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Why are Some Isotopes Radioactive? Mark as Favorite (56 Favorites)

ACTIVITY in Periodic Table, Half Lives, Subatomic Particles, Radioactive Isotopes. Last updated July 30, 2020.


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


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


Teacher Preparation: minimal
Lesson: 45-55 min



  • 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):

For The Student


Questions to consider

  • Where does radiation come from?
  • What causes some isotopes to be radioactive?
  • What characteristics would help us predict whether a radioisotope was radioactive?

Part I. What makes some particles radioactive?

Radioactive elements have an unstable nucleus. The chart below shows you which elements are commonly radioactive. The shaded elements are radioactive.

Image Source: EPA.gov

Here is another view of the same information. The blue elements are commonly not radioactive while the other shades are. (The different colors represent different ranges of half-lives.)

Image Source: Wikimedia Commons
  1. What pattern do you notice? What’s similar about most of the radioactive elements?

Look at the data presented in the graph below. Here, the number of protons and the number of neutrons for every known atom has been plotted. The black points represent stable (nonradioactive) atoms. The colored points are radioactive. The straight line is the line created by a 1:1 ratio of protons to neutrons. Notice where the stable atoms are.

Image Source: Wikimedia Commons
  1. Think about the atom the arrow is pointing to on the graph above. What would have to happen to the number of neutrons for this stable atom to become radioactive?

Part II. Test your hypotheses

Use the PhET simulation Build an Atom to discover or prove what properties of an atom make it radioactive. (Note: stable means that it is NOT radioactive. Unstable indicates that it would be radioactive.)


  1. Select “Atom” on the main screen, rather than “symbol” or “game”.
  2. Make sure the stable/unstable box is checked in the bottom right-hand corner.
  3. Create a hydrogen atom with 1 proton, 1 neutron, and 1 electron. In the table below, record whether the resulting atom is stable or unstable. What do you need to do to change the stability?
  4. Hit the rest arrow and build a new atom (or ion) of your choosing. Record what you build and its stability. Continue creating atoms (or ions) that are stable and unstable to try to determine what the root cause of stability is.
  5. Record your experimentation below:
Description of the particle created Nuclear Stability


  1. What conclusion(s) can you draw?
  2. Before we can be sure of a conclusion, we must make sure it works in all circumstances.
    1. Create a STABLE particle with at least 5 protons.
    2. According to your conclusion above, what should you do it to make it unstable?
    3. Try it. Do you need to adjust your conclusion? If so, what new conclusion might you draw?
    4. Create a stable particle with 7 protons.
  • What element is it?
  • Make it unstable and record how you did it.
  • Make a second unstable particle of the same element and record how you did it.

Part III. Final Conclusions

What characteristic of some isotopes causes them to be radioactive? What would have to happen in the nucleus of these isotopes for them to become stable? Use support your claim with evidence from the data above and your simulation. Be sure to clearly explain how your evidence supports your claim.