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.
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
Multiple Valid Lewis Structures
• Sometimes more than one Lewis structure
can be drawn for the same molecule.
• For example, ozone (O3).
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.
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.
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?
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.