips, which represent the stable isotope PB-206.Record the number of green/yellow chips in Data Table 1. Also, record the remaining number of red/black chips in Table 1.
3. Place the remaining black/red chips plus the green/yellow replacement chips in the bag and repeat steps 1 and 2 until only one black/red chip remains and all others have been replaced with green/yellow chips (this may take anywhere from between 4-7 shakes). Be sure to record the total number of green/yellow chips, and the total number of remaining red/black chips in the bag for each shake.
4. Share your data with the class by writing it on the board.
5. Copy down the class averages in a new table.
6. Graph the averages calculated for your class.
Data
Table1: Number of Parent and Daughter Elements after each shake
Shake # / Black/red / Green/yellow0
1
2
3
4
5
6
7
Table 2: Average Number of Shakes/Chips Remaining
Shake # / Average Total # Black/red chips / Average Total #Green/Yellow Chips
0
1
2
3
4
5
6
7
Analysis Questions
1. In your model for radioactive decay, what do the following items represent?
a. The black/red chips?
b. The green/yellow chips?
c. The bag?
d. Each shake?
2. Approximately what percentage of black/red atoms were removed and replaced after each shake?
3. What was the average number of shakes required to produce a single black/red atom?
4. If the bag represent bone which contained 100 atoms of C-14 at the time of the organism’s death, calculate the age of the bone when 1 atom of C-14 remains, based on your average results. Show your calculations below.
5. Using your graph as a reference, determine between which time intervals (shake numbers) there was the greatest decrease in parent element and the greatest increase in daughter element. Explain your answer.
6. Explain why carbon-14 is not used to determine the absolute age of fossils more than about 75,000 years old.