Representing Molecules
Resonance
Exceptions to the Octet Rule
Formal Charge
Bond Length
•The average
distance between
two bonded atoms
•The distance
between nuclei at
their minimum
potential energy
Bond Dissociation Energy
• Also called “bond energy”
• The energy required to break a bond
and form neutral isolated atoms
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
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”
Ozone, O3
•Draw a Lewis structure for ozone (the
oxygen atoms are in a line)
Both structures are equally correct!
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
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!
Problem
• A problem to try:
– Draw resonance structures for the
nitrate ion, NO3-
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
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
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
Calculate formal charges
• Which is the most appropriate
structure for the molecule CCl2O?
Problem
• Below are two different Lewis
structures
for
nitrous
acid
(HNO
).
2
..
..
H-O-N=O:
..
..
..
H-N=O
..
:O:
..
• Which is the better Lewis structure
based only on formal charge
arguments?
..