This is the In Class Activity that I use to review the concepts of Lewis Dot Structures, LDS, (connectivity, resonance, formal charges, etc.) learned in General Chemistry and to introduce new ideas of resonance contributions to the character of the molecule. The question itself is apparently very simple, but the discussion that it produces can be quite rich and brings in both new and old ideas of LDS, providing both a good review and a good segue into advanced ideas of Lewis Dot Structures.
- A student should be able to predict the connectivity of a Lewis Dot structure based on a formula; largely, this involves choosing the appropriate atom as the central atom based on electronegativity trends.
- A student should be able to draw/choose the best LDS for a molecule based on the ideas of formal charges.
- A student should be able to draw resonance structures for the LDS of a molecule , including less favorable structures, but eliminating any impossible/illegal structures (This is the first difference between what I do in Gen Chem and what I do with my sophomores/juniors; I want my students to draw all possible resonance structures, even those that are unfavorable and would therefore have been ruled out in Gen Chem; this requires the students also to differentiate between what is only poor, e.g. a structure with an unsatisfied octet, and what is actually not allowed, e.g, a structure with five bonds to nitrogen)
- A student should be able to rank the relative favorability/stability of the various resonance structures available to a particular compound.
- A student should be able to write a representative equation showing the relative contributions of all of the possible resonance (This is the "new" idea" that I am introducing at the sophomore/junior level; it is not expected that students get this far on their own in the In Class Activity, but some students who have been reading ahead may approach this)
A Periodic Table
I do this In Class Activity in my sophomore/junior level inorganic (Descriptive) course before I ever say a word about Lewis Dot Structures. I gave the students about 8 minutes to work on this activity individually. I had the students change writing utensils after their initial independent work, so that I could see what they had gotten on their own before the group discussion. But I allowed them (expected them) to correct their work and add to it as we discussed the problem as a group. Collecting this data allows an assessment of what the level of recall of this material was on the part of the students, but also allowed me to see a flaw in the way that I framed the question, which I will correct in future uses of the assignment, and which I discuss on my answer key.
I assess this in two ways:
- I collect the handouts to see the initial individual responses of the students. Since this is an In Class Activity, students know that they are graded based on participation, and can therefore be trusted not to change their answers. This assessment gives me an understaning of their backgrounds and recall of Lewis Dot Structures from Gen Chem. (It also led to the discovery of an interesting twist that I had not anticipated).
- I give a similar question on an upcoming exam with a different molecule.
A detailed description of the student responses is given in the key, but I was shocked by how poorly students performed on the simple LDS drawing. I was most surprised that 8 of 18 gave an unexpected connectivity that I had not considered.