Isotopes and atomic mass

The title of the Project: Isotopes and atomic mass

This virtual laboratory is intended for use in chemistry classes on the following topics:

  • 7th grade. Chapter III. “Atoms. Molecules. Substances.”
  • 7th grade. Chapter VII. “The relative atomic mass. The simplest formulas”


  • Understand the concept of isotopes and how they differ from each other.
  • Learn how to identify the key components of an atom (protons, neutrons, and electrons).
  • Explore the relationship between isotope stability and abundance in nature.
  • Discover how to calculate the average atomic mass of an element based on its isotopic composition.

Practical part

  1. This simulation offers an interactive way to learn about isotopes and average atomic mass. You will be working in two environments: “Isotopes” and “Mixture.” Begin by exploring the “Isotopes” screen.

Virtual experiment No.1: Isotopes

In this screen you investigate what isotopes are by building various isotopes.

  1. When the screen opens a model of a Hydrogen atom is shown and mass number is displayed on the balance. Familiarize yourself with the tools on the screen, including the atom model, neutron bucket, and element display.

  1. Drag neutrons to create different Hydrogen isotopes. Observe how the isotope name and the number of protons, neutrons, and electrons change.Analyze the differences between isotopes of Hydrogen, such as Hydrogen-1, Hydrogen-2, and Hydrogen-3. To help make these comparisons, track the number of protons, neutrons and electrons in the atom using the display on the left side at the top, which updates as neutrons are added or removed from the atom.

  1. Note that the only way you can change the number of protons and electrons is to select the different element from the periodic table. You can choose to explore any element from the first two periods.

  1. Notice how isotope stability is indicated by a stable or shaking nucleus. The abundance in nature panel is helpful for achieving this challenge. 

  1. Select the Carbon on the periodic table.It will display the most abundant isotope. Change the number of neutrons to discover the relationship between isotope stability and isotope abundance. Now switch to the “Mixture” screen to explore calculating average atomic mass.

Virtual experiment No.2: Mixture

Here, you can create mixtures of isotopes for elements from the first three periods.

reset button
erase button
in this mode the number of atoms are given in the buckets
in this mode the number of isotopes can be regulated with a slider
  1. Once you select an element to explore, you will be given samples of the element’s stable isotopes in the buckets to mix in the play space. You can drag them by clicking the left button of the mouse. 

  1. You may notice that some elements have only one stable isotope, while others have multiple.

  1. Challenge yourself to create a mixture that matches the average atomic mass displayed on the periodic table for an element. 
  1. For example, let’s try Boron, which has an average atomic mass of 10,811 amu. Place atoms from the bucket onto the play space, and the percent composition and average atomic mass update dynamically.

  1. Use the sliders to quickly add up to 100 atoms of each isotope and adjust your mixture.

  1. Compare your created mix with the “Nature’s mix” to see how it reflects the real-world isotopic composition.


Through this simulation, students have gained valuable insights into isotopes, their properties, and their impact on an element’s average atomic mass. They can now confidently identify isotopes, explain their differences, and calculate average atomic mass based on isotopic composition.