The Next Step
The next step requires students to calculate the average weight of one atom of Eggium. This can be done two different ways, and I encourage you to do it each way to show students that they yield the same answer.
Ask the students how they could calculate the average weight of one atom of Eggium. Let them brainstorm ideas without correcting them. Eventually someone will suggest that they add up the masses of each egg and divide by ten. As you demonstrate this be sure to group the elements of similar mass together. For example, your calculation may look like:
103/10 = 10.3
The atomic mass of eggium is 10.3.
Next, show students that they may take the percent natural abundance and multiply it by the weight of each isotope. If these are added together they will give the same result for atomic mass. The calculation would look like this:
Most students will wonder why they should go about this long process to get the average when it was so much easier the first way. Lead a discussion about this. Be sure that students understand that:
- It would be impossible to count every atom of an isotope, so we take representative samples.
- Atoms are present in such large quantities that it is much easier to use percentages than actual quantities of atoms.
- They are calculating the atomic mass for an element, and this is the number that is reported on the periodic table of elements.
The next logical step in this lesson would be to have students calculate the actual atomic mass of an element (using natural abundance data) and compare it to the atomic mass listed on the periodic table. If their value deviates from the periodic table value, it would be a great time to calculate percent error as well.
Now your students will know where the atomic mass values on the periodic table came from!