This simple yet informative activity will show your students how atomic mass is calculated. Only requiring a few minutes of prep and minimal supplies, it will pay you back tenfold with the understanding it generates for this difficult concept.

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### Getting Started With Eggium

You may use this activity to introduce the concept of atomic mass or as a followup to a lesson on atomic mass. Either way, you will need to gather the following supplies:

- 6 to 10 refillable plastic easter eggs in three different colors (these represent Eggium nuclei)
- dried peas (or appropriate substitute)
- small dried pasta pieces (or appropriate substitute pieces)

Before class begins you will make each Eggium nucleus. Each nucleus will contain 5 protons (peas) and a variable amout on nuetrons (pastas), depending on the color of the egg. Here is one possibility:

- Make 2 Blue eggs: 5 peas & 4 pastas
- Make 5 Yellow eggs: 5 peas & 6 pastas
- Make 3 Pink Eggs: 5 peas and 5 pastas

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### In the Classroom

Hand out one egg to a small group of students. Instruct them to determine the mass number for the nucleus, as well as the atomic number. Remember, the mass number is really a count of all of the particles in the atom that have mass, or a sum of the protons + neutrons. The atomic number is equal to the number of protons in the atom.

After they have successfully acquired the indicated information, have them put their information into isotope notation. For our purposes, the chemical symbol for Eggium can be Eg.

Now you are ready to have students share their data. Have a representative from each group write the isotope notation on the board.

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### Class Discussion

Check the isotope notations written on the board. Each element should have a subscript 5 in the lower left hand corner to show the number of protons in the element Eggium. The top left superscript should either be a 9, 10, or 11.

Ask students:

- Are all of the eggs the same element? How do you know? (Yes, they all have the same number of protons.)
- What do you call atoms that have the same number of protons but variable numbers of neutrons? (isotopes)
- Other than having more or fewer numbers of neutrons, name one other thing that is different between isotopes? (they have different atomic masses)
- Do all isotopes naturally occur with the same frequency? (No, there are more of the yellow isotopes than the others.)
- How could we find the average weight of our isotopes? (Accept all ideas.)

Send students back to their groups to calculate the atomic mass of their Eggium atom. Allow them to try whatever idea sounds best to them at this point.

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### Atomic Mass Calculations

Allow students to share the atomic masses they calculated for the element Eggium. You may prefer to have them write their isotope notations on the board. After this, help the students analyze their results by asking questions:

Are all of the atoms of Eggium the same weight? (no)

Why do some atoms weigh more than others? (they have different numbers of neutrons)

What is consistent from atom to atom? (they all have the same number of protons)

What is the term scientists use for atoms with the same number of protons but varying numbers of neutrons? (isotopes)

Do some of the isotopes occur more often than others? (Yes)

Help students calculate the percentage of each isotope. If you have 10 eggs and 2 of them are a certain isotope, then your percentage for that isotope would be 20%. The students have just calculated the natural abundance of the isotopes for your Eggium element.

Introduce students to the term natural abundance. Natural abundance describes how often a certain isotope occurs in nature, usually in percentage form.

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### 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:

9+9+10+10+10+11+11+11+11+11=103

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:

(20%x9)+(30%x10)+(50%x11)=10.3

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!