In this lesson students learn how to model and name molecular compounds. I scaffold the lesson so that first they learn how to draw single bonds and then I introduce double and triple bonds. Toward the end of class I remind students that molecular or covalent bonds are another way that atoms can satisfy the octet rule.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms because how covalent bonds form is intricately tied to their valence electron configuration.
It aligns to the NGSS Practice of the Scientist of Developing and using models because in order to understand covalent bonding students have to model electron sharing.
It aligns to the NGSS Crosscutting Concept of Patterns because different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena. In this case, only by looking at the nanoscale can we understand how and why atoms are bonding.
In terms of prior knowledge or skills, students should have a basic understanding of bonding, as is taught in this lesson.
There are no materials needed for this lesson besides the worksheets that accompany it. However, a molecule building kit will greatly enhance a student's experience with this material by giving him or her manipulatives and by helping students to see unpaired valence electrons and bonding sites.
Do Now: At the start of class I ask students to study diagrams like the ones found at this website, specifically the HCl, CO2, and C2H2, and write down what they notice and what they wonder about.
I reason that this is a good way to start class because I want students to start thinking about covalent bonds and how they relate to unpaired electrons. This site, like similar images in the student textbook, do a great job of this.
Activator: After I have taken attendance I ask students to share out what they observed and wondered.
Some things they noticed are that electron configurations are involved, that sharing is happening, that different elements have different colors for their electrons, that there are different types of bonds (single, double, triple).
This approach works well because everyone can offer something that they observe, and some students can ask questions that will raise the topics to be covered in the lesson. In this case, some students' questions include:
Why are there different bonds?
Why are there different colors of electrons?
Can all atoms make triple bonds?
These questions are a great segue into the mini-lesson.
Mini-lesson: For the first part of the lesson I explain that molecular or covalent bonding happens when atoms share electrons. They do this to satisfy the octet rule.
I then model what I expect students to be able to do with molecules containing single bonds as shown in this video about how draw Lewis dot structures for single bonds. Students take notes during this presentation using the first row of the Naming and Modeling Molecular Compounds practice sheet.
I note that the difference between the Lewis dot structure and the structural formula is that the latter only shows shared pairs, and it does so with lines, much like how the models look when we build the molecules with our kits.
Finally, I describe the naming system, which uses numeric prefixes to describe how many atoms of each element are in the compound. I note that when the first element only contains 1 atom, the prefix mono- is not used. I also note that the second compound has an ending of "-ide" just like we saw in ionic compounds.
This instructional choice reflects my desire to show students how to draw and name single bonds, and sets them up for their first learning activity.
Guided Practice: After I have shown them, I ask them to try doing this for a new compound.
I chose this particular focus to give students a chance to figure out what they understood from the lesson, and where they need clarification. Based on feedback like that which is found in this student's explanation of nitrogen trichloride, I feel confident that students are ready to practice this skill.
Student Activity: During this time students continue to complete the practice problems for single bonds. I walk around the room to ensure that students are getting support if they need it and to offer suggestions when I see students making mistakes.
This work is appropriate for this lesson because it helps them to grapple with and practice the skill of drawing and naming compounds with single bonds.
Catch and Release Opportunities: The biggest mistake I see is that students are forgetting that some atoms, most notably H, only have one bonding site. I ask students to take a moment to build the molecule they are drawing, and/or to pay attention to how many single unpaired electrons, symbolized by holes in the modeling kits, that each atom has. Hydrogen cannot have two bonds, yet many students think that it can.
Stopping class to discuss this idea is important because enough students are doing it that I want to call attention to it in an efficient way, rather than talk with each student individually.
Mini-lesson: Throughout the preceding Application period I noticed that some students were moving more rapidly through their work. I worked with one such student on the mechanics behind double and triple bonds and then had him explain it to the class.
This is a great strategy because it gives students a break from the teacher being the only source of information, and it shows that the task is within their grasp when one of their own is already teaching the material.
This video of a student teaching about double and triple bonds shows what the student presentation looks like. During this time, students take notes on the next assignment, which is Lewis Dot Structures for Double and Triple Bonds. In it, notice that I wonder if a connection is being made to the octet rule. At this point in the lesson, the connection is nonexistent, and this information informs one of my catch and release moments in the next application section of the lesson.
Guided Practice: I ask students to work the C2H2 problem and then we review it. Most students are able to draw the triple bond, but for students who are not comfortable, I work with them in a small group and conduct a think aloud. This differentiation is important. I do want to release students as they are ready. If I release them too soon, they are ill-prepared to practice, but I also want to release some sooner because they are more able or willing to figure things out on their own.
Student Work: During this time I encourage students to make traction with double and triple bonds. I explain that homework for tonight is to finish all the classwork. I walk around and remind students that when they have unpaired electrons, these need to be shared between two atoms that are already sharing.
Catch and Release: I remind students that the octet or duet rule needs to be followed for all of the atoms.
To wrap this lesson up I ask students to explain to a partner what they know about the relationship between double, and triple bonds using the molecules O2, N2, and C2H2. Here is one student's explanation.
Ending class this way gives students a chance to teach each other, and teaching is a higher level of learning. My hope is that students come away with a pretty solid understanding of the covalent bonds.
This sample of student work is informative. It is clear that the student has a basic understanding of single, double, and triple bonds. She did not fall for my trap of trying to make double bonds out of butane, and she was able to draw the correct Lewis structures for most of the compounds. Acetic acid gave her some trouble because she did not follow the directions for how the atoms were connected; she recognized something was wrong, but did not know how to fix it. Still, overall I was pleased with the student's evidence of learning.