Isotopes and Atomic Mass – Interactive PhET Simulation

Explore isotopes and relative atomic mass

Most elements occur naturally as a mixture of isotopes — atoms of the same element with the same number of protons but different numbers of neutrons.

This interactive PhET simulation allows you to investigate how the masses and relative abundances of isotopes determine the relative atomic mass of an element.

Experiment with the isotope abundances and make predictions before checking the result. In particular, notice how changing the proportion of each isotope moves the relative atomic mass towards the isotope that is more abundant.

Isotopes and Atomic Mass is provided by PhET Interactive Simulations, University of Colorado Boulder, and is used under the CC BY-NC 4.0 licence.

Try these challenges

1. Build isotopes of one element
Choose an element and create different isotopes by changing the number of neutrons. What stays the same, and what changes?

2. Identify what makes isotopes different
Compare two isotopes of the same element. Explain why they have the same chemical identity even though their masses are different.

3. Change the isotope abundance
Adjust the abundance of one isotope. Predict what will happen to the average atomic mass before checking the result.

4. Make one isotope dominant
Set one isotope to be much more abundant than the others. Where does the relative atomic mass move? Explain why.

5. Create equal abundances
Choose two isotopes and make their abundances equal. Predict the relative atomic mass. How close is it to the midpoint between the two isotope masses?

6. Test a weighted average
Set two isotopes to very different abundances, for example 90% and 10%. Predict which isotope will have the greater influence on the relative atomic mass.

7. Work backwards
Choose a target relative atomic mass between two isotope masses. Adjust the abundances until you get close to that value. What does this tell you about how relative atomic mass is calculated?

The key question

Keep asking:

How do isotope mass and isotope abundance combine to determine relative atomic mass?

The relative atomic mass is a weighted mean, so the more abundant isotope contributes more strongly to the final value.

 
 
 
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