Summary

In this activity, students will use clues to identify atoms in a given molecular model, then draw its corresponding Lewis structure. Students will analyze both models to determine model limitations and ultimately discover the VSEPR theory.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• Scientific and Engineering Practices:
  • Developing and Using Models

Objectives

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

• Compare benefits and limitations of Lewis structures and ball‑and‑stick models.
• Identify central atoms, lone pairs of electrons, and estimated bond angles.
• Use VSEPR theory to compare bond angles around different central atoms.

Chemistry Topics

This activity supports students’ understanding of:

• Lewis structures
• Atomic radius
• Covalent bonding
• VSEPR theory

Time

Teacher Preparation: 30 minutes

Lesson: 45-60 minutes

Materials

• 1 Modeling kit for every 2 molecule models (each student group gets a molecule model.) If kits are not available, anything can be used that you can build with different colored balls representing hydrogen, carbon, oxygen and fluorine, using standard colors.

Safety

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

Teacher Notes

• Prerequisite knowledge: For students to successfully identify the mystery molecule, they should already be familiar with atomic radius and periodic trends, Lewis structures, the duet and octet rules, exceptions to the octet rule, and formal charge.
• Depending on the level of the students and the length of classes, this activity could take 1-2 class periods to complete.
• Molecule parameters (One example is included in the answer key):
  • Contain only carbon (black), hydrogen (white), oxygen (red), and fluorine (green). Colors are for traditional model kits and can be modified, if needed. If model kits are not available, you can use colored Styrofoam balls or gumdrops with toothpicks. (If making models without a prefabricated model kit, it is a good idea to make multiples of each model if you have more than one class, as these will not hold up well to students moving them around as they examine the bonds and angles.)
  • At least three central atoms, with at least one having 2D/planar geometry (linear or trigonal planar), and another having 3D geometry (tetrahedral, trigonal pyramidal, or bent).
  • At least one oxygen atom is central, with single bonds to two other atoms.
  • Complexity can be added by increasing the number of central atoms or adding branches (like substituting an X in the model below with a CH₃.)

• Classroom management tips:
  • 6 lab tables with groups of 2–4 students, each table receives a different molecule
  • Students may struggle at first because of the puzzle‑like nature of identifying the molecule. Allow them time (about 10 minutes, depending on the class) to work through the identification step, and then review the identities of the atoms in their molecules before allowing each group to move through the rest of the questions at their own pace.
  • Some students may need additional clarification on terms such as bond angle, lone pairs, and three‑dimensional, even though these are defined in the activity.

• Modification option – if formal charge has not been taught, the activity can be adjusted by telling students that the least electronegative atoms typically occupy the central positions.