Over several decades of teaching General Chemistry I have utilized various methods to get students to draw appropriate Lewis Dot Structures. About 10 years ago I learned about a process which I call the "Account Ledger Method". In this method all valence electrons are assumed to initially belong to the molecule (and placed in a ledger) and not to specific individual atoms. As the molecule is put together those electrons are distributed systematically and removed from the ledger. Later the electronic need of the individual atoms and the number of electrons remaining in the ledger may not match up. In those cases multiple bonds are invoked. The process continues until the electronic needs of the molecule matches the number of electrons in the ledger.
I have found this method gets students away from the “shuffling” of electrons between atoms as they draw Lewis Dot Structures and avoids the invocation of dative bonds. The basic process is given in the first slide and an example of its use is given in the remaining slides.
<p>The ultimate goal for these Five Slides About is for students to learn how to draw appropriate Lewis Dot Structures. This includes learning how to draw an appropriate skeleton structure, learning which atoms typically have lone pairs, and learning how to distribute valence electrons such that all atoms in a molecule have a complete octet; this latter goal may include the utilization of multiple bonds.</p><p>Although the concepts of Formal Charge and Resonance are not included here, they may follow directly from this approach.</p>
These slides can be used as the portion of a lecture on Lewis Dot Structures as typically given in General Chemistry. I often will use them as well as part of a review at the beginning of Inorganic Chemistry.
For a PenCast showing the determination of SO3 using this method, click on the following url:
For a PenCast showing the determination of CO2 using this method click on the following url:
For a PenCast showing the determination of NOF2+ using this method, click on the following url:
Students are evaluated by how well they draw Lewis Dot Structures. These can be from any typical source (textbook questions, quizzes, exams) as the utilization of this method should not change the way a faculty member evaluates proficiency of this common topic.
Since beginning to use this method about 10 years ago I have found a significant improvement in student homework, quizzes and exams. Those who learn this method typically have fewer instances of "extra" or "missing" electrons in their final structures; they also most often include the correct number of multiple bonds. These results are particularly in contrast to those students who insist on using methods learned in high school, such as those which include putting valence electrons on each respective atom and then trying to connect those atoms together.
Thanks for adding the pencasts, it really brings this to life.