Molecular Geometry, Bonding and Balloons

Introduction

The structure and three-dimensional shape of molecules depends to a great degree on the number of valence electron pairs and their mutual repulsion. Use balloons and a special wire holder to quickly model different types of molecular geometry.

Concepts

• Covalent bonding
• Hybrid orbitals
• VSEPR theory
• Sigma and pi bonds

Materials

Safety Precautions

Although the materials in this demonstration are considered nonhazardous, please observe all normal laboratory safety guidelines.

Disposal

The balloons will usually stay inflated for several days. The orbital holder can be stored and used many times over.

Prelab Preparation

Balloon Orbital Holder

1. Using long-nose pliers, make a loop about ½-inch in diameter at one end of the wire (see Figure 1).
   {12780_Preparation_Figure_1}
2. Use electrical tape to join the two wires and to cover the sharp end of the wire (see Figure 2).
   {12780_Preparation_Figure_2}
3. Bend the loop over so that it is perpendicular to the wire handle as shown in Figure 3.
   {12780_Preparation_Figure_3}
4. Inspect the fishing leader. One end has a clasp that can be opened and closed. The other end has a swivel.
5. Thread the six fishing leaders with their swivel ends onto the key ring.
6. Slip the key ring with the fishing leaders on it over the wire handle on the wire holder (see Figure 4).
   {12780_Preparation_Figure_4}
7. Insert the fishing leader clasps through the hoop of the wire holder (see Figure 5).
   {12780_Preparation_Figure_5}

Balloons

1. Blow up balloons for Parts 1 and 2 of the procedure.
   1. Part 1. VSEPR and Molecular Geometry

      Equally inflate six pear-shaped balloons.

   2. Part 2. Hybridization

      Equally inflate four pear-shaped balloons, one round balloon and six long balloons. Note: The long balloons only need to be inflated to 20–24" long. Fold over and tape down the excess.

Procedure

An optional student worksheet is included. Use the worksheet during the demonstration if students are already familiar with connecting Lewis structures to molecular geometry and also to hybridization.

Part 1. VSEPR and Molecular Geometry

1. Use different numbers of the round and pear-shaped balloons to demonstrate molecular geometry according to the Valence Shell Electron Pair Repulsion (VSEPR) model.
2. Attach each balloon to be used to a clasp on a fishing leader. Feed the tied-off knot of each balloon through the clasp (see Figure 6).
   {12780_Procedure_Figure_6}
3. Once all the balloons are attached, pull the key ring down along the wire handle until the balloons are just touching. The balloons will spread themselves out in the correct three-dimensional shape.
4. For two atoms around a central atom, AB₂, attach two balloons. Pull the ring and the balloons will adopt a linear arrangement (see Figure 7).
   {12780_Procedure_Figure_7}
5. For three atoms around a central atom, AB₃, attach three balloons to illustrate the trigonal planar geometry (see Figure 8).
   {12780_Procedure_Figure_8}
6. For four atoms around a central atom, AB₄, attach four balloons to illustrate the tetrahedral shape (see Figure 9).
   {12780_Procedure_Figure_9}
7. For five atoms around a central atom, AB₅, attach five balloons to illustrate the trigonal bipyramidal shape (see Figure 10).
   {12780_Procedure_Figure_10}
8. Finally, for six atoms around a central atom, AB₆, attach six balloons to illustrate the octahedral shape (see Figure 11).
   {12780_Procedure_Figure_11}

Part 2. Hybridization and the Formation of Multiple Bonds

The “orbital holder” is the wire holder with the key ring and six fishing leaders attached.

1. Show the class one round balloon and six long balloons. The balloons will represent the atomic orbitals of carbon. There are one 2s and three 2p atomic orbitals.
2. Attach four pear-shaped balloons to the fishing leader clasps on the key ring on the wire holder. Pull the ring to get the arrangement of the 4sp³ hybrid orbitals (see Figure 12).
   {12780_Procedure_Figure_12}
3. Repeat steps 1 and 2 to show sp² hybridization. Show the class one round 2s and two long 2p orbitals. Attach three pear-shaped balloons and two long balloons to the orgital holder. The arrangement shows the trigonal planar arrangement of the three sp² hybrid orbitals with the unhybridized 2p orbital perpendicular to the plane of the sp2 hybrid orbitals (see Figure 13).
   {12780_Procedure_Figure_13}
4. Repeat step 2 using two pear-shaped and four long balloons to show sp hybridization (see Figure 14).

Student Worksheet PDF

12780_Student1.pdf

Lab Hints

• Practice with the use of the orbital holder in orienting the balloons to show the proper geometry.
• Several enhancements can be done to extend the demonstration to show sigma (σ) and pi (π) bonding between carbon atoms. Obtain an additional orbital hanger to construct two carbon atoms with either three sp² hybrid orbitals and one p orbital or two sp hybrid orbitals and two p orbitals. Attach Velcro^® tabs to the ends of each facing orbitals of the two carbon model. Bring the hangers together. Connect the facing p orbital Velcro tabs and the facing sp² hybrid orbitals to model the sigma bond(s) between the hybrid orbitals and the pi bonds of the p orbitals (see Figure 15).

Answers to Questions

Write the Lewis structures and predict the shape of the following molecules and ions.

1. XeF₄
   {12780_Answers_Figure_16}

   Square–planar

2. SO₄^2–
   {12780_Answers_Figure_17}

   Tetrahedral

3. SF₄
   {12780_Answers_Figure_18}

   Seesaw

4. I₃^–
   {12780_Answers_Figure_19}

   Linear

5. (Optional) How many bonds are formed between the carbons in acetylene, C₂H₂? What hybrid orbitals are involved in each bond? How many sigma bonds and pi bonds are present?

   There are three bonds between the carbons in acetylene. One bond is a sigma bond involving the sp hybrid orbitals and two bonds are pi bonds involving the remaining 2p orbitals of each carbon.

Discussion

Molecular geometry and hybridization are important bonding concepts that require a visual understanding. The use of balloons to represent orbitals and electron pairs can greatly facilitate this learning. The use of the orbital holder makes the rearrangement of balloons much easier and quicker than tying them together manually. Once the concept of molecular geometry has been demonstrated with balloons, further study of bonding concepts with the use of traditional molecular models is successful.

References

Special thanks to Steve Long of Rogers High School, Rogers, AR, for providing Flinn with this procedure, along with thanks to Bruce Mattson, Creighton University, Omaha, NE, for directions to assemble the orbital hanger.