Summary

In this activity, students use periodic trends and data to make predictions about what makes an isotope radioactive. They will then verify or refine their predictions using a PhET simulation.

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:

• Describe what characteristics of an isotope tend to make it radioactive.
• Explain the stabilizing function of neutrons in the nucleus.

Chemistry Topics

This lesson supports students’ understanding of:

• Atomic structure
• Subatomic Particles
• Nuclear Chemistry
• Radiation
• Periodic Trends

Time

Teacher Preparation: minimal
Lesson: 45-55 min

Materials

• Student handout
• Internet access
• PhET Simulation: Build an Atom

Safety

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

Teacher Notes

• This activity is intended to be an introduction to radiation. It could be taught after discussing isotopes and average atomic mass but before talking about specific types of radiation.
• When I do this activity, my students do not fully know what radioactivity is yet. They typically have an idea that it is dangerous, but they don’t really know what it is. We do this activity first, and then in following lessons students look at alpha and beta decay to learn what it is. I refer to this initial activity a lot to emphasize the idea that the ratio of protons to neutrons can cause an atom to have an unstable (radioactive) nucleus.
• This is setup as an inquiry activity. Initially students may not see the connection, and be wrong, but I like to remind them that it is part of the scientific process. When they look at the next graph, they usually get the hint that we should be focusing on the number of protons and neutrons. It’s great if students see the relationship to mass on their own. With this in mind, there are portions of the activity that I don’t recommend that teachers evaluate/grade for accuracy.
• Since the images are color-coded, it is useful to allow students to have access to a digital copy of this activity even if you provide a paper copy for them to record their answers on.
• Students should notice that larger (mass) atoms are more likely to be radioactive than smaller ones.
• Students should also notice that there is a line on the Number of Neutrons vs Number of Protons graph where the stable isotopes are. For any number of protons, having too many or too few neutrons causes an atom to be radioactive.
• As the atomic number increases, the number of neutrons required for the nucleus to be stable increases. We can see that because the curve representing the stable isotopes veers away from the straight line representing a 1:1 proton:neutron ratio.
• The PhET simulation will help them clarify and verify this conclusion from the graph. Students can create stable atoms and then add or remove neutrons and record the results.
• One example is included, but students are encouraged to use the tools to find the patterns themselves. This should allow for students to make connection themselves.
• Images on the student handout can be found in the public domain (listed in order that they appear on the student worksheet):
  • Periodic Table: Radioactive Elements: EPA website.
  • Periodic Table displaying range of half-lives: Wikimedia Commons, CC, © Lexi Sioz
  • Graph of Decay: Wikimedia Commons, CC