Why is radioactive decay so predictable

Nuclear Chemistry

Pre-Lab Questions

1. Define radioactivity decay.

The spontaneous transformation of an unstable atomic nucleus into a lighter one, in which radiation is released in the form of alpha particles, beta particles, gamma rays, and other particles.

2. What is predictable about radioactive decay? What is unpredictable?

What is predictable about radioactive decay is how long it will take for the half-life of the mass of the material to decay. What is unpredictable about radioactive decay is that it is random and spontaneous and how long a particular molecule takes to decay.

3. Describe how half-life is used to determine the geologic age of a rock.

To determine the age of a rock, measure the ratio of the remaining parent atom to the amount of daughter atom and by this you will know how long the molecule has been decaying.

Table 2: Radioactive Decay Data

Trial

Skittles® “S” Up (Parent Atoms)

Skittles® “S” Down (Daughter Atoms) for each Trial

Skittles® “S” Down (Daughter Atoms) Cumulative Total

0

61

0

0

1

30

31

31

2

7

23

54

3

3

4

58

4

2

1

59

5

1

1

60

6

1

0

60

7

0

1

61

Post-Lab Questions

1. Create a graph using your data from Table 2 and a computer program such as Microsoft Excel®. If you do not have a graphing program installed on your computer, you can access one on the internet via the following links: http://nces.ed.gov/nceskids/createagraph/ or http://www.onlinecharttool.com. On the x-axis plot “Trial Number.” On the y-axis plot “Parent Atoms” and “Total Daughter Atoms.”

2. Suppose the isotope your Skittles® represented was uranium-238 and the trials represent the number of half-lives. How old was the sample at the end of your tests? Use Table 1 in the Introduction to help you answer this question. Include your calculations.

4.5 x 7 = 31.5 billion years’ old

3. Suppose the isotope your Skittles® represented was uranium-238 and the trials represent the number of half-lives. Use the ratio of daughter to parent atoms to calculate the age of the sample in Trial 3. Use Table 1 in the Introduction to help you answer this question. Include your calculations.

Ratio: 1:127

(7)(4.5) = 31.5 billion years old

4. Suppose the isotope your Skittles® represented was uranium-238 and the trials represent the number of half-lives. Calculate the age of the sample after three half-lives. (Hint: this calculation should be different than Question 2). Does this match your answer to Question 3? Why or why not? Use Table 1 in the introduction to help you answer this question. Include your calculations.

5. Identify and describe similarities and differences between this experiment and radioactive decay in nature.

Shows half life but not time.

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Trial 1 Trial 2

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Trial 3 Trial 4

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Trial 5 Trial 7

Radioactive Decay

Parent Atoms 0 1 2 3 4 5 6 7 61 30 7 3 2 1 1 0 Total Daughter Atoms 0 1 2 3 4 5 6 7 0 31 54 58 59 60 60 61