Lewis Structures

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Objective

Students will be able to write Lewis Structures for covalent compounds as demonstrated by filling in notes, making molecules, and performing whiteboard practice.

Big Idea

Lewis Dot Structures can be used to show how covalent compounds share electrons between atoms.

Introduction

In this lesson students learn how to write Lewis Structures for covalent compounds.  This builds on their prior knowledge of how to write Lewis Structures for Ionic Compounds from Unit 3 Lesson 4 and students knowledge of Covalent Compounds from Unit 3 Lesson 6.

  • 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 5: Energy and Matter with students understanding that Lewis Structures show how elements combine in covalent compounds.

Within this lesson I use several resources:

  1. For the making molecules practice 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

10 minutes

To engage students in the lesson I give them an opportunity to review three concepts which are found on the first seven slides of the PowerPoint. Students are expected to fill in answers at the top of their Unit 3 Lecture 3 Notes Graphic Organizer.

  1. First students are asked to name several covalent compounds.  Most students remember these fairly well from the previous lesson and absent students catch on quickly.
  2. Students perform Periodic Table Aerobics.  To see how I do this look at the Kinesthetic Learners in Chemistry Reflection.
  3. Students review how to find valence electrons and write simple Lewis Dot Structures.  This is review from Unit 2, but also from Unit 3 Lesson 4 where students were writing Lewis Dot Structure for the formation of ionic bonds.

Explain

30 minutes

In this part of the lesson I teach students how to perform Lewis Dot Structures for covalent bonds. 

  • Students fill in their notes on their Notes Graphic Organizer as I go through slides 8-12 on the PowerPoint.
  • I first explain the idea of how electrons are shared and how this is shown with a line. 
  • I then go through the rules to write Lewis Structures and then several examples.
  • This Video shows how I explain how to form double bonds using the example of Sulfur dioxide.
  • This is a copy of one student's filled in notes.

 

Elaborate Part I

30 minutes

To reinforce the idea of Lewis Dot Structures I have students perform whiteboard practice with their partners.

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.

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.

Here are two examples of common student mistakes:

In this first example lewis-mistakes1.JPG.the students did not have 8 valence electrons around Sulfur.

In this second example lewis-mistake-2.JPG the students had too many valence electrons around Sulfur and only used 22 total electrons instead of 24.

Elaborate Part II

45 minutes

As a second way to reinforce Lewis Structures I have students perform a making molecule activity.

For this activity I give one model set to each pair of students.  Student then fill in their activity paper by performing several steps:

  1. Students first determine the total number of valence electrons for the compound.
  2. Students then write the Lewis Structure for the compounds
  3. Students then build the molecule using the balls and sticks and draw a picture of it.
  4. Finally students name the compound.

To begin the activity I go over the directions with the students and do the first example with them.  I then give them time to work through the different compounds.  As student groups complete each compound I go over the answers.  Here is a video showing how I do this.

This is a copy of one student's filled in activity.   This student did the work correctly except for naming of the last molecule, phosphate ion.  This year I did not teach my students polyatomic ions so I let the naming using prefixes (phoshorous tetraoxide) slide rather than using the proper ion name.

Evaluate

For the final part of this lesson students perform a Lewis Structure Practice Worksheet.  If they have time after completing their modeling molecules practice I have them start the paper in class, but they then finish for homework.

I stamp the homework for completion when they come to class the following day and review using the Answer Key.

After all of the practice in class most students do well on the homework.  The biggest mistakes continue to be completing using the number of electrons and making sure each atom has 8 electrons, except Hydrogen that has 2.  This is especially true when students need to use double or triple bonds to satisfy the octet rule!