Resonance Structures We have assumed up to this point that
there is one correct Lewis structure. There are systems for which
more than one Lewis structure is possible: Different atomic
linkages: Structural Isomers Same atomic linkages, different
Resonance Structures (cont.) The classic example: O 3. Both
structures are correct!
Resonance Structures (cont.) In this example, O 3 has two
resonance structures: Conceptually, we think of the bonding being
an average of these two structures. Electrons are delocalized
between the oxygens such that on average the bond strength is
equivalent to 1.5 O-O bonds.
Structural Isomers What if different sets of atomic linkages
can be used to construct correct LDSs: Both are correct, but which
is more correct?
Formal Charge Formal Charge: Compare the nuclear charge (+Z) to
the number of electrons (dividing bonding electron pairs by 2).
Difference is known as the formal charge. #e- 7 6 7 7 6 7 Z + 7 6 7
7 7 6 Formal C. 0 0 0 0 +1 -1 Structure with less F. C. is more
Formal Charge Example: CO 2 e - 6 4 6 6 4 67 4 5 Z + 6 4 6 6 6
46 6 4 FC 0 0 0 0 +2 -2-1 +2 -1 More Correct
Beyond the Octet Rule There are numerous exceptions to the
octet rule. Well deal with three classes of violation here:
Sub-octet systems Valence shell expansion Odd-electron systems
Beyond the Octet Rule (cont.) Some atoms (Be and B in
particular) undergo bonding, but will form stable molecules that do
not fulfill the octet rule. Experiments demonstrate that the B-F
bond strength is consistent with single bonds only.
Beyond the Octet Rule (cont.) For third-row elements (Period
3), the energetic proximity of the d orbitals allows for the
participation of these orbitals in bonding. When this occurs, more
than 8 electrons can surround a third-row element. Example: ClF 3
(a 28 e - system) F obey octet rule Cl has 10e -
Beyond the Octet Rule (cont.) Finally, one can encounter odd
electron systems where full pairs will not exist. Example: Chlorine
Dioxide. Unpaired electron
Summary Remember the following: C, N, O, and F almost always
obey the octet rule. B and Be are often sub-octet Second row
(Period 2) elements never exceed the octet rule Third Row elements
and beyond can use valence shell expansion to exceed the octet
rule. In the end, you have to practice..a lot!
VESPR Background Recall from last lecture that we had two types
of electron pairs: bonding and lone. The Lewis Dot Structure
approach provided some insight into molecular structure in terms of
bonding, but what about geometry? Valence Electron Shell Pair
Repulsion (VESPR). 3D structure is determined by minimizing
repulsion of electron pairs.
VESPR Background (cont.) Example: CH 4 Must consider both
bonding and lone pairs in minimizing repulsion. Lewis Structure
VESPR Background (cont.) Example: NH 3 (both bonding and lone
pairs). Lewis Structure VESPR Structure
VESPR Applications The previous examples illustrate the
strategy for applying VESPR to predict molecular structure: 1.
Construct the Lewis Dot Structure 2. Arranging bonding/lone
electron pairs in space such that repulsions are minimized.
VESPR Applications Linear Structures: angle between bonds is
180 Example: BeF 2 180
VESPR Applications Trigonal Planar Structures: angle between
bonds is 120 Example: BF 3 120
VESPR Background (cont.) Pyramidal: Bond angles are