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Published byAnna James Modified over 9 years ago
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Variations in Lewis Structures
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Resonance Forms Some substances are not adequately defined by a single Lewis Structure For example, ozone ( O 3 ), can be drawn just as easily two different ways The two equivalent structures are called resonance forms Both are written side by side with a double headed arrow between
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In resonant forms, only the placement of electrons is different between the structures The resonant compound behaves almost like an average of the two forms
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Example Draw two resonant forms of NO 3 -.
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Bond Order Single Bond – 1 Double Bond – 2 Triple Bond – 3 In resonance form the bond order is the mathematical average of the bond orders Bond Order = Sum of Bond Orders # of Bonded Atoms Example: What is the bond order of NO 3 - ?
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Exceptions To Rule Of Eight The octet rule is not always obeyed If the number of electrons is odd, an octet cannot be achieved for every atom ClO 2, NO, NO 2
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Less Than Octet Some atoms have less than an octet around them In B and Be occurs often BF 3 These compounds tend to be reactive due to the lack of the octet
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More Than Octet Some atoms form compounds with more than an octet Observed only in elements in period 3 or higher These elements have unfilled d orbitals which can be used to accommodate electrons Larger atoms can often accommodate more electrons
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Examples Draw PCl 5 Draw ICl 4 - Which never has more than an octet around it: S, C, P, Br
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