Investigating Isotopes: Using M&M's as a Model for Calculating Average Atomic Mass
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.
Vocabulary words used throughout this activity include isotope, percent abundance, relative abundance, and average atomic mass.
Context for Use
Resource Type: Activities:Lab Activity, Classroom Activity
Grade Level: High School (9-12)
Description and Teaching Materials
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:
- Separate the almond, peanut, and plain isotopes.
- 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.
- Count the number of "atoms" of each isotope and record.
- 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.
- 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.
- Express the percent abundance as relative abundance (decimal percent).
- Calculate the average atomic mass of M&Mium by using the average mass and the relative abundance.
Teaching Notes and Tips
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.