# Where in your environment might scientists look for large concentrations of strontium?

El Centro College

ENVR 1402

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Objectives

1. Students will define the terms half-life and radioactive decay.

2. Students will model the rate of radioactive decay

3. Students will understand how radioactive decay is used to date artifacts.

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Background

If two nuclei have different masses, but the same atomic number, those nuclei are considered to be isotopes. Isotopes have the same chemical properties, but different physical properties. An example of isotopes is carbon, which has three main isotopes, carbon-12, carbon-13 and carbon-14. All three isotopes have the same atomic number of 6, but have different numbers of neutrons. Carbon-14 has 2 more neutrons than carbon-12 and 1 more than carbon-13, both of which are stable. Carbon-14 is radioactive and undergoes radioactive decay.

Radioactive materials contain some nuclei that are stable and other nuclei that are unstable. These unstable nuclei spontaneously give off particles. This process, called radioactive decay, changes the nucleus of the material into a more stable form. Not all of the atoms of a radioactive isotope (radioisotope) decay at the same time. Rather, the atoms decay at a rate that is characteristic to the isotope and range from seconds to millions of years. This fixed rate of decay is a known as its half-life. Carbon-14 has a half-life of 5730 years, which means that if you take one gram of carbon-14, half of it will decay in 5730 years.

The ratio of the amounts of carbon-12 to carbon-14 in a human is the same as in every other living thing. After death, the carbon-14 decays and is not replaced. The carbon-14 decays, with its half-life of 5,730 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing such as dinosaurs. Radiocarbon dates do not tell archaeologists exactly how old an artifact is, but they can date the sample within a few hundred years of the age.

Activity 1: Radioactive Decay Simulation

Introduction

In this activity you will perform a simplified simulation of radioactive decay to determine the half-life of a given substance.

Procedure

1. Count the number of items in your bag. Record it as the count for 0 time in your data table.

2. Shake the bag for 10 seconds & pour contents out.

3. Count the number of pieces with the mark showing. These items are still radioactive.

4. Return these to the bag.

*All pieces without a mark have decayed & should be discarded.

5. Record this number in your data table.

6. Repeat steps until all of the items have decayed and the bag is empty. Extend the data table as needed.

Results

Data Table 1. Radioactive Decay

 Time (sec) Radioactive particles 0 10 20 30 40 50 60 70 80 90 100

Did each group get the same results? Why or why not?

(Please feel free to delete the graph “paper” below and copy/paste a table into Word)

Title:

To calculate the half-life, look at the total amount of the substance given on the y-axis. In this case it is ________. Divide that number by 2, which gives _________. Draw a vertical line. The half-life is where it bisects the x-axis. Here the half-life is _________________________.

Conclusion

1. Explain how scientists use radioactive decay to date fossils and artifacts?

2. Strontium is chemically similar to calcium. If you lived in a city where there had been a nuclear accident, you and your family might be exposed to strontium-90, which is the principal health hazard in radioactive fallout because it can easily get into the water supply or milk and then be ingested by people. Write about how the strontium-90 might accumulate in your body and how it might affect you.

3. How is the half-life of Strontium-90 28.8 years important? Would you prefer a longer or shorter half-life? Explain

4. Suggest ways that government agencies, such as your state’s department of health, might test for strontium-90.

5. Where in your environment might scientists look for large concentrations of strontium?