This Lewis structure and VSEPR problem is based on a paper from Inorganic Chemistry in 2010 reporting the crystal structure of the carbonyl diazide molecule. This relatively simple molecule provides an interesting application of the predictive powers of Lewis structures and VSEPR theory to molecular structure, backed up by experimental data on bond distances and bond angles. Before tackling carbonyl diazide, the students warm up by considering the structures of hydrogen azide and the isolated azide ion. The reference to the original paper is Inorg. Chem. 2010, 49, 9694-9699.
In answering these questions, a student will:
- Practice drawing multiple resonance structures for a molecule, assigning formal charge, and identifying major vs. minor resonance contributors based on considerations of formal charge distribution
- Gain experience applying VSEPR to predict the molecular structure and approximate bond angles in different molecules
- Relate the predictions of relatively simple theories to actual experimental results obtained from X-ray crystallography
This was assigned as a problem to be worked at the board by small groups during a conference session for a second-year inorganic chemistry course. I have also used it in a conference for general chemistry students.
This was done as an in-class activity during my conference section, so I did not collect or formally evaluate student work. I made the solutions available after class for students to check on their own time. During the conference, I circulated among the groups, answering questions and gently guiding them towards the right answer if necessary. The solutions to this activity are posted as a Problem Set learning object type (see Related Activities above) and can be downloaded by VIPEr Faculty Users that are logged in.
Once students get the hang of coming up with multiple resonance structures, this activity goes relatively smoothly. The common “hang-up” is they draw resonance structures that put + formal charges on adjacent nitrogen atoms. I point out why this is highly unlikely to be an important resonance contributor if they have other “better” resonance structures. One option for follow-up would be to ask a related question on azides on a problem set or exam after this activity. See the links provided above under Related Activities above.