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Lecture 2

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CAS CH 203
John K Snyder

Week 2: February 12 – 14 th Resonance and Electron Pushing What is it? Resonance is a theory proposed by Linus Pauling to accurately depict molecules that could not be drawn with a single Lewis structure. For example, consider the following acetate ion (chemical formula C H 2 3: 2- Which one is correct? Chemical testing shows these bonds are shorter than single bonds but long than double bonds; also, both oxygens have a slight negative charge. So, how does one know which oxygen be drawn with negative charge and which should have the lone pair? The answer, of course, lies in resonance structures. Resonance structures are a combination of different Lewis Structures that contribute to make a real hybrid molecule. In other words, two or more ‘fake’ Lewis Structures combine to make the ‘real’ picture. Don’t be fooled by the method of drawing resonance structures and multiple different figures: resonance structures DO NOT actually exist! My chemistry teacher in high school described resonance as such: You want to show someone a unicorn. But you can’t find a unicorn. So instead you show them a horse and a rhinoceros and tell them a unicorn is a mixture of both. Other people like the 2 colors of paint mixing to make a new color metaphor; I’m personally fond of the unicorn. How to draw Resonance Structures The following list of rules describes a good checklist you can go through for each resonance structure to make sure it is correct. With practice these rules will become second nature. 1. Connectivity of the atoms does not change. Resonance is the sharing of electrons in π orbitals (double bonds) and p orbitals (lone pairs). Never break sigma (single) bonds! 2. All contributing structures have the same number of valence electrons. 3. Obey the rules of covalent bonding. No Texas Carbons! Priority Rules Not all
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