Resonance
O3
CO32-
Octet Rule Exceptions
BF3
BeH2
¯
Expanded Octets
More Exceptions
PCl5
ClF4
SiF5¯
BrF5
SF6
NO2
1
Molecular Shape
Most molecules are not flat as they appear in twodimensional Lewis structures
Molecules have three-dimensional shapes
Shape can be predicted using Valence Shell
Electron Pair Repulsion (VSEPR) theory
Shape via VSEPR
First obtain the Lewis structure
Then apply the VSEPR rules:
All valence shell electron pairs (electron charge
clouds) are treated equally whether they are
bonding or nonbonding
Stereochemically significant electron pairs
Double and triple bonds are treated as single
bonds for predicting molecular shapes
Electron pairs orient in space to get as far away
from neighbors as possible
Shape via VSEPR
Electron pairs orient in space to get as far away from
neighbors as possible
Two pairs
180o
CO2
linear
O
Three pairs
BF3
C
120o
B
Four pairs
CH4
Central atoms surrounded only by bonding pairs
NH4+
PF5
O
F
F
Shape via VSEPR
trigonal planar
F
H
109.5o
tetrahedral
C
H
H
H
2
Molecular Shape
E
A
E
Linear (Angle = 180)
E
E
A
E
E
E
A
Shape via VSEPR
E
E
E
Stereochemically significant pairs?
Trigonal bipyramidal
Trigonal planar (angles = 120)
Molecular shape is determined by the positions of
atoms
Therefore NH3 will be pyramidal with a
triangular base
Trigonal pyramid
N
E
E
E
A
A
E
E
E
E
Central atoms surrounded by bonding and lone pairs
Example, NH3
Lewis dot structure prediction?
E
E
E
H
octahedral
Tetrahedral (angles = 109.5)
H
H
Molecular Shape
X
A
X
A
X
Shape via VSEPR
X
H2O
X
trigonal planar
bent
Structure is ______________
X
A
X
X
X
tetrahedral
A
X
X
X
trigonal pyramidal
A
bent
ClF2+
X
X
Structure is ______________
3
Molecular Shape
X
X
X
A
X
X
A
X
Shape via VSEPR
X
X
A
A
X
X
X
X
X
X
trigonal bipyramidal
see-saw
t-shaped
linear
X
X
X
A
X
X
X
X
X
A
X
X
X
X
A
Central atoms with expanded octets and lone pairs
ClF4+
ICl4¯
BrF5
X
X
X
square pyramidal
octahedral
square planar
Effects of Lone Pairs
X
A
X
A
X
Effects of Lone Pairs
X
X
trigonal planar
X
X
X
A
X
X
A
X
A
A
X
X
bent
X
X
X
X
X
X
trigonal bipyramidal
see-saw
t-shaped
linear
X
X
X
A
X
X
X
tetrahedral
A
X
X
X
trigonal pyramidal
A
bent
X
X
X
A
X
X
X
X
X
A
X
X
X
X
A
X
X
X
octahedral
square pyramidal
square planar
4