« 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?

Simulation Activity: Isotopes & Calculating Average Atomic Mass Mark as Favorite (28 Favorites)

ACTIVITY in Isotopes, Atomic Mass, Subatomic Particles. Last updated October 07, 2019.


In this simulation, students first learn how the average atomic mass is determined through a tutorial based on the isotope abundance for Carbon. Students will then interact within a workspace where they will select the number of isotopes, the mass of each isotope as well as their abundancies in order to successfully build a mystery element. Finally they will use their choices to calculate the average atomic mass of the mystery element.

Grade Level

Middle and High school

NGSS Alignment

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

  • MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
  • Science & Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Developing and Using Models


By the end of this simulation, students should be able to

  • Explain how the atomic mass shown on the periodic table for an element was determined.
  • Define an isotope.
  • Accurately calculate the average atomic mass of an element given the atomic mass of each isotope and its abundancy.
  • Explain the meaning of a weighted average.
  • Identify which subatomic particle(s) affect the atomic mass of an atom.

Chemistry Topics

This simulation supports students’ understanding of

  • Atomic Structure
  • Isotopes
  • Average Atomic Mass
  • Subatomic particles


Teacher Preparation: minimal

Lesson: 30-45 minutes



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

Teacher Notes

  • The tutorial section of this simulation could be used as part of a teacher-lead lecture about isotopes and calculating the average atomic mass or it could be used as an introduction, before students being the student activity.
  • The tutorial is presented so that students can have a better understanding of how isotopes and their abundancies impact the average atomic mass, as presented on the periodic table. The tutorial should help students make connections between the subatomic particles of an atom, and the quantity of each particle. In turn, this should help them better understand how the atomic mass of an atomic is determined. During the activity, the tutorial can be used by students as a guide/point of reference as they complete average atomic mass calculations on their own.
  • It is recommended that students should be exposed to the concept of isotopes, subatomic particles and atomic mass prior to using this simulation.
  • In this simulation the “Mystery Elements” were designed to have a heavy mass (500-506), so that they are not confused or mistaken for any of the existing elements on the periodic table.
  • Please note that the names and symbols of the possible mystery elements are all fictional.
  • For the purpose of this simulation, students are given the ability to choose the number of isotopes to use in each calculation and the mass values for each isotope. They will also need to choose the percent abundance for each of the isotopes, keeping in mind that the total value must equal 100%. These choices allow students to have the opportunity to complete many unique calculations, and not be limited in their investigation.
  • The final “Calculate” step should serve as an answer key for the student, based upon the combination of isotopes, mass, and abundancies that they chose. Students are expected to complete the calculation on their own, and then use the calculate button to check their work. Students will also be presented with a pie chart and an element symbol displaying the make-up of their mystery element, which is intended to serve as a visual aid as they comprehend the information.
  • The simulation can be found at either of the following links:

For the Student


In this investigation you will learn about isotopes and average atomic mass calculations. Through your investigation you will determine the average atomic mass for an element based upon the number of isotopes it has, as well as the mass and relative abundance of each isotope.

You can find the simulation here: teachchemistry.org/isotopes


Click on the “Tutorial” tab at the top of the screen. You should see two carbon atoms. Using the information on the screen, answer the following questions:

  1. What is the difference(s) between the two carbon atoms shown on the screen?
  2. What is the mass of each carbon atom on the screen?
  3. In your own words, explain the meaning of isotope. Use the carbon atoms shown on the screen as an example in your explanation.
  4. What is the meaning of abundance? What effect do the abundancies have on the element carbon, if any?
  5. Based on the abundancy values for each isotope, would you expect the average atomic mass of carbon to be closer to 12 or 13? Briefly explain your reasoning.
  6. Analyze the average atomic mass calculation for carbon. Where do the following values come from?
  7. Why is the mass shown on the periodic table for an element rarely a whole number?


Click on the “Practice” tab at the top of the screen. You should see the screen shown below:


  • You will need to build at least 4 different Mystery elements using the simulation.
  • Each mystery element will have a different number of isotopes, as indicated in the table below.
  • For the name, masses and abundancies of the isotopes, you will choose those from the options on the screen and record your selections in the table below.
  • Based on your selections, you will calculate the average atomic mass for your mystery element, and then use the “Calculate” button on the screen to check your answer.
  • An example is completed in the table for your reference.

Name of Element

Number of isotopes

Mass and abundancies

Average Atomic Mass Calculations for Element

Example Chemistrium)


Ch – 501, 25%
Ch – 504, 40%
Ch – 505 , 35%
501 x .25 = 125.25
504 x .40 = 201.60
505 x .35 = 176.75503.60
Add for final mass=503.60






  1. What information is displayed in the pie chart within the calculations section? Why is this helpful?
  2. How is calculating a weighted average different than calculating a non-weighted average (like a mean calculation)?
  3. Could any of the mystery elements that you created using the simulation represent an actual element from the periodic table? Explain why or why not.

Additional Practice

  • If you are in need of additional practice, use the table below to organize your selections and calculations.

Name of Element

Number of isotopes

Mass and abundancies

Average Atomic Mass Calculations for Element