Investigating Isotopes and Average Atomic Mass Using Pennies
Summary
In this activity students apply what they have learned about weighted averages, isotopes, and systems of equations to a new situation...coins in a sealed container. They learn some historical information regarding the composition of pennies and then are faced with a challenge: to determine the number of pre-1982 and post-1982 pennies that are contained in a sealed container without opening the container.
CollapseLearning Goals
Goals
1. The students will be asked to develop their questioning and observation skills...namely, what information do I know?, what do I need to know?, how can I obtain this information?
2. The students will be asked to develop their critical thinking skills. There is more than one way to solve this challenge. They must determine their own methods.
3. The students will review weighted averages, isotopes, and systems of equations and use these concepts to help them solve a new problem.
Key Concepts
1. Atomic mass is the weighted average of all of the existing isotopes of an element.
2. Weighted average mass is an average in which each quantity (in this case mass) to be averaged is assigned a weight (in this case relative abundance).
3. An isotope is an atom that has the same number of protons as another atom, but that has a different number of neutrons giving it a different mass.
Vocabulary
weighted average
mass
isotopes
1. The students will be asked to develop their questioning and observation skills...namely, what information do I know?, what do I need to know?, how can I obtain this information?
2. The students will be asked to develop their critical thinking skills. There is more than one way to solve this challenge. They must determine their own methods.
3. The students will review weighted averages, isotopes, and systems of equations and use these concepts to help them solve a new problem.
Key Concepts
1. Atomic mass is the weighted average of all of the existing isotopes of an element.
2. Weighted average mass is an average in which each quantity (in this case mass) to be averaged is assigned a weight (in this case relative abundance).
3. An isotope is an atom that has the same number of protons as another atom, but that has a different number of neutrons giving it a different mass.
Vocabulary
weighted average
mass
isotopes
Context for Use
This is a great 30-40 minute activity for an average size 20-30 student high school chemistry class. It should be used after students are familiar with the parts of the atom, average atomic mass, isotopes, and percent abundance equations. The equipment needed would include: balances, film canisters or other sealed container (1 per group of 2 to 4 students), pennies to put in the canisters, and extra pennies left outside the canisters for measurements. The activity can be easily adapted for use in other settings and ability levels by varying the amount of information provided/guided.
Description and Teaching Materials
This activity is presented as a challenge to the students after they have studied the parts of the atom, average atomic mass, isotopes, and percent abundance. The students are told the day before that they will be given a numbered sealed canister. They will be given the mass of the empty canister and the fact that the canister contains 15 pennies. Their goal will be to use mass to determine the number of pre-1982 pennies and the number of post-1982 pennies contained in the canister WITHOUT OPENING THE CANISTER. They will have a balance and extra sample pennies available for their use. After they determine the number of each type of penny contained in their canister, they will be asked to compare the pennies to isotopes.
Student Handout for Isotopic Pennies (Microsoft Word 28kB Aug25 09)
Teaching Notes and Tips
1. The amount of hints or guided questioning you use for this activity should be determined by the level of students you are working with, but provide as little as possible.
2. In the past I have used a set number of pre and post pennies for odd numbered canisters and a different number of pre and post for even numbered canisters. This makes it so everyone in the class has to do their own work, but you still have a quick way to check their results.
3. Make sure every canister is numbered and that the students receive the mas of their empty canister so that they do not have any excuse to open the canister.
4. Some students will solve this challenge algebraically using total mass, some will solve it algebraically using average mass, and some will use a guess and check method. Resist the urge to make them all solve the problem the same way...a lot will come out in discussion.
2. In the past I have used a set number of pre and post pennies for odd numbered canisters and a different number of pre and post for even numbered canisters. This makes it so everyone in the class has to do their own work, but you still have a quick way to check their results.
3. Make sure every canister is numbered and that the students receive the mas of their empty canister so that they do not have any excuse to open the canister.
4. Some students will solve this challenge algebraically using total mass, some will solve it algebraically using average mass, and some will use a guess and check method. Resist the urge to make them all solve the problem the same way...a lot will come out in discussion.
Assessment
The instructor can judge whether or not the learning goals have been achieved in three ways: the "walk-around" method while the activity is taking place, the student work that is collected (see student handout for a sample of what is required and expected) and viewed (did they determine the correct combo of pennies? did they show and explain their method? did they understand and describe how the activity relates to isotopes?), and post-activity discussion.
Standards
9.2.1.1.1 Parts of an atom
9.2.1.1.4 isotopes
9.2.1.1.4 isotopes