Shapes and Angles
How do we see the world around us? Many people are happy to take a picture with their camera or mobile device to preserve a memory of an event or place. Painters, photographers, and others engaged in creating visual media will take things a step (or several steps) further. Gradually, the clear and obvious representation gives way to a mode that draws more on impression and less on what seems to be reality.
Amazing But True
- How do we depict a molecule? Renaissance painters such as Albrecht Dürer would paint the atoms and the bonds in much the same way we do in the basic chemistry books. Once the ability to indicate three-dimensional perspective was developed, Piero della Francesca and others of his time would portray the shape of the molecule. However, as we probed more deeply into molecular structure, the electron distribution becomes less clear, and we need to go to the abstract expressionism of Jackson Pollock and his confreres to start to get a handle on the reality of the molecule.
- Two challenges face any attempt to represent molecular structures in a static medium such as the printed page. First, where are the electrons? We can use Lewis structures up to a point. Then we need to draw the electron cloud shapes dictated by quantum theory. Even then, it becomes necessary to remind ourselves that this picture only represents a moment in time. Additionally, physical studies that measure bond lengths suggests strongly that some molecules exist as resonance structures, where we cannot definitely state the location of electrons.
- The second challenge asks the question: what is the three-dimensional shape of the molecule? VSEPR theory allows a first-approximation answer, but we cannot show the flexing, bending, and twisting that occurs in the actual molecule. Changes in locations of pi orbitals (in any double- or triple-bonded resonance form) will exert some effect on the overall structure.
- Watch a video on drawing resonance structures at the link below:
Show What You Know
Use the links below to learn more about resonance and VSEPR issues. Then answer the following questions.
- Which is a more significant structure: a resonance form with a +1 charge or one with a 2+ charge?
- Are the different resonance forms in equilibrium with one another?
- What is the basic idea behind VSEPR theory?
- For what two groups does VSEPR not work?