Lewis Dot Structures...A Closer Look

Lewis dot diagrams

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     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."  

     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.
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Comments 7

Dave Doherty's picture
Dave Doherty | Tue, 02/16/2016 - 15:33

Hi Chad,

Understanding and teaching the meaning and limitations of models in chemistry is one our great challenges. Making connections between models (and the concepts that they represent) is perhaps even more difficult.

It's obvious that Lewis Structures are models.

Perhaps less obvious is that electronegativity is also a model.  We like to think of it as a property of elements but, at best, it is a derived quantity -- it is not a physical "observable" (it's not something that we can calculate using quantum mechanics).

To address one of your points, Linus Pauling defined a quantitative scale of electronegativity specifically for the purpose of quantifying bond character on a continuum (ionic to covlaent).  That's the model.  See the pictures below for his thoughts on how to characterize bonds based on the differences in electronegativity (these are taken from Pauling's wonderful General Chemistry textbook, which is still available today in paperback from Dover).  He most definitely intended the electronegativity scale as a tool for defining bond character.

How can we connect Lewis Structures to electronegativity and bond energies?  I have some thoughts on this but, because I was unable to find the TIMU and AMTA discussions that you referenced, I can't address your questions.

Can you provide more specific links to the referenced material?

Thanks,

Dave Doherty 

 

If you have any trouble seeing the images, they image can be seen here:

https://twitter.com/Atomsmith1/status/699378318483832835

page2zoom.jpg page1zoom.jpg

 

 

 

Chad Husting's picture
Chad Husting | Tue, 02/16/2016 - 17:22

David - Thanks so much for the feedback.  Here is the link for the activities...

http://www.targetinquirymu.org/TIMUactivities

Here is the interesting thing I found.  Suppose you take three Lewis dot structures, two that are incorrectly drawn and one that is correctly drawn.  Now...for each of the structures add up the heats of formations of each bond in the the Lewis structure and then add up all of the electronegativity differences in that structure.  Do it for each three.  What I have found almost without fail is that the one structure with the greatest sum of the heats of formation also has the greatest sum of electronegativity differences.  This is the one which is the correct Lewis structure.  I found in the research that I did that Pauling did account for bond energies when developing electronegativity.  It is almost as if as the atoms form the bonds attempt to get together in an environment that provides the greatest bond energy and , in theory, the lowest potential energy well (if we consider forming bonds negative enthalpy).  The sum of the electronegativity differences seem to parallel the sum of the heats of formations.  It seems to me that it is a bit easier to add up the electronegativity differences.

     Thanks for the comments and feedback.  Any and all comments are sincerely appreciated.  I am sure it will positively impact my teaching and help my students.

Deanna Cullen's picture
Deanna Cullen | Tue, 02/16/2016 - 17:38

Just FYI...You need to register an account with the Target Inquiry group on the link Chad provides to be able to access the teaching materials. Then search for "Lewis Dot Structures...A Closer Look". The activities are well worth the few minutes required to register an account.

Dave Doherty's picture
Dave Doherty | Thu, 02/18/2016 - 08:50

I applied.  Hopefully they won't object to me not being a teacher...

20160218:  Just got my TIMU account set up and downloaded the Activity.  A lot to think about.  Will do so, then post another reponse. -DD

Chad Husting's picture
Chad Husting | Sun, 02/21/2016 - 07:06

Could not agree more.  I actually used that paper (and I cite it in the activity).  Here is a quote from the paper you mentioned...

We suggest that covalent bonding and associated energy changes be introduced first, emphasizing that elements form bonds because the resulting system is more stable (not because elements “want octets”). This is a central concept that underlies much of chemistry, and might help alleviate the commonly held idea that bonds release energy when they break (41). Next, physical and computer molecular models can be used to introduce the 3-D structures involved. We believe it is important for students to also develop an early understanding of the three-dimensional structure of molecules, beginning with simple molecules such as hydrocarbons, water, ammonia, and alcohols. 

Thanks for the comments...they are greatly appreciated...

Deanna Cullen's picture
Deanna Cullen | Sun, 02/21/2016 - 10:49

I have been using an activity for the past two years that I found through the AMTA Modeling Instruction materials to introduce the differences between structures of atomic, molecular and ionic substances. The activity cites an article, “Teaching With Crystal Structures” published in the September 2009 issue of The Science Teacher written by Dennis Smithenry and uses freely available software from the Mercury[1] website. This has provided a sound base for further discussions relating to intermolecular forces.