Summary

In this activity, students are given clues about various sister isotopes in order to complete a crossword puzzle. Students will become familiar with isotope names, symbols, and mass numbers as they consider descriptions of the isotopes and information about their uses to solve the puzzle.

Grade Level

High School

Objectives

By the end of this activity, students should be able to

• Explain the meaning of sister isotopes.
• Demonstrate an understanding of radioactive isotopes and related concepts, including: radioactive decay, half-lives, and alpha decay.

Chemistry Topics

This activity supports students’ understanding of

• Nuclear Chemistry
• Radioactive Isotopes
• Half-Lives

Time

Teacher Preparation: minimal
Lesson: 20 minutes

Materials

• Student handout

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This puzzle was created to support the celebration of the 2025 Earth Day theme, Our Power, Our Planet^TM. It highlights the various isotopes that are used in studying our impact on Earth so we can protect our Earth. Earth Day is celebrated on April 22.
• This activity was selected for the Chem Fun section of the March 2025 issue of Chemistry Solutions.
• Many of the clues in this puzzle are written such that students could recognize the likely isotopes if they pay attention to the whole clue. Some examples:
  • References to a molten salt reactor should make students think of sodium and chlorine.
  • The alpha decay of a samarium isotope should lead them to subtract 2 from the 62 protons in samarium to get an isotope of neodymium.
  • The mention of fertilizers should make them think of the most common elements in fertilizers, like nitrogen and phosphorus.
• Students can use the internet to search for isotopes whose names would fit into the number of spaces allotted for each clue.
• If you want to make the puzzle a little more difficult, you can remove the list of the mass numbers from the student activity.
• If the student activity is used as is, note that one mass number is repeated as it is for two different isotopes.
• An Answer Key Document has been provided for teacher reference.