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Chapter 3
Molecular Shape and Structure
Introduction to Stereochemistry
and Advanced Bonding Theory
Lewis structures were able to provide a 2D picture of
molecule; while good, it is incomplete!
All molecules can’t be flat!
When present, lone pairs must have ‘special’ rules
for placement;
Large atoms vs.
. small atoms: does it make a
difference how they are placed?
ALL OF THESE HAD TO BE RESOLVED
Introduction to Structure
3-1

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Consider CH
2
Cl
2
: if flat, TWO possible isomers
must exist:
The Dichloromethane Dilemma
3-2
C
Cl
H
Cl
H
C
H
H
Cl
Cl
cis
trans
HOWEVER, only ONE
CH
2
Cl
2
structure
is seen!
V
alence
S
hell
E
lectron
P
air
R
epulsion
Theory
• VSEPR adds 3D component that Lewis lacked
• Based on repulsion of electrons and groups (
X
)
around central atom (
A
);
•
Thesis
: items (
X
’s) around
A
will try to get as far
from one another as possible;
• Both multiple bonds and unshared electrons are
treated the same as single bonds to arrive at 3D
structure
3-3

3
VSEPR:
Electronic Geometry
AX
2
AX
3
AX
4
AX
5
AX
6
3-4
VSEPR:
Electronic Geometry (examples)
3-5

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VSEPR: Molecular Geometry (Shape)
• VSEPR provides 3D component based on Lewis
structures;
• Electronic geometry (
aka
VSEPR geometry) based
on
AX
n
;
• If central atom has lone pairs, structure observed
will differ from predicted (lp’s can’t be seen);
• Observed structure, the
MOLECULAR
GEOMETRY
or
SHAPE
, based on VSEPR geometry.
3-6
VSEPR:
Molecular Geometry (Shape)
2
From AX
3
3
From AX
4
4
From AX
5
5
From AX
6
1
From AX
2
1
2,3
2
3
3
4
4
5
5
5
4
3-7

5
VSEPR:
Molecular Geometry
(Examples)
3-8
Lewis Structure
Electronic Geometry
Shape
3-9

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Lone Pair Effects
Unshared electrons, since not involved in a bond, can be
thought of as “taking up more space” than a bonding pair of
electrons; this leads to bond angle anomalies:
3-10
For repulsions: lp-lp > lp-bp > bp-bp
Cl
C
Cl
Cl
Cl
Theory: 109.5
Actual: 109.5
H
N
H
H
Theory: 109.5
Actual: 107
H
O
H
Theory: 109.5
Actual: 104.5
lp-lp Repulsion = angle expansion
lp-bp Repulsion = angle compression
Lone Pair Effects: The Strange Case of AX
5
Note that trigonal bipyramid has
TWO
different
sets of bond
angles and two different sets of positions; because of this,
positions will have different energies
3-11
X
A
X
X
X
X
axial
axial
equitorial
equitorial
equitorial
So: WHERE do lone pairs go?