Valence Shell Electron Pair Repulsion Theory (VSEPR)

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Objective

Students will be able to determine the shape of molecules using VSEPR theory as evidenced by taking notes, performing a molecule lab, and doing whiteboards.

Big Idea

Valence Shell Electron Pair Repulsion Theory (VSEPR) allows chemists to infer the shape of molecules.

Introduction

In this lesson students learn how to determine the shape for covalent compounds using VSEPR theory.  This builds on students' prior knowledge of how to write Lewis Structures for covalent compounds from Unit 3 lesson 7.

  • This lesson aligns with NGSS Performance Expectation: 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.
  • This lesson aligns with NGSS Science and Engineering Practice 2: Developing and using models because students learn to model the formation of covalent bonds through performing Lewis Structures.
  • This lesson also is related to NGSS Crosscutting Concept 6: Structure and Function with students seeing how a 2-D Lewis structure can be used to determine the 3-D shape of a molecule which more closely relates to how the molecule functions.

Within this lesson I use several resources:

  1. For the VSEPR lab I have students use model kits.  These kits can be found online from various resources.  I order mine from Flinn Scientific.
  2. For the whiteboard practice students need access to whiteboards, erasers, and dry-erase markers.

Engage

5 minutes

To begin the lesson and review concepts from the previous lesson I have students perform a quick Lewis Structure question with a partner on a piece of paper or whiteboard.  For this class I had them perform the lewis structure for water.

Here is a picture of one group's responses.

After students complete the question and I make sure that they have done it correctly I talk to them about how we will be moving from 2-D lewis structures to 3-D shapes of molecules today.

Explain

30 minutes

During this portion of the lesson I present notes related to VSEPR.  The notes include:

  • The basic shapes for molecules ranging from 2 to 6 atoms around the central atom
  • The angles for each type of shape
  • How polarity is related to shape

The notes I present are found on slides 1-11 of the PowerPoint and students fill in their notes on the first page and a half of partially empty slides.   This is a copy of one student's filled in notes.

While teaching the content I make sure to stress to students to only look at the CENTRAL atom to determine the shape. 

Here is a video of my explanation of the Trigonal Pyrimidal shape with students.

Elaborate Part I

20 minutes

After I have gone over the content with students I have them perform four practice questions which are located on the last four slides of the PowerPoint and Student notes

I have students perform these four questions of varied complexity to give them an opportunity to practice different types of shapes with various polarities.

I present the questions one at a time.  I give students time to work on the example and go over the answers with them as they complete each question. 

To help students I remind them to look back at their notes and make sure to look at the CENTRAL atom to determine the shape.

If students get done quickly I have them also add in the bond angles for each question.

This is a copy of one student's notes with the answers to the practice questions on the bottom of the second page.

Elaborate Part II

45 minutes

In this portion of the lesson I give students time to practice what they have learned by doing a VSEPR Lab

Students work with a partner using a molecular model kit.

In the lab students do the following:

  1. Determine the total number of valence electrons.
  2. Write the Lewis Structure.
  3. Put the molecule together using the model kit.
  4. Write the shape.
  5. Write the bond angles.
  6. Write if it is polar or non-polar.

Before students start the lab I go over the directions with students and point out that each ball color represents different atoms.  I also remind students to look back at their notes or on the first page to help with shapes, angles, and polarity.

As students are working I walk around and help them out with making the molecules and make sure that they are working with their partners.

I also periodically go over some of the examples with students on the document camera.  This is a video where I go over an example with students.

Here is a copy of one student's completed lab.

 

Elaborate Part III

20 minutes

As a last way to go over shapes with students in class I lead students in VSEPR whiteboard practice.

I have students work in pairs and each pair needs one whiteboard, one dry erase marker, and one dry eraser.  I have students work with partners to encourage them to discuss why they think that a certain answer is correct.  I periodically remind them to switch partners in terms of who is doing the writing throughout the whiteboard session. 

This is the PowerPoint I use.  Note that there are many different types of examples with varied complexity to help ensure that all students are learning the shapes.

1. I put up a problem on the PowerPoint and then have students hold up their answers. 

2. I either give thumbs up or down and if they get it incorrect they should retry. 

3. After most students answer I go onto the next answer.  If it is one that many get wrong I go over why the correct answer is correct either by myself explaining or having a student explain how they determined the answer.

This takes students some time but overall they did well with the whiteboards after having done the other two elaborate sections.  Again the most common mistakes are students getting confused about looking at the central atom to determine the number of bonded and lone pairs and students getting the incorrect Lewis Structure and therefore the incorrect shape.

For more information on how I use whiteboards, refer to my reflection on Student Ownership and Whiteboards in my lesson Metallic and Covalent Bonds Formation and Naming.

Evaluate

For homework I have students perform practice questions on a VSEPR worksheet

I stamp the homework for completion the following class and go over using my answer key.

For the homework students do fairly well with the most common mistakes being incorrect Lewis structures which then relate to incorrect shapes, etc.