Radiometric dating – the “half life” of M&M’s
background: All radioactive elements are unstable and eventually decay to a stable form. Each radioactive element has a unique rate of decay, called its half-life. Half-life is the average time it takes for half of the radioactive elements in a sample to decay, or change into different elements (isotopes). We can use M&Ms to represent radioactive elements. M&Ms have two sides, just like radioactive elements have 2 conditions. “M” side up is radioactive, “M” side down is decayed to a stable element. When the half-life of an element is known, it can be used to date fossils and artifacts. Since all living things have carbon in them, and there is a naturally occurring form of radioactive carbon, fossils can be carbon dated. The amount of radioactive carbon in a fossil is compared to the normal amount that is always in the air. By seeing how much less radioactive carbon the fossil has, scientists can decide the age of a fossil.
Problem: Can we use M&Ms to model radioactive dating? Can radioactive dating tell how old something is?
Hypothesis: What is the scientific hypothesis about the amount of radioactive element in an actual fossil and its age? Write an IF…THEN hypothesis on the answer page.
Materials:
- 100 M&Ms - Paper plate - Paper cup
Procedure (put one slash through the numbers as you complete each step):
1. Wash your hands if you will be handling the M&Ms.
2. Be sure you start with 100 M&Ms. Also make sure you can see an “M” clearly on one side.
3. Turn all 100 M&Ms in your plate “M” side up. “M” side up means it is a radioactive M&M. Record the data for time 0 in the Table 1 under Trial 1. You a starting with 100 so 0 are decayed.
4. Shake the plate gently for 3 seconds so that some of them flip over but don’t spill them. One round of shaking signifies 10 million years.
5. Take out and count decayed M&Ms (“M” side down). Place the decayed M&Ms in your cup – you will need them for Trials 2 & 3. DON’T EAT THEM YET!
6. Each time you remove radioactive isotopes, you’ll have to total them and subtract that number from 100 each time.
7. Stop once 50 of the M&Ms have decayed, that is the half life for that set of M&Ms.
8. Repeat steps 2-7 three times.
SAMPLE DATA:
For the first round, the student started with 0 decayed M&Ms.
Time / DecayedM&Ms
0 / 0
10mil / 5
20mil / 11
30mil / 22
40mil / 50
5 upside down (“M” side down) M&Ms resulted & were removed from the first shake.
The second shake resulted in 6 upside down M&Ms that were removed (6+5=11 total decayed M&Ms).
How many M&Ms were upside down on the plate on the 3rd shake? ______
What is the half-life for this set of M&Ms? ______
NAME ______PERIOD _____
M&M Radioactive Dating Answer Page
Hypothesis: ______
______
Table 1 – Recording Decay Figure 1 – Years VS. Decayed isotopes
Trial 1 / Trial 2 / Trial 3Years (millions) / Decayed
M&Ms / Decayed
M&Ms / Decayed
M&Ms
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Group # / AVG. / Group # / AVG.
1 / 7
2 / 8
3 / 9
4 / 10
5 / 11
6 / 12
Table 2 - Average Half-Life What is the average half-life for your M&Ms? ______
Record this number on the class data table and then in Table 2. Graph the data from Table 1 into Figure 1. Be sure to label your axes. Use different line types to represent different trials. Use Table 2 to record class data for the average M&M half-life.
Compare your graphs with those of another group in the space below.
______
Analysis
1. What do we call an element that decays into a different element (Carbon 14 à Carbon 13)?
______
2. Define the term “half-life.” ______
______
3. Describe what you are simulating by removing upside down M&Ms from the plate.
______
______
4. Can we use the “M&M isotope” to determine the age of fossils that are 500 million years old?
YES / NO – Explain. ______
______
______
5. How accurate would your data be if you shook the plates for different amounts of time?
______
______
6. Would our data be more accurate with 200 M&Ms? Explain. ______
______
______
7. How could this experiment have been changed in another way to change our accuracy?
______
______
8. Explain the results of Table 2. ______
______
______
9. Radiometric dating is one way to determine the age of a fossil or rock layer. Describe relative dating (not taking your cousin to prom.) ______
______
______
10. Using radioactive carbon (carbon dating) can also be used to date fossils. How?
______