representing molecules resonance exceptions to the octet rule formal charge
TRANSCRIPT
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Representing MoleculesResonance
Exceptions to the Octet RuleFormal Charge
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Bond Length•The average distance between two bonded atoms•The distance between nuclei at their minimum potential energy
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Bond Dissociation Energy• Also called “bond energy”
• The energy required to break a bond and form neutral isolated atoms
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Trends in Bond Energy• C-C vs. C=C vs. C≡C
– Higher bond energy for multiple bonds compared to single bonds
• Triple bonds are shorter on average than single bonds– There can be a lot of variation in bond
length, depending on other bonds for those atoms
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Bond Energy & Bond Length• Higher bond dissociation energy is
linked to lower chemical reactivity
• Longer bond lengths lower bond dissociation energies– “Short bonds are strong bonds”
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Ozone, O3
•Draw a Lewis structure for ozone (the oxygen atoms are in a line)
Both structures are equally correct!
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Resonance Structures• Sometimes, more than one correct
Lewis structure can be drawn for a compound– A single Lewis structure may be
inadequate for describing the situation– “resonance” structures
• Same arrangement of atoms, same numbers of single and multiple bonds
• Link the multiple structures with a double headed arrow
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Resonance• The actual structure is a “blend”,
somewhere in between the Lewis structures
• Alternating single/double bonds would have different bond lengths– In the real molecule, all the bond lengths
are identical!
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Problem• A problem to try:
– Draw resonance structures for the nitrate ion, NO3
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Exceptions to the Octet Rule• Elements in row 3 or below on the
periodic table can have expanded octets– i.e., they can have more than 8
electrons, because they may use d orbitals in bonding
• B, Be may be “electron deficient”– Fewer than 8 electrons around central
atom
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Nonequivalent Lewis Structures
• If a molecule or polyatomic ion exceeds the octet rule, or if different skeletal arrangements (i.e. isomers) are possible, non-equivalent Lewis structures may be drawn.
• Million dollar question: Which Lewis structure best describes the actual bonding situation?– Use “formal charge” to evaluate
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Formal charge• FC = # valence electrons – (# of lone
pair electrons) – (# bonds)– Calculated for each atom– Sum of formal charges must equal
overall charge of species
• The most appropriate Lewis structures have:– Lowest formal charges (zero is best)– Negative formal charge on the most
electronegative element
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Calculate formal charges• Which is the most appropriate
structure for the molecule CCl2O?
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Problem• Below are two different Lewis
structures for nitrous acid (HNO2).
H-O-N=O: H-N=O
:O:
• Which is the better Lewis structure based only on formal charge arguments?
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