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ACTIVITY in Isotopes, Atomic Mass, Subatomic Particles, Kitchen Chemistry, Kitchen Chemistry  High School. Last updated January 29, 2024.
Summary
In this activity, students will learn about isotopes and be introduced to basic average atomic mass calculations. They will use simple numbers and M&M candies to model ratios that approximate real world atomic mass values on the periodic table.
Grade Level
Middle School, High School
NGSS Alignment
This activity will help prepare your students to meet the performance expectations in the following standards:
 5PS11: Develop a model to describe that matter is made of particles too small to be seen.
 MSPS11: Develop models to describe atomic composition of simple molecules and extended structures.
 Scientific and Engineering Practices:
 Using Mathematics and Computational Thinking
 Developing and Using Models
 Analyzing and Interpreting Data
Objectives
By the end of this activity, students should be able to:
 Define isotope
 Discuss the mass number in relation to an atom’s protons and neutrons
 Calculate weighted averages
 Determine atomic mass
Chemistry Topics
This activity supports students’ understanding of:
 Atomic Structure
 Isotopes
 Atomic mass
 Subatomic Particles
Time
Teacher Preparation: 15 minutes
Lesson: 3040 minutes
Materials (per group)
 M&M’s of different sizes, flavor & color
 4 red pretzel M&M’s and 1 red chocolate M&M (representing Boron)
 1 green pretzel M&M and 3 green chocolate M&M’s (representing Chlorine)
Safety
 Do not consume lab materials, even if they’re otherwise edible products.
 Food in the lab should be considered a chemical not for consumption.
 Lab tables should be washed before and after activity.
 Students should wash hands before and after activity.
 Be mindful of food allergies and take proper precautions.
Teacher Notes
 Prior to the activity, students should be familiar with basic atomic structure (protons, neutrons, electrons) and have been introduced to the concept of isotopes.
 Isotopes are atoms of the same element with different numbers of neutrons. Having additional neutrons makes an element slightly heavier.
 Elements can have a variety of isotopes, and a sample of an element will contain a mixture of those isotopes. Since each isotope has a different weight, it becomes useful to look at the average weight of an atom – this is called atomic mass. It is a weighted average and usually contains several decimal places. An individual atom won’t have a mass equal to the atomic mass, but individual atoms are rarely dealt with. The actual number of protons and neutrons in an individual atom/isotope is called mass number.
 Students may be paired or work alone. Green & Red M&Ms can be purchased around the holidays more easily!
 Other colors of M&Ms can be used, but the Student Handout will need to be adjusted to reflect the colors used.
 Candy can either be purchased by teacher or brought in by students.
 Students will be using different sized M&M’s to represent different isotopes of an element. The color represents the element (red is boron vs. green is chlorine for example), the size (bigger or smaller) represents the qualitative weight of the isotope (small green is an atom of chlorine 35 vs. large green is an atom of chlorine 37 for example).
 The M&M’s are used as visual aids to represent real world ratios in terms of smaller or larger atoms. Students will use real mass numbers to calculate the atomic mass of two elements.
 This could be done at a higher level by incorporating elements with more than two isotopes. This could be simplified by working through the calculation of average atomic mass with the students.
 The AACT simulation Isotopes & Calculating Atomic Mass could be used as an extension activity for students after completing this introductory handson activity.
 If you wish to allow students to eat the M&M’s after completing the activity, do not complete this activity in the lab or have a separate supply of candy separate from that used in the activity that can be eaten outside the lab.
For the Student
Lesson
Background
 Isotopes are atoms of the same element with different numbers of neutrons. The number of neutrons will affect the mass of the atom.
 Elements can have a variety of isotopes, and a sample of an element will contain a mixture of those isotopes. Since each isotope has a different weight, it becomes useful to look at the average weight of an atom; this is called average atomic mass. It is a weighted average and usually contains several decimal places. An individual atom won’t have a mass equal to the average atomic mass, but individual atoms are rarely dealt with.
 The actual number of protons and neutrons in an individual atom is called a mass number.
Prelab Questions
 What is an isotope?
 What is the mass number of an element?
 What is the average atomic mass of an element?
Problem
How do isotopes affect the average atomic mass of an element?
Materials
 4 red pretzel M&M’s and 1 red chocolate M&M (representing Boron)
 1 green pretzel M&M and 3 green chocolate M&M’s (representing Chlorine)
Safety
 Lab tables should be washed before and after activity.
 Students should wash hands before and after activity.
 Do not consume lab materials, even if they’re otherwise edible products.
 Food in the lab should be considered a chemical not for consumption.
Procedure
 Make a pile of 4 red pretzel M&M’s and 1 red chocolate M&M. **The different sized candies represent different isotopes of the same element (same color candy).
 Boron has two isotopes, represented by the red M&M’s. Boron11 has 6 neutrons and Boron10 has 5 neutrons. Boron11 is the red pretzel M&M because Boron11 is heavier than Boron10, and the pretzel M&M is bigger.
 Assume that each red pretzel M&M has a mass of 11 units (5 protons and 6 neutrons) and the red chocolate M&M has a mass of 10 units (5 protons and 5 neutrons).
 The average atomic mass (sometimes called atomic weight) is a weighted average. This means it takes into account how many of each isotope exists in nature. For Boron, 4 out of 5 atoms of Boron are Boron11, and 1 out of 5 atoms is Boron10.
 To calculate the average atomic mass of Boron, multiply the number of red pretzel M&M’s (4) by its mass (11) and number of red chocolate M&M’s (1) by its mass (10). This calculation should be recorded in the data table.
 Divide the total mass of all the M&M’s of a certain color by the total number of M&M’s in that sample.
 Repeat this process with the green M&M’s representing the two isotopes of Chlorine. Assume that the green pretzel M&M has a mass of 37 units (17 protons and 20 neutrons) and each green chocolate M&M has a mass of 35 units (17 protons and 18 neutrons). Make note that the number of pretzel and chocolate M&M’s are different in this sample than the previous one.
 To calculate the average atomic mass of Chlorine, perform the same type of calculation as described in steps 5 & 6 but with the data from the green M&M’s.
Data & Calculations
Pretzel  Chocolate  Average  
# of M&M’s × mass  #of M&M’s × mass 
Total
mass


RED (Boron)  
GREEN (Chlorine) 
Analysis
 Compare your calculations of average atomic mass (atomic weight) to the values for Boron and Chlorine on a periodic table. Were they similar? Explain why or why not.
 If you round off the average atomic mass of Boron to the nearest whole number, does it give you the mass number of the most common isotope? How about for Chlorine? Explain why or why not.
 Most average atomic masses are NOT whole numbers. Why do you think this is so?
 An element needed in your diet is Magnesium. The three most common isotopes of Mg have a mass of 24, 25, and 26. Using your periodic table, predict which of these is the most common isotope, and explain your choice.
 What is the difference between mass number and average atomic mass?
 If a sample of the element Chemistrium (Ch) contain 100 atoms of Ch12 and ten atoms of Ch13 (for a total of 110 atoms in the sample), what is the average mass of Chemistrium? Calculate here:
Extension
Research the practical application of any 2 isotopes and report on them below. Include the mass number, how the isotope is used, and what is special about that particular isotope.