Resonance

A molecule or polyatomic ion for
which 2 or more dot formulas with
the same arrangement of atoms can
be drawn is said to exhibit
RESONANCE.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3
Resonance Structures


Another way to represent this is by
delocalization of bonding electrons:
(the dashed lines indicate the 4
pairs of bonding electrons are
equally distributed among 3 C-O
bonds; unshared electron pairs are
not shown)

See p. 256
VSEPR
valence shell electron pair repulsion
Molecular Shape


Lewis structures (electron dot
structures) show the structure of
molecules…but only in 2 dimensions
(flat).
BUT, molecules are 3 dimensional!

for example, CH4 is:
Molecular Shape


Lewis structures (electron dot structures)
show the structure of molecules…but only in
2 dimensions (flat).
BUT, molecules are 3 dimensional!
 but in 3D it is:
a tetrahedron!
= coming out of page
= going into page
= flat on page
Why do molecules take on 3D shapes
instead of being flat?


Valence Shell Electron Pair
Repulsion theory
“because electron pairs repel one another,
molecules adjust their shapes so that the
valence electron pairs are as far apart from
another as possible.”
Why do molecules take on 3D shapes
instead of being flat?


Valence Shell Electron Pair
Repulsion theory
Remember: both shared and unshared
electron pairs will repel one another.
Non-Bonding
Pairs
H—N — H
H
Bonding
Pairs
5 Basic Molecule Shapes

Linear

Example: CO2
5 Basic Molecule Shapes



Bent or angular
Example: H2O
Notice electron pair repulsion
5 Basic Molecule Shapes

tetrahedral

example: CH4
5 Basic Molecule Shapes



Pyramidal
Example: NH3
(note: unshared pair of electron
repels, but is not considered part of
overall shape; no atom there to
contribute to the shape)
5 Basic Molecule Shapes

Trigonal planar or planar triangular

Example: BF3
Geometry and polarity


Three shapes will cancel out
polarity.
Shape One: Linear
Geometry and polarity


Three shapes will cancel out
polarity.
Planar triangles
120º
Geometry and polarity


Three shapes will cancel out
polarity.
Tetrahedral
Geometry and polarity


Others don’t cancel
Bent
Geometry and polarity


Others don’t cancel
Trigonal Pyramidal