One of my favorite lessons each semester in general chemistry is teaching Lewis dot structures. For a few bucks and an hour several years ago, I made some fantastic kits for introducing students to Lewis dot structures.
A very brief lesson for folks who don’t know what this is. A Lewis structure is a sort of a drawing of a molecule that shows how the atoms are connected. They’re very important for predicting the shape and reactivity of a molecule. They’re also a big step from chemical formulas. For instance, HC2H3O2 becomes <include picture>
So it’s pretty different.
The goal for new students is to follow these rules:
1. Give every element (except hydrogen) eight electrons
2. To have the right number of electrons in each structure.
The kits are designed to focus students on those two rules, and structured to make following those rules as natural as possible (scaffolding FTW)(citation on scaffolding?). Each atom is represented by a note card cut into a plus sign. On the front is an element symbol, and eight Xs to show the eight places to put electrons. On the back, as a memory aid, is the valence configuration and number of valence electrons.
<image of front and back of a card>
Here’s how it works. I usually group up students by counting off, into groups of 3-5 (cite Bligh). Each group gets a kit, and is instructed to find two chlorines. Then, they should decide, in total, how many valence electrons they need, and count off that many beads. Each chlorine gets seven, so they should count off 14 beads. I ask them to put each bead on an X, so that each chlorine has it’s seven electrons.
<image of that>
Next, we review the octet rule, that every atom wants to have eight electroms, and that a covalent bond means sharing pairs of electrons. I then ask the groups to share electrons such that both of their chlorines have eight electrons.
<image of that>
Ta-da! The wonderful thing is, this looks just about like the Lewis structure of Cl2:
<image of that>
One issue many students have when drawing a Lewis structure is that they add or subtract electrons without really noticing. By making the electrons physical objects (I have 14 beads and need to put them on these elements), it’s a lot harder to invent or disappear two beads, so they end up with the right number of electrons. The other thing the kits really help with is the octet rule. There are eight Xs, so it’s very clear when you’ve got eight electrons.
From Cl2, we move on to H2, the only exception I teach in this lesson. Hydrogen is too small to get eight electrons, so it only gets two. Then we go through a couple of other small molecules, to illustrate the concept.
<Image of student work with Cl2, H2, CH4>
Double bonds pose a challenge, and one which the kits aren’t a natural fit for. Still, they make it work, and it helps them remember that double bonds always come from moving electrons to places they might not feel natural being.
<Amy and Liz on O2>
Finally, the lesson ends with several challenge molecules: N2H2, NH3, N2, CH2O, and NO3-. I let the class work as they will, and the most recent time I taught it, they spontaneously put answers on the board without my asking for it.
<several student images>