If you look at any chemistry textbook, you will see Lewis structures introduced long before electronic and molecular geometries. This makes sense since you need Lewis structures to determine molecular geometry. Unfortunately, research has shown that students often do not recognize that the purpose of drawing Lewis structures is not to create the structure itself but to use it as tool to understand the properties of the molecule (Cooper, Grove, Underwood & Klymkowsky, 2010).
While chatting with my colleague across the hall about teaching Lewis structures and geometries, he showed me an intriguing activity he uses to introduce the VSEPR model before ever having students draw a Lewis structure. I had my doubts about this model, to which my colleague replied, “I’m not saying this is the best way to teach this topic. Actually, I guess I am saying it is the best way.” As if that argument was not convincing enough, I did a little digging into the issue. I found that research (cited above) supports the practice of connecting 3-dimensional representations to 2-dimensional Lewis structures and not the other way around. I tried this approach and was amazed with how quickly my students were able to grasp the complex concepts of the VSEPR model, Lewis structures and polarity.
I started by reviewing basic atomic structure with my students. Atoms are made of protons, neutrons and electrons with electrons on the outside. I then asked students what the natural tendency of electrons is when near each other. They quickly replied, “they repel each other.” With that idea in mind, I issued a challenge to my students: “construct 3-dimensional models of molecules with 2, 3, 4, 5 and 6 atoms around a central atom with styrofoam spheres. Keep in mind that electrons repel each other and therefore will be most stable in an arrangement that minimizes repulsions.” I tried to avoid anthropomorphic language like “the electrons do not want to be near each other” because it is just as easy and more chemically correct to relate the phenomenon to stability. Students checked their geometries with me before filling out this worksheet (you may download below). Students gave each geometry their own name to help them remember what it looks like and I gave them the actual names later.
The first two geometries were pretty easy for students to come up with because they are planar. When they got to “central atom + 3” (tetrahedral), every group first constructed this planar geometry:
and were flabbergasted when I told them that it was incorrect. In every class, one group would eventually create a 3-dimensional geometry and the rest of the class would follow. As groups finished their worksheet, I asked them to rebuild one of their configurations so I could use it as my example piece. By the time every group finished, I had a full set of molecular models.
We then went through each model as a class, identified the bond angles and gave each geometry a real name. I also showed students how to draw 3-dimensional structures with wedge-dash notation (essentially a Lewis structure). The amount of information my students were able to recall the next day about their geometries was pleasantly surprising.
I then made the transition from electronic geometry to molecular geometry through water. I wrote the formula for water on the board and asked students to draw what they thought the molecule looked like. Of course they all drew this:
What they drew was completely consistent with their current model so students were surprised to see the actual geometry of water is bent (I showed students a styrofoam sphere model). With the backbone of the VSEPR model established, students were able to determine there must be something repelling the hydrogen atoms to create the bent geometry. At this point, I had students construct a Lewis structure of water with me to introduce the concept of “lone pairs.” I gave students very few “rules” for drawing Lewis structures except that the valence electrons are the only ones that participate in bonding and it is energetically favorable for electrons of opposite spins to pair together (electrons are rarely found unpaired). We drew the Lewis structure with a bent geometry to make sure it represented the actual water molecule.
I then compared the geometry of water to the tetrahedral model my students had made the day before. Instead of 4 atoms attached the central atom, 2 atoms and 2 lone pairs were attached to the central atom. Students concluded that both models had 4 “electron domains” attached to the central atom. With the idea of “electron domains” in mind, I set my students off on the “Molecular Shapes” PhET. Students made a table of geometries by replacing bonds with lone pairs (keeping the number of electron domains the same). The table looked like this:
Number of domains
around central atom
(no lone pairs)
|One lone pair||Two lone pairs||Three lone pairs||Four lone pairs|
|3 TRIGONAL PLANAR||XXXXXXXXX||XXXXXXXXX||XXXXXXXXX|
|5 TRIGONAL BIPYRAMIDAL||XXXXXXXXX|
This table (you can download a copy below) became the basis for all future Lewis diagrams. After visualizing the possible 3-dimensional structures of molecules, my students were able to easily construct Lewis structures with correct geometries.
My students now have an understanding of the purpose of Lewis structures and do not see molecular geometries as something to simply be memorized. The two tools are one in the same. Now that we are learning about polarity, it is incredibly simple for my students to sketch the geometry of a molecule and determine if it is polar or non-polar and the implications of such.