VSEPR Theory
– Predict 3-dimensional shapes of molecules
– Valence shell electron pair repulsion theory
– Each “group” of valence e- around central atom
located as far away from other groups as possible
• e- repel each other
• No. electron groups = no. bonds + no. lone pairs
• Single, double and triple bonds each count as one
group
– Double bond is composed of two orbitals, but
both are pointing to the same atom, i.e., both
are pointing in the same direction
Fig. 10.5
2
3
4
5
6
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramidal
Octahedral
Two electron groups
F
Be F
O C O
• Molecular shape – where the atoms are located,
linear
• The class of both molecules is AX2, where A
stands for the central atom and each X is an atom
attached to the central atom
Fig. 10.6
Three electron groups
– Trigonal planar arrangement of three electron
groups
– Two different classes possible
• AX3 – atoms attached to all 3 corners
• AX2E, where E = lone pair on central atom
Examples of AX3: BF3, CO32-
F
F
B
2-
O
F
O
C
O
Molecular shape also trigonal planar,
120° bond angles
Example of AX2E: NO2-
O N O
O N O
Molecular shape bent
O
N
O
Fig. 10.7
Four electron groups
– Point to 4 corners of a tetrahedron
– Three different classes
• AX4
• AX3E
• AX2E2
Both of these molecules are tetrahedral?
H
H C
H
1. True
2. False
H
H N H
H
Example of AX4: CH4
H
H
H C
H C
H
H
H
H
3-D
Molecular shape is tetrahedral,
bond angle = 109.5°
Example of AX3E: NH3
representation
H N H
H
Arrangement of e- groups – tetrahedral, molecular
shape – trigonal pyramidal
Example of AX2E2: H2O
H O
H
Arrangement of e- groups – tetrahedral, molecular
shape – bent
Fig. 10.8
Bond angle in H2O only 104.5°, not 109.5°
– Lone pairs of e- repel other orbitals more
strongly
• “lone pair repulsion”
– Bonding pair of e- are partially “neutralized” by
a second positively charged nucleus
Lone pair orbitals are bigger because they are not
attracted to two nuclei
H O
H
Bond angles in NH3 only 107.3°
Also see greater repulsion from double bonds
– They contain more electrons
– Molecular shape – trigonal planar
H 122°
116° C O
H
Five electron groups
Trigonal bipyramidal arrangement of e- groups
• AX5 class – PCl5
• Molecular shape – trigonal bipyramidal
• 2 axial, 3 equatorial positions
• Axial 90º angles to 3 nearest neighbors
• Equatorial 90° angles to 2 axial neighbors
120° angles to 2 equatorial neighbors
Cl
Cl P Cl
Cl
Cl
Fig 10.10
In structures where one or more lone pairs, i.e.
AX4E, AX3E2, AX2E3, lone pairs occupy equatorial
positions where there is more space
SF4
AX4E class
• Molecular shape – seesaw
F
F S
F
F
Six electron groups
–
–
–
–
Octahedral arrangement of e- groups
SF6 AX6 class
Molecular shape – octahedral
All positions identical
F
F S F
F
F
F
AX5E
– Molecular shape is square pyramidal
– BrF5
F
F
F
Br
F
F
AX4E2
– Lone pairs point in opposite directions to get as
far apart as possible
– Molecular shape is square planar
– ICl4-
Cl
Cl
I
Cl
Cl