Chapter8 - 8 Understanding Lewis Structures Triple Bonds...

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8 Understanding Lewis Structures Triple Bonds Students sometimes get the idea that triple bonds are not desirable. That is not true. Many molecules have triple bonds. Look at these two examples. HC CH (0) (0) The only way to make the carbon have an octet of electrons is to have a triple bond. The formal charges (in parenthesis) are both zero. This is a very happy molecule. Many of you are familiar with this molecule. C 2 H 2 is acetylene and is used in gas torches for cutting metal and welding. Carbon monoxide has two possible Lewis structures.
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2 Chapter 8 Understanding Lewis Structures CO (-1) (+1) (0) (0) A B Structure B has good formal charges but the carbon atom does not have an octet. Structure A has a carbon with an octet but bad formal charges. Note the +1 formal charge on the very electronegative oxygen atom in structure A . (Bad, Bad, Bad!!) If carbon were allowed to violate the octet rule ( like B or Be ) then structure B might be preferred. However, carbon is not allowed to violate the octet rule. The octet rule is more important in this case than the formal charges. Structure A with the triple bond will be preferred even though the formal charges are worse. I Try to avoid positive formal charges on the most electronega- tive atom. Sometimes, however, it is necessary as in the case of CO. An Analogy of Resonance Imagine painting a picture of a f ower using only two inks. One
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An Analogy of Resonance 3 is a light tone and the other dark. Suppose further that we are only alowed to use one ink at a time. With the light ink by itself we can paint most of the petals. With the dark ink we can paint the rest of the petals, the leaves and stem. But neither the light toned painting nor the dark toned painting represents the real f ower. The real f ower
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4 Chapter 8 Understanding Lewis Structures is a hybrid or mixture of the two top paintings. The reason
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This note was uploaded on 11/21/2011 for the course CHEM CHEM 1201 taught by Professor Rupnik during the Spring '11 term at LSU.

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Chapter8 - 8 Understanding Lewis Structures Triple Bonds...

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