MOLECULAR
GEOMETRY
1
MOLECULAR GEOMETRY
VSEPR
• Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion between
electron pairs.
Molecule adopts
the shape that
minimizes the
electron pair
repulsions.
2
3
Electron Pair Geometries
Figure 9.12
4
No. of e- Pairs
Around Central
Atom
2
Example
F—Be—F
Geometry
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
No. of e- Pairs
Around Central
Atom
2
5
Example
F—Be—F
Geometry
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
No. of e- Pairs
Around Central
Atom
2
6
Example
F—Be—F
Geometry
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
7
Electron Pair Geometries
Figure 9.12
Structure Determination by
VSEPR
Ammonia, NH3
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
H
H
••
H N H
H
lone pair of electrons
in tetrahedral position
N
H
8
3. The 4 electron pairs are at the
corners of a tetrahedron.
Structure Determination by
VSEPR
Ammonia, NH3
There are 4 electron pairs at the corners
of a tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
The ELECTRON PAIR GEOMETRY is
tetrahedral.
9
The MOLECULAR GEOMETRY — the
positions of the atoms — is
PYRAMIDAL.
Ammonia, NH3
The electron pair geometry is tetrahedral.
lone pair of electrons
in tetrahedral position
N
H
H
H
10
11
The electron pair geometry is
TETRAHEDRAL.
Water, H2O
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
12
Water, H2O
The electron pair
geometry is
TETRAHEDRAL
The molecular geometry
is BENT.
Geometries for Four e-Pairs
Figure 9.13
Electron-Pair repulsion DECREASE angle
13
14
Formaldehyde, CH2O
1. Draw electron dot structure
2. Count BP’s and LP’s at C
•
•
H
O
C
•
•
H
3. There are 3 electron “lumps” around C
at the corners of a planar triangle.
•
•
O
•
•
The electron pair geometry is
PLANAR TRIGONAL with
120o bond angles.
C
H
H
15
Formaldehyde, CH2O
•
•
O
The electron pair
geometry is PLANAR
TRIGONAL
•
•
C
H
H
The molecular geometry
is also planar trigonal.
16
Structures where Central Atom do
NOT contain 4 Electron Pairs
Often occurs with Group 3A
elements and with those of 3rd
period and higher.
17
Boron Compounds
••
•
•
•
•
Consider boron trifluoride, BF3
••
•
•
F
••
The B atom is surrounded by
only 3 electron pairs.
Bond angles are 120o
Geometry described as
planar trigonal
F
B
•
•
•
•
F
••
Compounds with 5 Pairs
Around the Central
Atom
90Þ
F
F
P
Trigonal bipyramid
F
120Þ
F
F
5 electron pairs
18
19
Sulfur Tetrafluoride, SF4
• Number of valence electrons = 34
• Central atom = S
90Þ
••
F
F
F
••
S
••
F
••
••
•F•
• •
••
F
S
••
•• F
••
••
•
•F
••
Electron pair geometry
120Þ --> trigonal bipyramid
(because there are 5
pairs around the S)
••
••
•• F
••
• ••
•F
••
••
S
F
••
••
Sulfur Tetrafluoride,
SF4
•F•
• •
••
Lone pair is in the equator
because e’s require more
room.
90Þ
F
••
S
F
F
F
120Þ
20
Molecular Geometries for
Five Electron Pairs
Figure 9.14
21
22
Compounds with 6
Pairs Around the
Central Atom
6 electron pairs
90Þ
F
F
S
F
Octahedron
F
F
F
90Þ
Molecular Geometries for
Six Electron Pairs
Figure 9.14
23
Electronic Geometries
Molecular
Geometries
24