In this activity, students can look inside the model that resembles the atom and find information that reinforces what an isotope actually is. Furthermore, the quantitative data forces students to examine beliefs about different types of averages and what the numbers really mean. This takes a bit of effort to set up but is inexpensive and can be used year after year.
About 30-40 minutes for the activity. This depends on how many balances are available to share and how many different isotopes are made for a single element. We had 5 balances for 12 or 13 teams of two students. Students can work in groups of 3 as well.