4 Jan 2008

In Lewis' Own Words

Literature Discussion

Submitted by Nancy Scott Burke Williams, Scripps College, Pitzer College, Claremont McKenna College

This is G. N. Lewis' classic paper explaining his "octet rule" and the idea of bonds being represented as shared electron pairs.  It's beautifully written, and is a lot of fun for students to read.  Highlights include his description of atoms as being concentric "cubes" of eight electrons at the vertecies, philosophical discussions of the importance of letting experimental observations guide the development of theory, and the sense that students gain that Lewis developed this theory completely in the absence of an orbital or Bohr-type model of the atom.  It wonderfully captures the way in which a brilliant mind wrestled with the problems of developing a bonding theory for the periodic table.  Students understand the paper pretty easily, and are capable of picking out little "gems" on a first read of their own which they bring to a discussion.  

Learning Goals: 

Students should be able to address the following questions:

(1) Explain Lewis' original "cube model" and his subsequent "tetrahedron model" of the atom.

(2) Why does Lewis abandon his first for his second model in this paper, even though it contradicts the prevailing theory of the day.

(3) Compare and contrast what Lewis actually proposed with the "Lewis Dot Model" which youlearned in genchem. 

(4) Please discuss the usefulness of  low-level theory which is easy to use vs. high-level theory which is hard to use, and articulate the value (or lack thereof) of doing things at a "simplistic" level.

Implementation Notes: 

I hand out this paper in advance of one class and have the students bring the answers to the questions in the handout as well at three questions of their own, which they give to another student. By the next class, they try to answer those student questions, and we use all of the questions so generated as a basis for a full class discussion.

Time Required: 
5 minutes on one day, plus one additional day for discussion (students read beforehand)
Evaluation Methods: 

See implementation notes. What I evaluate is the sorts of questions and answers the students generate both on their own and in the discussion.

Evaluation Results: 

I've been very impressed by the level of discussion this generates. The students love the paper for its readability, the "outdated" ideas, and the cleverness of the hypotheses put forth. They are very good at putting themselves in Lewis' position, knowing what he did, and the discussion quickly developed (this year) to a surprisingly deep discussion of the nature of scientific truth, the iterative scientific method, and the role of theory and experiment. The students "get" these models, and can see how powerful they are even though they are "wrong". I finished by emailing them Bronowski's incredibly powerful monologue on scientific knowledge: 


Creative Commons License: 
Creative Commons Licence


I like it! I'm going to use a version of this idea this semester. Thank you!

This went *really* well. I organized the discussion just a little differently. I gave the students these four questions and the paper, and told them that in one week I would ask pairs of them to lead class discussion on each question (since I have 8 students in the class this worked perfectly). I also used our Moodle to set up a forum (first time I have done so) and asked them to each post a question of their own before class, also for our discussion. It just went beautifully. Students told me afterwards that it was exciting and different to read Lewis' writings directly, and I believe it really was a great way to tie into our introductory unit on bonding models and summarize/integrate ideas about bonding that they had gained from prior organic and physical chemistry courses. Having them present in pairs helped alleviate anxiety and stage fright; in future discussions they can lead individually.

Thanks again to Nancy for sharing this!

I used this as an introduction to discussion of Lewis structures in a second-year inorganic course. Students are very familiar with Lewis structures from Gen Chem, and this was a great way to review the basics without having to go through lots of examples they'd seen before. They really seemed to enjoy seeing how Lewis arrived at his ideas, and how the Lewis Structure fits into the broader context of chemical theory at that time. Several students commented on the conversational tone of the paper, and most seemed to find it very accessible. We had interesting discussions about the role of evidence in supporting new ideas, the use of others' work and theories to support your own, and the ways that scientific discourse builds off of previous results. Students also seemed interested in how the formality and tone of scientific writing has changed over time. 

Students read the paper in advance and answered the reading questions above, which I collected at the beginning of class. We used about half of the class period (30 mins) to discuss the major conclusions and implications of the paper, and then began working examples of more advanced Lewis structures (ones that exceed the octet, etc) and discussing the VSEPR model and bond geometry. 

Based on student answers to the reading questions, most understood the paper quite well, though a few failed to identify the cubic and tetrahedral models as distinct, and therefore struggled with question 2. I think that several students also failed to recognize the geometric implications of using tetrahedra instead of cubes in Lewis' bonding model until I drew tetrahedra sharing a corner, a side, and a triangular base on the board to represent single, double, and triple bonds. 

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