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Molecular Polarity
• Individual bonds tend to be polar, but that doesn’t mean that a molecule will be polar overall.
• To determine molecular polarity, you need to consider the 3-D shape and see if polarity arrows cancel or not.
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Note that all these molecules have polar bonds. Two of these molecules are nonpolar because their symmetry causes the charges to cancel out:
Copyright by the Glencoe Division of Macmillan/McGraw-Hill School Publishing Company
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Multiple Valid Lewis Structures
• Sometimes more than one Lewis structure can be drawn for the same molecule.
• For example, ozone (O3).
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Resonance Forms
• Resonance forms are also known as resonance structures.
• Resonance forms have the same relative placement of atoms, but different locations of bonding and lone e- pairs.
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Resonance Hybrid• Neither resonance form is a true picture of the
molecule.• The molecule exists as a resonance hybrid, which
is an average of all resonance forms.• In a resonance hybrid, e- are delocalized over the
entire molecule.
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Important Resonance Forms
• If all resonance forms have the same surrounding atoms, then each contributes equally to the resonance hybrid.
• If this is not the case, then one or more resonance forms will dominate the resonance hybrid.
• How can we determine which forms will dominate?
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Exceptions to the Octet Rule
• We’ve already discussed expanded valence cases, but there are other exceptions as well. e- deficient atoms like Be and B, e.g. BeCl2 and
BF3. Compounds w/ odd # of e-’s: free radicals.
Examples include NO and NO2. Expanded valence – when d orbitals are used
to accommodate more than an octet.