MnSTEP Teaching Activity Collection > MnSTEP Activities > Investigating Isotopes: Using M&M's as a Model for Calculating Average Atomic Mass

Investigating Isotopes: Using M&M's as a Model for Calculating Average Atomic Mass

Karen Hohenstein, Delano High School, Delano, MN, based on a presentation at a Chemistry Teachers Workshop at the Science Museum of Minnesota (original author unknown).
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In this chemistry laboratory activity, students will be given a random sample of the fictitious element "M&Mium." This sample contains at least three different "isotopes" of M&Mium (examples include plain, peanut, almond, peanut butter, etc). The students will design and carry out a procedure to determine the average "atomic" mass of the element M&Mium.

Learning Goals

This activity is designed for students to gain an understanding of the relationship between the mass and relative abundance of an isotope and its effect on the average atomic mass of the element. The activity will require students to design a procedure and utilize critical thinking skills. Students will also have to work cooperatively with other students, collect data, and analyze data. In order for students to complete this activity, they need to have a clear understanding of the concept of average atomic mass; average atomic mass is not simply the mathematical average of the masses of each isotope, but instead depends upon the amount of isotope present.

Vocabulary words used throughout this activity include isotope, percent abundance, relative abundance, and average atomic mass.

Context for Use

This lab activity is appropriate for mainstream or honors high school chemistry students (grades 11-12). The activity can be completed in one class period. Necessary equipment includes an electronic balance (2 or 3 are ideal with larger classes) and small paper cups. Prior to this activity, students should have an understanding of the atom, its constituent particles, and the masses of each. Students should also understand the term isotope. Students should have previous experience calculating percents. They should also understand the relationship between percent abundance and relative abundance. This activity should be used after the concept of calculating average atomic mass has been introduced. This activity would be used early in a chemistry course—typically in the first couple of units. This activity could be adapted for lower ability students by providing additional procedural steps (rather than having the students develop them on their own).

Subject: Chemistry:General Chemistry:Atomic Structure
Resource Type: Activities:Lab Activity, Classroom Activity
Grade Level: High School (9-12)

Description and Teaching Materials

To set up the activity, place electronic balances around the room. Purchase Dixie cups or other small paper or plastic cups—four cups are required for each pair of students. In a large (gallon sized) plastic bag, combine 2 large bags of plain M&Ms, 2 large bags of peanut M&Ms, and 1 large bag of almond M&Ms. Mix well. This bag of M&Ms should be enough for approximately 70 students working in pairs.

To introduce the activity, review with the students a calculation where average atomic mass is determined from an actual element's isotope data. Discuss with the students how a mass spectrometer is used to determine isotope data (the number of isotopes, the mass of the isotopes, and the percent abundance of each). Because of the cost and lack of availability of a mass spectrometer, explain to students that a model will have to be used to study isotopes. Then show the students the bag of M&Ms and explain that each M&M piece will serve as an "atom" and that our sample "element" contains three common isotopes: plain, peanut, and almond.

As listed in a handout for students, review the goals of the activity: the overall goal is to calculate the average atomic mass of an M&Mium "atom" (like the example calculation students reviewed earlier in the lesson). In order to complete this calculation, students will have to design a procedure to determine the average mass of each isotope and the percent abundance of each isotope. As the students determine the method they will use, they should record the steps or procedure, and create a data table to organize their data. Remind students that they are allowed to eat the sample of M&Mium when they are finished collecting data, so the M&Ms must remain on a paper towel or in a cup, but never on the surface of the lab tables.

After this introduction, students may come up and obtain a small cup of M&Ms and three extra cups and begin designing their procedure.

For teacher reference, a suggested procedure is a follows:
  1. Separate the almond, peanut, and plain isotopes.
  2. Determine the total mass of each isotope (the mass of all the almond isotopes together, the mass of all the peanut isotopes, etc.) and record.
  3. Count the number of "atoms" of each isotope and record.
  4. Divide the total mass of each type of isotope by the number of "atoms" of that isotope to determine the average mass of each isotope. Record.
  5. Determine the percent of each isotope present in the sample by dividing the number of atoms of an isotope by the total number of M&Ms and multiplying by 100.
  6. Express the percent abundance as relative abundance (decimal percent).
  7. Calculate the average atomic mass of M&Mium by using the average mass and the relative abundance.
As students are working, it may be necessary to provide hints to students who have trouble determining the procedure. Once the students have completed the calculation, they should record their average atomic mass on the board for comparison with other groups. In their conclusion, students should address the two following questions: Explain any differences between the atomic mass of your M&Mium sample and that of your neighbor. Explain why the difference would be smaller if larger samples were used.

Teaching Notes and Tips

A common area of confusion is the final calculation of average atomic mass. Two mistakes are often encountered: 1) students will try to take a mathematical average of the three isotope masses rather than taking into account the abundance of each isotope; and 2) students have a tendency to use the total mass of the isotope rather than the average mass of each isotope. This will lead to an average atomic mass that is much too large.

One point that needs to be reinforced and clarified: students have a hard time changing the percent abundance into relative abundance, even though it only requires moving the decimal two places to the left.

In terms of safety, eating in the lab is usually not permitted; therefore if students are going to eat the M&Ms, they need to be careful not to allow the M&Ms to come into contact with the surfaces of the lab benches or directly with the pans on the electric balances. If peanut butter or crispy M&Ms are used in place of plain, peanut, or almond, it may be more difficult for students to distinguish the isotopes by size.

In the past, I have used a similar activity but provided students the actual procedure to be used. The current activity is more inquiry based because it requires the students to design the procedure; this change results in a higher level of thinking and a deeper understanding of the concepts involved.


Throughout the activity, I would walk around the room to assess students understanding of the activity and provide guidance where necessary. Students may be provided with a handout to complete the activity and record their data, or they may complete the activity in a laboratory notebook. Students will be expected to record their materials list, procedure, and organize their data into a table. They should also clearly show how they calculated average atomic mass. As students post their results on the board, it will be apparent which groups may need assistance with their calculations. A conclusion paragraph should be written that addresses the questions mentioned earlier.


9-12 II.A.3—Properties of an element and its isotopes

References and Resources