Summary

In this activity, students analyze a series of graphs and data points to discover a pattern, and realize the meaning of a half-life. During this investigation, students will make connections between the concepts of nuclear decay, radiation and the Law of Conservation of Mass.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data

Objectives

By the end of this activity, students should be able to:

• Define half-life.
• Determine the half-life of a radioisotope from a mass versus time graph.
• Model the process of radioactive decay to illustrate half-life and conservation of mass.

Chemistry Topics

This activity supports students’ understanding of:

• Nuclear Chemistry
• Half-lives
• Radioactive Isotopes
• Radiation
• Conservation of Mass

Time

Teacher Preparation: minimal
Lesson: 35-40 minutes

Materials

• Student handout

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This activity is intended to introduce students to the concept of a half-life by looking at mass vs time data. Students should already be familiar with nuclear decay (for example: alpha decay and beta decay).
• For all data, both the graph and the data table are given. Some students find the pattern in the data more easily when they do not have to estimate the y-value as they answer the questions. However, the visual representation of the graph helps them understand the overall process.
• In the first set of data students are asked to look at data for sodium-24. The questions ask the student to determine the percent mass remaining after each half-life (15 years). However, the word half-life is not used. The students are asked to look at the data at 15 year, 30 years, 45 years, and so on and to find the pattern in the change of mass. (The amount remaining is always half of whatever the previous answer was).
• Students then look at similar data for iodine-131. They are asked to find how long it took to for ½, ¼, etc. of the mass to decay. They should notice that the answer is always a multiple of 8 days.
• They are then told that the half-life of sodium-24 is 15 years and iodine-131 is 8 days and asked to define a half-life. They may have trouble with finding the right wording for the definition, but they should come to the conclusion that half of the substance decays in a half-life.
• They are given a third data set and asked to determine the half-life.
• One misconception students often have is that the atom is simply gone once it decays. Even though they are familiar with radiation and have written nuclear equations, they do not always apply that here without prompting. To help clarify this misconception, students are given another graph. This time it includes the isotope decaying (silicon-32) and the isotope into which it decays (phosphorous-32). Students will notice that as the mass of silicon decreases the mass of phosphorous increases. You may want to point out that the slopes of these line change similarly and that at any point the percent by mass of sodium plus the percent by mass of phosphorous is equal to 100.
• Finally the students are asked to complete a particulate model of this for the decay of nickel-63 to copper-63. Students should represent both the atoms of nickel and the atoms of copper in the sample at each point in time.
• An Answer Key document has been provided for teacher reference.