Download MOLECULAR GEOMETRY Structure Determination by VSEPR

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