Radioactive Decay Lab: Half-Life Model

Radioactive Decay Lab: Half-Life Model

Just because an element is radioactive, does not always mean it is dangerous. Radioactive elements have an unstable nucleus and decay into a more stable element over time. The rate of this radioactive decay varies with the isotope (or form) of the element. The rate of this decay is expressed as the half-life of the material. A half-life represents the amount of time it takes half a sample to decay into its stable product. To understand the concept of a half-life we are going to model radioactive decay using Skittles to represent radioactive atoms.

Skittles that land with the “s” side up are the radioactive parent element: Skittilium (Sk). Skittles that land with the blank side up have decayed to the non-radioactive daughter product: Blankium (Bl). The half-life of Skittilium is 10 seconds.

Skittilium =Radioactive parent material Blankium = Non-radioactive daughter product

Procedure:

1. Open your napkin on your desk, and pour your skittles onto the napkin so that none of them touch the desk. Count out the Skittles, and place them back in the cup. Record the exact number of Skittles you are starting with in the Data Table below. DO NOT eat any yet, they’re all radioactive!

2. Place your hand over your cup, and shake your cup for 10 seconds (the half life for Skittilium) and then pour the Skittles onto the napkin. Do not turn any over! This represents the passing of one half-life, or decay period. Count all of the non-radioactive Blankium atoms and record the number in your Data Table. Put the Blankium to the small cup, they are no longer radioactive!

3. Count the number of radioactive Skittilium atoms remaining and record the number in the Data Table. Put them back in the red cup. Shake them for 10 seconds and dump them back onto your napkin. This represents a second half-life, or decay period.

4. Again, remove the Blankium atoms that have decayed, record how many are left. Count the radioactive Skittilium atoms that remain and record this number.

5. Repeat steps 3 and 4 until all of your Skittilium have decayed or it has gone through 8 half-lives.

Data Table:

# of Half-Lives / Total Time
passed / Number of Radioactive
Skittilium atoms remaining / Number of Non-radioactive Blankium atoms remaining
0 /

0 seconds

(the total # of Skittles you started with)

/ 0
1 / 10 seconds
2 / Add to the total in the previous box every time
3
4
5
6
7
8

Use two different colors to graph your data.

Plot your Skittilium (Sk) data on the graph above as a LINE GRAPH. Color the key

Using a different color, plot your Blankium (Bl) data on the graph above as a LINE GRAPH. Color the key

Lab Questions

1. Describe the pattern in your data as the lab progressed. Use your data table and graph as evidence in your description.

2. Iodine-131 has a mass of 131. It is a radioactive isotope that can be used to treat certain thyroid conditions. It has a half-life of 8 days. Hermione’s friend, Hagrid, has a thyroid condition and is given 40 mg of I-131 to drink. Assuming that all the I-131 stays in his gigantic body, how many mg of I-131 will remain after 4 half-lives? (Use a data table below to help answer the question.)

# of Half-Lives / Total Time
passed / Amount of I-131 remaining (mg)
0 / 0
1
2
3
4

3. Tris will soon undergo some medical testing that will require her to swallow a radioactive isotope as a tracer. She is nervous about having a radioactive substance in her body even though the radiologist has told her not to worry because the isotope has a half-life of only 7 minutes. If you were Four, explain to her: a) what it means to be radioactive, b) what an isotope is, c) what a half-life is, and d) the significance of the half-life being 7 minutes.

Uranium-235 is found in most igneous rocks. Geologists can compare the ratio of U-235 atoms to Pb-207 produced from it and determine the age of the rock. The half-life of U-235 is 704 million years. Using this decay rate, scientists have dated the volcanic intrusion in the diagram below to be 110 million years old. Using this information, what can you infer about the age of rock layers C and D?