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(valence shell electron pair repulsion)
simple model to predict 3D shape
“e– groups” want to be as far apart as possible
(same as baloon arrangements)
x e– Arrangement
due to same charge repulsion
x Atom Arrangement
10: 2
Prob
FUP:
10: 6, 7
E of C: 10: 34 - 40
valence e–’s occur as “e– pairs”:
x non-bonding pairs (lone pairs), LP
x “bonding pairs” (= single, double, triple bonds),
BP
x odd e– = “1 pair”
various terminology
“e– pair” = effective pair = e– group
HMWK 7
VSEPR Principle:
# of e– groups predicts shape
Adv Rdg 10: 3
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Pet. Fig. 10.12 VSEPR Illustration; see also SB Fig. 10.2
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VSEPR Model
CB IV
Molecular Geometry
(VSEPR)
Ref
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atom arrangements ...
2.) if lone pairs present
then atom arrangement different
from e– arrangement
Useful Symbolism:
A X m En
# of BP’s
e.g., SF4
belongs to A X4 E type
# of LP’s
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General Note
Molecular Shapes
Summary
LP’s ( & multiple bonds)
tend to require more space
# of
e– groups
than single BP’s
? bonding angles maybe distorted
2
from “normal” values
e.g., H2O :
3
o
4
? LP’s occupy more spacious (“roomy”)
positions;
5
important for “5 & 6 e– group” types
Following:
overview and details of various A Xm En types
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“type”
“name”
AX2
AXE
linear
“linear”
AX3
AX2E
trigonal
planar
bent
AX4
AX3E
AX2E2
trigonal
pyramidal
bent
tetrahedral
AX5
AX4E
AX3E2
trigonal
bipyramidal
“see-saw”
T shape
AX6
AX5E
AX4E2
square
pyramidal
square
planar
octahedral
AX2E3
linear
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Practice ..
Selected Practice
1.) O3;
Lewis :
O
O
O
AX2E type for central O
O
4.) XeF2; Lewis :
O
F
O
H
2.) NH3; Lewis :
N
N
H
H
H
for central N: electron arrangement: tetrahedral
atom arangement: trigonal pyramidal
3.) PF5; Lewis :
F
F
P
F
F
e dots omitted on F atoms
F
F
AX5 type for central P
Xe
F
for central Xe: electron arrangement: trigonal bipyramidal
atom arangement: linear
Note: lone pairs occupy more spacious equatorial positions
H
H
AX3E type for central N
e dots omitted on F atoms
F
AX2E3 type for central Xe
for central O: electron arrangement: trigonal planar
atom arangement: bent
Xe F
F
F
P
F
F
for central P: electron arrangement: trigonal bipyramidal
atom arangement: trigonal bipyramidal