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chm115_lecture16

chm115_lecture16 - Chemistry 115 Lecture 16 Outline Chapter...

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16-1 Chemistry 115 Lecture 16 Outline Chapter 8 Finish Lewis Dots Covalent Bonds Electronegativity EXAM II: Thursday, March 10 8:00pm, Elliott Hall of Music Chapters 5, 6, 7 Review Session: Wed 7:30 pm WTHR 200 Recitation : Electronegativity VSEPR, molecular shapes Molecular polarity
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Still follows octet rule The steps in converting a molecular formula into a Lewis structure H - C - C - O - H H H H H ( Lewis Dots, continued) Note that many larger molecules have more than one central atom. Example, Ethanol: CH 3 CH 2 OH. Place carbons, and oxygen next to one another. The hydrogen atoms can only have one bond each. Add multiple bonds as necessary to follow octet rule
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Resonance Structures Some molecules have more than one Lewis Dot structure. OR Both are correct. These are called resonance structures. We use a ↔ to depict this situation. One of the most famous cases is benzene, C 6 H 6 . You can convert from one form to the other by moving an electron pair, or a bond. Neither is correct by itself. Instead, the actual molecule is an average of the two: The dotted lines indicate a partial bond . Each bond has a bond order of 3/2. (3 bonded electron pairs/2 bond linkages). What do you do in a case such as ozone O 3 ? O = O – O O – O = O O – O – O
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How about for CO 3 -2 ? Each is equally likely. O C O O -2 O O -2 C O O O -2 C O Example 3: Nitrogen dioxide anion, NO 2 - N 5 + 12 + 1 = 18 18 - 4 = 14 N O O - N O O - N O O - or Which one is correct? Both. Electrons are delocalized over the 2 bonds. N - O O O 2 Insufficient electrons for octet on N. Fix with double bond:
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-2 0 A B C Rules: Smaller FC preferred Like charges on adjacent atoms to be avoided More neg. FC should be on more EN atoms 0 +1 -1 0 0 0 -1 So, from FC, first structure (A) not important. C preferred over B.
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