« Return to AACT homepage

AACT Member-Only Content

You have to be an AACT member to access this content, but good news: anyone can join!

Need Help?

What are Isotopes? Video Questions Mark as Favorite (51 Favorites)

ACTIVITY in Introduction, Elements, History, Review, Atoms, Isotopes, Alpha/Beta/Gamma Decay, Subatomic Particles, Radioactive Isotopes, Electrons. Last updated January 29, 2024.


In this lesson, students will watch a video and answer questions about isotopes. They will learn about the discovery of isotopes, the difference between chemical and nuclear reactions, different kinds of radioactive decay, and some uses of radioactive isotopes.

Grade Level

Middle School, 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.


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

  • Define isotopes and explain the differences and similarities in subatomic particles for isotopes of the same element.
  • Explain the difference between a chemical reaction and a nuclear reaction.
  • Describe the changes that occur in an atom in three types of nuclear decay.
  • List uses for radioactive isotopes.

Chemistry Topics

This lesson supports students’ understanding of:

  • History of Chemistry
  • Atomic number
  • Mass number
  • Subatomic particles
  • Isotopes
  • Radioactive isotopes
  • Alpha/beta/gamma decay


Teacher Preparation: minimal

Lesson: 10 minutes



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

Teacher Notes

  • The “What are Isotopes?” video is part of the ACS video series Chemistry Basics. The entire series can be found here.
  • The running time of this video is about four minutes. It goes through information pretty quickly, so you may want to pause the video at certain points or play the video through twice.
  • This video is intended for students to watch, and for teachers to integrate into their curriculum.
  • The student questions/answers are presented in sequential order in the video.
  • An answer key has also been provided for teacher reference.

For the Student



While watching the ACS Chemistry Basics video about isotopes, answer the following questions:

  1. What are isotopes?
  2. What element did Thomson and Aston study when they discovered that the same element had different “types”?
  3. What machine did Aston invent to study isotopes, and what did he use it for?
  4. Why is the mass of electrons usually ignored when determining the mass of an atom?
  5. What part of the atom is involved in chemical reactions?
  6. What part of the atom is involved in nuclear reactions?
  7. What holds the positive protons together in the nucleus?
  8. What are three types of radioactive decay?
  9. What are three uses for radioactive substances?