Lecture04 - Individual resonance forms DO NOT EXIST they represent the extremes The actual chemical species is a hybrid of all the resonance forms

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Lecture Summary 04 August 30, 2006 Chapter 2 - Polar Covalent Bonds; Acids and Bases Formal Charge When atoms have more or less number of bonds than their valency requires, they must have a charge that is not zero. A formal charge is a way of keeping track of electrons and who owns them in a molecule. FC = (# valence electrons) - (# bonds) - (# nonbonded electrons) H 3 C N O O FC = 6 - 2 - 4 = 0 FC = 6 - 1 - 6 = -1 FC = 5 - 4 - 0 = +1 Resonance Whenever you have a pi-bond adjacent to another pi-bond, lone pair, free radical or empty orbital, the electrons in the system can spread out. It is impossible to indicate this situation precisely using normal Lewis structures. Thus, we draw the Lewis structure extremems of what actually exists somewhere in the middle. These are called Resonance Forms or Resonance Structures.
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Unformatted text preview: Individual resonance forms DO NOT EXIST- they represent the extremes. The actual chemical species is a hybrid of all the resonance forms. Resonance forms differ only in the distribution of pi or non-bonding electrons. Atoms do not move. Individual resonance forms must obey normal valency rules. You cannot break the octet rule. The actual hybrid is more stable than the individual resonance forms and the individual structures do not have to be equal in energy. H 3 C C O O H 3 C C O O H 3 C C O O negative charge spread equally between the two oxygens H H H H H H Acetate Ion bonds are equal Benzene © 2006 Gregory R . Cook page 1 Chem 341 North Dakota State University...
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This note was uploaded on 12/06/2011 for the course CHEM 341 taught by Professor Sun during the Fall '08 term at ND State.

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