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Discovering Molecular Shapes (4 Favorites)

ACTIVITY in Covalent Bonding, Molecular Structure, VSEPR Theory, Molecular Geometry, Lewis structures. Last updated January 10, 2019.


Summary

In this activity, students will use tactile methods (manipulation of connected strings) and a computer simulation to discover how electron-electron repulsion determines the 3D VSEPR geometric shapes of simple covalent molecules. It will allow them to practice drawing Lewis structures as well as deepen their understanding of the connection between a molecule’s structure and its shape.

Grade Level

High School

NGSS Alignment

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

  • Scientific and Engineering Practices:
    • Developing and Using Models

Objectives

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

  • Match the VSEPR molecular shape names with 3D images or representations.
  • Predict the VSEPR 3-D shape around a central atom based the Lewis structure of the molecule.
  • Describe the role of electron-electron repulsion in molecular shape.

Chemistry Topics

This activity supports students’ understanding of

  • VSEPR Theory
  • Molecular Geometry
  • Lewis Structures
  • Molecular Structure
  • Covalent Bonding

Time

Teacher Preparation: 5-10 minutes

Lesson: 80 minutes

Materials (per group)

  • Six 1.25 m or so pieces of string or rope tied together at one end (see photo)
  • Computer with Internet access PhET Molecule Shapes Simulation
  • *If a computer is not available, pictures of the different electron geometries and VSEPR molecular shapes
  • Camera (or paper/ pencil) in order to capture (sketch) string positions
  • Periodic table

Safety

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

Teacher Notes

  • The teacher should plan to measure, cut and tie strings for each group in advance of the activity.
  • Prior to engaging in this activity, students should be proficient in drawing Lewis structures for simple covalent compounds.
  • This activity can be done in full (to include shapes with central atoms containing expanded octets) or as a mini activity (no expanded octets). The latter may be more suitable for lower level classes.
  • This activity is intended to allow students to “discover” the 3D shapes that simple molecules adopt (based on the number of atoms and lone pairs attached to a central atom) and the role of electron-electron repulsion in molecular shape. Due to this, these concepts should not be discussed before engaging in this activity.
  • Once students have explored the different molecular shapes, they will then link them to the names of the shapes. Due to this fact, the names should not be introduced before engaging in this activity.

  • As written, the activity directs students to take pictures of the shapes that they build with the string (see photo for exemplar), but it can be amended for students to sketch their shapes if access to cameras/cell phones is not possible. The purpose of this is to have a reminder for students of the shapes they created when they are discussing their findings and when they are trying to determine the shapes of subsequent sets of molecules in part II.
  • Prior to beginning the activity, students should be reminded not to advance to the next molecule until the group reaches consensus and the group decision has been recorded (as a photo or a sketch).
  • Group size should be 4-6 students depending on the level of the activity chosen. A minimum of 4 students is required for the shortened, mini activity and a minimum of 6 students is required for the full activity that includes expanded octets.
  • Students will first be given an opportunity to create the shapes for a representative linear molecule (BeH2), trigonal planar molecule (BF3) and tetrahedral molecule (CCl4). After the groups have been given some time to investigate these shapes and come to consensus as a small group, the class will share their findings and come to consensus as a large group. If desired, the teacher can elect to check student groups individually and not engage in a whole class discussion.
  • During this initial discussion (either in a large group or with individual groups) the teacher should encourage the students to articulate the rationale behind their shapes. The goal is for them to realize that the guiding principle is having the bonds as far away from each other as possible as to minimize electron-electron repulsion.
  • During the analysis section, students will predict the shapes of molecules containing multiple bonds and the check their assignments using the model option of the PhET Molecular ShapeSimulation. They can build each molecule in the simulation by adding the appropriate number of lone pairs as well as double, single, and triple bonds to a generic central atom and then check their assignments by selecting the molecular geometry name box. If individual or group access to a computer is an issue, the teacher can lead the students in checking their work in one large group or provide an alternate mechanism for students to check their shape name selections.
  • Students may need some support for the naming of the shapes (especially the expanded octet shapes). If they are struggling, the teacher can suggest they rebuilt a molecule using the strings, identify explicitly which strings are lone pairs then hold those strings close to the knot (central atom) to examine just the molecular shape of the molecule. In the case of the expanded octets, students may need to be reminded of their thoughts (in an earlier part of the analysis) about whether lone pairs will go in an equatorial or axial position.
  • A follow up or extension activity could involve using a molecular model kit to build models of the molecules used in this activity.
  • An answer key for the analysis questions has been included as a separate document available for download.

For the Student

Lesson

Background

Molecular shape is critical to the function of many biologically important molecules such as proteins. In addition, it can help explain differences in the attractive forces between molecules so it can help explain a host of physical properties including viscosity, surface tension and boiling point. The purpose of this activity is to investigate the 3-D shapes of simple covalent molecules with the goal of understanding the possible shapes and the factors that influence them.

Prelab Questions

  1. Draw Lewis structures for the following molecules; where appropriate include resonance structures.
Molecules For Mini Activity Additional Molecules for Full Activity
(including expanded octet)

a. BeH2

b. BCl3

c. CF4

d. NF3

e. H2S

f. CH2O

g. HCN

h. CO2

i. NO3-

j. SO2

k. SO3-2

l. AsF5

m. SF6

n. SF4

o. BrF3

p. XeCl2

q. BrF5

r. XeF4

  1. Is it energetically favorable for electrons to get near each other? Why? (1-2 sentences)

Objective

We are going to use string to model the 3-D shapes of simple molecules. The questions we are examining in this activity include:

  • What types of 3-D shapes are possible?
  • What factors influence the 3D shape around a central atom?
  • What is the role of electron-electron repulsion in molecular shape?

Procedure

  1. Your group has several pieces of string tied together into a knot.
  • The knot is the central atom in each molecule.
  • Each string will represent the bonding between the central atom and a surrounding atom.
  1. Using your Lewis structure as a guide, have members of your group hold the strings to create the 3D shape of BeH2

NOTES:

  • Only use the strings you need. The others can be left hanging from the knot.
  • Don’t forget to think about pre-activity question #2 as you decide where the bonds should be relative to each other.
  1. Once your group has come to consensus about the shape of the molecule, take a photo of your string representation of it.
  2. Repeat for each molecule shown below, first using your Lewis structure and the strings to create the 3-D shape then taking a photo of the result.
    1. BCl3
    2. CF4
      *Full activity only:
    3. AsF5
    4. SF6
  3. Once all groups have completed the activity for these shapes, the class will compare their results and come to a class consensus of 3D shape of each and the key factor in determining the shape.
  4. After the discussion, you will return to your small group and repeat for each molecule shown below, first using your Lewis structure and the strings to create the 3D shape then taking a photo of the result.
    NOTE: This time a string can represent a bond or a lone pair on the central atom
    1. NF3
    2. H2S
      *Full activity only:
    3. SF4
    4. BrF3
    5. XeCl2
    6. BrF5
    7. XeF4
  5. Find another group and compare your photos. Discuss any differences until you come to consensus about the shapes.

Analysis

Part I: Matching shapes with names
Based on what you have discovered so far, work with your group to answer the questions below

  1. Match the 3D molecular shape name with each molecule you investigated.
    NOTE: These are molecular shape names so look at where the atoms are relative to each other in your photos and what shapes they create, it will help you figure out which name goes with which shape.
Molecules Shape Names
BeH2
BCl3
CF4
NF3
H2S
Tetrahedral
Trigonal Pyramidal
Linear
Bent
Trigonal Planar

NOTE: if you are struggling, try rebuilding the molecule with string then holding any string that represents a lone pair close to the knot so you can just look at the molecular shape of the molecule.

  1. The F-C-F bond angle is 109° but the F-N-F bond angle is slightly smaller (about 102°). What does this tell you about the repulsion between bonds vs. between a bond and a lone pair of electrons?
  2. Based on this fact, what do you think the H-S-H bond angle is, 92°, 102° or 109.5°? (circle choice) Explain your thinking.

*Full activity only:

  1. Keeping your answer to #2 in mind, for molecules with expanded octets, where do you think it is more likely for the lone pairs to be, in the axial positions (above or below the central atom) or in the equatorial position (in the plane with the central atom that is perpendicular to the axial positions) (see figure 1)? Explain your thinking.

HINT: What is the angle between the axial position and each equatorial position? What is the angle between the equatorial positions?

  1. Find another group of students and share your answers to #4. When you have reached consensus, move on to #6.
  2. Match the 3-D molecular shape name with each molecule you investigated.
    NOTE: These are molecular shape names so look at where the atoms are relative to each other in your photos and what shapes they create, it will help you figure out which name goes with which shape.
Molecules Shape Names
AsF5
SF6
SF4
BrF3
BrF5
XeCl2
XeF4
Octahedral
Square Pyramid
Trigonal Bipyramidal
See-Saw
T-Shaped
Square Planar
Bent

NOTE: if you are struggling, try rebuilding the molecule with string then holding any string that represents a lone pair close to the knot so you can just look at the molecular shape of the molecule.

Part II: Extension Questions

  1. The following molecules differ from those you have explored so far in that they contain multiple bonds. Using what you have learned, can you predict the 3D shape name of each one based on its Lewis structure?
    1. SO2
    2. HCN
    3. H2CO
    4. CO2
    5. NO3
      *Full activity only:
    6. SO3-2
  2. For each of the molecules in #1, check your answers by building each molecule using the model option of the molecular shape PHET simulation ( https://phet.colorado.edu/en/simulation/molecule-shapes) and then checking the molecular geometry box to see the molecular shape name.
  3. Consider the following statement:
    The nature of the bonds (single, double, triple) does not impact the overall 3-D shape of a molecule; it is only the number of atoms and lone pairs attached to the central atom that are important.
    1. Is this statement true or false?
    2. Explain using at least two specific molecules you investigated in this activity to support your claim.
  4. Consider the following statement:
    There is no difference in repulsion between bonding electrons and between bonding electrons and lone pair electrons.
    1. Is this statement true or false?
    2. Explain using at least two specific molecules you investigated in this activity to support your claim

  1. Both butter and olive oil are composed of lipids with hydrocarbon tails but one contains hydrocarbon chains with mainly single bonds (saturated) while the other contains mainly double bonds (unsaturated).
    1. Which image (A or B) illustrates chains from a saturated fat and which represents chains from an unsaturated fat?
    2. In the saturated chains, what would be the name of the shape around the carbon atom if it were bonded to two other carbon atoms and two other hydrogen atoms?
    3. In the unsaturated chains, what would be the name of the shape around the carbon atom if it bonded to two other carbon atoms and one hydrogen atom?
    4. This structural difference is why butter is a solid at room temperature and olive oil is a liquid. Based on your answer to part a, explain why the lipids in butter can pack together more tightly and be a solid at room temperature while the lipids in olive oil must be cooled to a lower temperature in order to solidify.

Conclusion

Based on what you learned today, answer the following questions:

  1. What factors influence the 3-D shape around a central atom?
  2. What is the role of electron-electron repulsion in molecular shape?