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MOLECULAR GEOMETRY
methane
caffeine
see CD-ROM screen 9.12
Mar. 29, 2006
boron trifluoride
ammonia
VSEPR
• Valence Shell Electron Pair Repulsion theory.
•
Most important factor in
determining geometry is the
relative repulsion between
electron
Mar. 29, 2006
A molecule
adopts the shape
that minimizes
electron pair
repulsions
around an atom.
Page 1
see CD-ROM screen 9.13
No. of e- Pairs
Around Central
Example
Atom (σ + LP)
2
Geometry
linear
F—Be—F
180°
F
3
F
planar
trigonal
B
F
120°
109°
H
4
C
H
tetrahedral
H
H
Mar. 29, 2006
Structure Determination by
VSEPR
Ammonia, NH3
1. Draw electron dot structure
2. Count σ BP’s and LP’s = 4
3. The 4 electron pairs are at the corners of a
tetrahedron.
••
H N H
H
Mar. 29, 2006
Page 2
Structure Determination by
VSEPR
••
H N H
H
lone pair of electrons
in tetrahedral position
N
H
H
The ELECTRON
H
PAIR GEOMETRY
is tetrahedral.
The MOLECULAR GEOMETRY
— the positions of the atoms —
is PYRAMIDAL.
Mar. 29, 2006
Structure Determination by
VSEPR
Water, H O
2
1. Draw electron dot structure
2. Count σ BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
O
The electron pair
geometry is
TETRAHEDRAL.
H
H
Mar. 29, 2006
Page 3
••
H O H
••
The molecular
geometry is
bent.
Structure Determination by
VSEPR
•• O ••
H C H
Formaldehyde, CH2O
1. Draw electron dot structure
2. Count σ BP’s and LP’s = 3 Counts as 1 pair!
3. There are 3 electron pairs at
the corners of a planar
triangle.
Mar. 29, 2006
Structure Determination by
VSEPR
The electron pair
geometry is PLANAR
TRIGONAL with 120o
bond angles.
•• O ••
C
H
Mar. 29, 2006
H
The molecular
geometry is also
planar trigonal.
Page 4
Phenylalanine, an amino acid
1
H
C
H
H
C
C
C
C
H
C
H 2 H
O
3
C
C
C
O
H
N
H
H
H
Mar. 29, 2006
Phenylalanine
Mar. 29, 2006
Page 5
5
4
H
STRUCTURES WITH
CENTRAL ATOMS
THAT DO NOT OBEY
THE OCTET RULE
Mar. 29, 2006
Violations of the Octet Rule
••
••
••
F
••
F
••
••
B
••
F
••
••
Usually occurs with Group 3A
elements and with those of
3rd period and higher.
Consider boron trifluoride, BF3
The B atom is surrounded by
only 3 electron pairs.
Bond angles are 120o
This geometry
is described as
planar trigonal
Mar. 29, 2006
Page 6
Compounds with 5 or More
Pairs Around the Central Atom
90°
F
F
P
Trigonal bipyramid
5 electron pairs
F
120°
F
F
90°
6 electron pairs
F
F
S
F
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F
Octahedron
F
F
90°
Sulfur Tetrafluoride, SF4
• Number of valence electrons = 34
• Central atom = S
• Dot structure has 5 e- pairs
around S
Electron pair geometry = trigonal
bipyramid (because there are 5
pairs around the S)
Mar. 29, 2006
Page 7
••
••
•• F
••
•••
••••
F•
F S ••
••
•• F••
••
Sulfur Tetrafluoride, SF4
••
••
•• F
••
•••
••••
F•
F S ••
••
•• F••
••
The lone pair is in the
equator because it
requires more room.
90°
F
••
S
F
F
120°
F
Mar. 29, 2006
Page 8
The molecular
geometry is
called seesaw.