Most chemistry teachers somehow teach Lewis dot structures. These structures are the foundation for VSEPR theory, three dimensional models and ultimately how the structure allows us to predict what happens on a large scale. Here is the crazy part...there are a number of different "rules" that really do not make a whole lot of sense. Do a quick search...everyone has there own rules. Most of these rules work farely well. But, there is a problem. If structures start to get just a little complex, the rules fall apart. And then, there are the exceptions. The ultimate questions I get are, "Where do these rules come from?" "How are we suppose to know the exceptions."
Lewis Dot Structures...A Closer Look
Along with these Lewis dot structures, there is another topic that is strangely connected which happens to be electronegativity (try and stay with me on this one). 99.9% of textbooks talk about the "electronegativity scale" as a way to determine what type of bond is occuring. Is it pure covalent, polar covalent or ionic? No problem...find the difference and check the "rule" to see where it falls on the scale that identifies one of three bonds. Unfortunately, there are several exceptions and as always, it is difficult to explain this to students. They have a hard time understanding the idea that bonding might be more about a "continuum".
Here is a crazy idea. If you look at Linus Paulings orginal papers on electronegativity, the concept had nothing to do with covalent, polar covalent or ionic. As far as I can tell, and please correct me if I am wrong, using electronegavity to determine types of bond, I think, came from textbooks. Linus Pauling developed the concept of electronegativity along with bond energies. Then there is another piece of curious information. I was looking at a bonding activity from the American Modeling Teachers Association. One of the writers made a curious statement that said (I am paraphrasing), "Electronegativity, bond energies and Lewis dot structures correlate together. This is a work in progress." As luck would have it, I was in the middle working on a curriculum project for Project TIMU at Miami University. Here is the really odd thing. Take three Lewis dot structures that have all the same number and type of elements. One is the correct structure of the compound found in nature. The other two are structures that technically follow the "rules" but are not representitive of the compound as it is in nature. This is where it gets interesting. Add up all of the bond energies for each structure. Then add up all of the changes in electronegativity for each bond in that structure. If you compare each of the three structures, the structure with the largest negative sum of heats of formation is also the structure with the largest sum of the electronegativities AND most likely the one correct Lewis dot structure that is a model of what is found in nature. As far as I can tell, it works every time. It is almost as if as the atoms come together to form the compound, they want to form the most energetically stable arrangement. This is the largest heat of formation, it is deep in the potential energy well and electronegativities are based on heats of formations. They can help us tell which Lewis dot structure is the correct one when there are multiple possibilities.
Is this a better way of teaching these structures instead of memorizing a bunch of rules? I do not know but I am trying to find out. I wrote a TIMU activity (at the link above) called, "Lewis Dot Structures...A Closer Look". Take a look at the activity...I would love to know what you and especially your students think about it.