Background
Molecular geometry can be tricky for students to grasp. Two-dimensional representations of molecular geometries predicted by the valence-shell electron-pair repulsion (VSEPR) model can be difficult to comprehend. Three-dimensional models, however, give students the opportunity to view and manipulate models as needed, making molecular geometry much easier to understand.
In this activity, each balloon represents an electron domain (either a bonding pair or a lone pair of electrons) surrounding a central atom. When connected, balloons naturally adopt the lowest energy arrangement predicted by the VSEPR model.
You can perform this activity as a demonstration, or alternatively, as a student investigation.
Students may incorrectly believe that the balloons in this activity represent electron clouds around the nucleus of an atom (rather than electron domains around a centrally bonded atom). Address the electron domain misconception as needed. Consider incorporating subsequent activities that use plastic molecular models. These products have colored atoms and use connectors to represent electron domains. Each atom connects to others using the appropriate molecular geometry.
AP students also struggle with predicting correct molecular shapes and bond angles. Consider assigning this activity as pre-course work over the summer or incorporating it in the end of course review.
Materials
- 20 balloons(round, pack of 24)
Preparation and Procedure
Linear Geometry: Two Electron Domains
- Inflate and tie a balloon.
- Inflate another balloon to the same size and tie it.
- Tie the two balloons together.
Trigonal Planar Geometry: Three Electron Domains
- Inflate and tie three balloons.
- Tie the three balloons together.
Tetrahedral Geometry: Four Electron Domains
- Inflate and tie four balloons.
- First, tie them into pairs.
- Then, connect the two balloon pairs by twisting one pair’s connection around the other’s connection.
Trigonal Bipyramidal Geometry: Five Electron Domains
- Inflate and tie five balloons.
- Tie balloons together to form two sets of two balloons. You should have one remaining.
- Tie this remaining balloon to one of the balloon pairs.
- Connect the set of three with the set of two balloons by twisting at the points of connection for the two sets.
Octahedral Geometry: Six Electron Domains
- Inflate and tie six balloons.
- Connect them first into three pairs.
- Then, connect all six by twisting the pairs’ connections together.
Activity Variations
Create temporary classroom decorations using helium-filled balloons. Before creating each model, create a note card label and attach it to a long piece of string. (To aid in retrieval, the string should be six feet less than the height of your classroom ceiling). During the model assembly, tie the free end of the string to the connection point between balloons.
The helium in each balloon will lift it toward the ceiling and keep the models out of the way during other classroom activities. Alternatively, labels and string can be used to hang air-filled balloons from classroom walls.
You may choose to begin with six balloons in an octahedral geometry and show other molecular shapes by popping one balloon at a time.
