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For the midterm, should we always draw the resonance structures or will we be okay with drawing the most stable structure? I guess more clearly I am wondering if Dr. Lavelle will specify "draw the Lewis Structure and its resonance forms" or should we still draw the resonance structures if it only says "draw the lewis structure"?
I believe that if we have to draw the lewis structure regardless of where we will have to draw resonance. But if you are talking about the formal charge when you are addressing the most stable state, then yes you always draw the most stable unless it states otherwise.
If he is going to grade us on whether or not we know how to draw resonance structures, he will ask us to draw them. There may be resonance structures of the most stable lewis structure for a given molecule, but we shouldn't have to draw them unless specified.
How do we know when a molecule has resonance?
After optimizing formal charges on the Lewis structure, you will typically find resonance if there is an arrangement of single/double bonds (maybe triple, though we haven't seen that in class yet) that can be moved around without affecting the overall distribution of formal charges (the central atom's FC doesn't change and the outer atoms simply swap formal charges.) This usually happens with a central atom bonded to several atoms of the same elements.
Jessica Tran_3K wrote:How do we know when a molecule has resonance?
A molecule has resonance when it has a bond that can be drawn in a different place and not alter the overall formal charge of the atom or molecule itself. For example, if you have a carbon single bonded to two oxygen and single bonded to another, you can draw the double bond attaching to any one of the three oxygen, so the molecule has resonance. To display this just draw three lewis structures and in each one, the carbon is double bonding to a different oxygen. Resonance only works if the bond that can be moved around pertains to the same atom. If the carbon were double-bonded to an atom that was not oxygen then the molecule would not have resonance as moving the double bond to a different atom alter the formal charges.
The midterm did say to draw the resonance structure if appropriate, so I think in the future you would have to determine whether or not there is a resonance structure. For instance, if it doesn't matter which oxygen has the double bond and which one has the single bond it may look like you have a resonance structure on your hands.
KaleenaJezycki_4G wrote:For the midterm, should we always draw the resonance structures or will we be okay with drawing the most stable structure? I guess more clearly I am wondering if Dr. Lavelle will specify "draw the Lewis Structure and its resonance forms" or should we still draw the resonance structures if it only says "draw the lewis structure"?
Drawing resonance, most likely, is for the most stable/lowest energy structures. If this lowest energy structure has resonance, draw it, but if it doesn't say to draw the resonance, it's not necessary.
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