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LAB in Isotopes, Atomic Mass, Subatomic Particles, Experimental Design. Last updated October 14, 2019.


In this lab, students use a sample of pennies to mimic how average atomic mass is calculated.

Grade Level

High school


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

  • Understand how atomic masses on the periodic table are calculated based on isotope abundances.

Chemistry Topics

This lesson supports students’ understanding of

  • Isotopes
  • Atomic mass


Teacher Preparation: 30 minutes initially, less than 5 minutes once prepped

Lesson: 25 minutes


  • Samples of pennies
  • Balance


Students should wash hands before leaving the lab.

Teacher Notes

  • I give the kids their objective, but I don't tell them how to do it. They really have to think about the similarities between pennies and isotopes. It tests their understanding of atomic mass calculations as well.
  • Students need to know how many pennies are in the bag, so it may be easiest to prepare each container with the same number of pennies.

For the Student



The mass of an atom depends on the number of protons and neutrons in the atom. Atoms of the same elements can differ in the number of neutrons, which is how isotopes form. Isotopes of an element are the same in all ways except mass—some atoms of the element are heavier than others. The periodic table lists each element’s atomic mass, which is a weighted average based on the masses of the isotopes and their abundance in nature.

Prior to 1982, pennies were made entirely of copper. In 1982 the composition of a penny was changed to make its value less than one cent. Currently, a penny has a zinc core with a copper coating. Because of this change, pennies minted before 1982 have a different average mass than pennies minted after 1982. These average masses are listed in the data table below. In this lab, pennies will serve as a model for isotopes and you will determine the abundance of two kinds, or isotopes, of pennies in a sealed sample of “pennium.”


  1. Record the mass of an empty container in your data table.
  2. With your group, determine what other measurements you need to determine what percent of the pennies in a sealed container are pre-1982 and what percent are post-1982. List these measurements in your data table.
  3. Obtain a sealed container of pennies. Record the bag code in the data table. Collect and record the other measurements you need.

Average mass of a pre-1982 penny 3.114 g
Average mass of a post-1982 penny

2.502 g

Mass of empty container
Bag Code


Determine what percent of the pennies are pre-1982 and what percent are post-1982. Show all calculations.

Pre-1982 pennies = ________ Post-1982 pennies = ________


  1. How many pennies of each type would you expect to find in each container?
    Pre -1982 ________ Post-1982 ________
  2. Identify two experimental errors that could alter your percentages.
  3. What is one way that these pennies are similar to isotopes?
  4. What is one way that they are different?
  5. If “pennium” was an element on the periodic table, what measurement would be listed as its atomic mass?
  6. How does your atomic mass of “pennium” indicate which isotope is most common?