[spirochete]

After working with countless organic chemistry undergrads, I've noticed that many people come in to organic with a very poor background in drawing lewis structures. Often students think their skills are adequate, but they still lack the ability to consistently assign formal charge and draw structures in a fluid manner required to proceed with the course.

Practically all general chemistry text books teach a laborious method for drawing Lewis Structures where all of the valence electrons are counted, with a series of following steps we all know and love (sarcasm). Formal charge is treated as an after thought, and often neglected entirely. Does any chemist actually follow this method?

I would prefer to teach drawing lewis structures with an emphasis on formal charge, where a student knows that the structure is complete because the formal charges add up to the total real charge. Common bonding patterns would be emphasized (for example oxygen makes 2 bonds when neutral). I would also like to emphasize trends in formal charge, for example how replacing a bond with a lone pair causes the formal charge to be more negative.

Is this crazy? Anyone have any experience teaching it this way in general chemistry? The common way that structure drawing is taught in gen chem feels terribly wrong to me.

[mikasaur]

So my girlfriend has a PhD in OChem which is what got me interested in learning more about chemistry. When I started (re-)learning about how to draw Lewis structures (I had learned it before in high school/college) I was amazed at how much faster and more accurate she was than me. I couldn't see how she could so quickly draw out the structure for something like [NCNH]^- when I had to sit there counting valence electrons and figuring out where bonds would go.

The biggest boon to me has been learning common bond patterns (4 for C, 3 for N, 2 for O). After that I taught myself the trends in formal charge like you said, e.g. a N with four bonds will be positive while a C with 3 bonds would be negative. I can now draw out organic molecules very quickly and easily identify impossible compounds.

I think the most important part of learning anything is learning the "why" behind the lesson. I think counting valence electrons and going through the laborious process that entails might do a better job of teaching students that each atom contributes to the valence electrons available for bonding. And they're shared across all of the constituent atoms. Once you understand THAT then you can move on to the shortcuts that you propose.

[Babcock_Hall]

I sometimes teach how to draw Lewis structures using ions, such as nitrite, nitrate, and carbonate, so that the formal charges can be compared with the overall charge of the chemical species.