Beacon Lesson Plan Library

The Average Atom - Isotopes

Jeri Martin


When looking at a periodic table, students notice that atomic masses are decimal numbers. These masses are an average of all the isotopes of that element. Students investigate a model of isotopes by completing the Isotopes Model activity.


The student solves multi-step real-world problems involving whole numbers, fractions or decimals using appropriate methods of computation, such as mental computation, paper and pencil, and calculator.

The student measures accurately with the measurement tools to the specified degree of accuracy for the task and in keeping with the precision of the measurement tool.

The student determines the mass number and atomic number of an atom from the number of protons and neutrons.

The student uses appropriate procedures for safety in the classroom, home, and community.


Per group of 2 students:
-Lima beans (20 small, 20 medium, 20 large)
-1 Sandwich-size reclosable plastic bag
-Balance (triple beam or electronic)
-Periodic table

For each student:
-Protons, Neutrons, Electrons worksheet (See Associated File)
-Atoms KWL Chart (See Associated File)
-Isotopes Rubric (See Associated File)
-Isotopes Model Lab (See Associated File)


1. Make student copies of the Protons, Neutrons, Electrons worksheet, Atoms KWL Chart, Isotopes Rubric and Isotopes Model Lab. (See Associated File)
2. Prepare equipment for lab activity. (Place the 60 lima beans in the sandwich-size reclosable bag.)
3. Gather needed materials.


1. Give each student a copy of the Isotopes Rubric (See Associated File) and give a short explanation of the assessment. Explain that even though this lesson is on atoms and isotopes, safety rules must be followed and students must work together to achieve the information.

2. Give students a copy of the Atoms KWL Chart. (See Associated File)

3. Discuss with students the K (what do you KNOW) and W (what do you WANT to know) from the chart. Note: The L (what did you LEARN) will be completed at the end of this activity.

4. Give students a copy of the Protons, Neutrons, and Electrons worksheet. (See Associated File) Review the background information on the sheet with them. Have students complete the sheet and turn in.

5. Have students follow the procedures on the Isotopes Model Lab. (See Associated File) As students work on the lab, circulate and offer feedback and assistance. When the lab is complete, have students turn it in.

6. Have students complete the L section of their Atoms KWL Chart and turn in.


1. The Isotopes Rubric (See Associated File) is marked as students go through this activity.
2. The Atoms KWL Chart, Protons, Neutrons, Electrons worksheet and Isotopes Model Lab are collected. The entire Isotopes Model Lab, the Protons, Neutrons, Electrons worksheet and the L section of the Atoms KWL Chart are assessed using the Teacher Copies provided in the associated file.


1. Repeat the Isotope Model Lab, but have the students select other items that could be used to represent a model of an isotope. (Example: If you wanted to show a model of an isotope of a metal you could use different size nails.)
2. The procedures could be modified to allow it to be used as a cooperative group activity for ESE/ESOL students or if there are not enough supplies for groups of two students to have a set.

Web Links

Gives an overview of what isotopes are.
Chemistry for Kids

Reviews atoms, isotopes & ions.
CPO Catalog

This site reviews isotopes and shows models of isotopes of different elements. Since it contains colorful graphics, it may take a few minutes to load.

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