VSEPR: Valence Shell Electron Pair Repulsion Theory
Predicts an approximate shape/geometry of a given Lewis structure
Assume that each bond (single or multiple) and each lone pair on a central atom
occupies a region of space; the bonds and lone pairs are arranged in such a way about
the central atom so as to minimize repulsions among them.
bonds +
lone pairs
2
3
4
5
6
linear
trigonal
planar
tetrahedral
trigonal
bipyramidal
octahedral
O
O
O
O
O O
O
O
arrangement
O O O
O
O
O O
O
O
O
O
O
O
O
O
O
O
Some of the “corners” of the arrangement may be taken up by lone pairs on the central
atom. This will determine the actual molecular geometry, defined by the atom positions.
Each combination of bonds and lone pairs can be represented by AXnEm with A =
central atom, X = atoms bound to A, E = lone pair on A.
BP + LP
2
lone pairs
0
AX2: linear
3
AX2E: bent
F
3
F
B
N
F
AX4: tetrahedral
H
H C
H
H
AX5: trigonal
bipyramidal
F
5
F P
F
F
F
F
S
F
O
F
F
AX2E2: bent
H
H N
H
H
AX4E: see-saw
F
F
F
I
F
F
AX4E2:
square planar
F
F
F
F
Xe
AX2E3:
linear
F
Xe
F Cl
AX5E: square
pyramidal
F
H
AX3E2:
T-shaped
F
F
F
O
F
S
F
Advantages
• based on a simple,
qualitative premise
• fast and intuitive
• qualitatively accurate
• especially useful for
organic compounds
Cl
AX3E: pyramidal
F
AX6: octahedral
6
2
O C O
AX3: trigonal planar
4
1
F
F
Disadvantages
• requires a valid Lewis
structure, so cannot be
used if Lewis isn’t valid
– no TM complexes
– no odd-e– molecules
– many structures not
correctly predicted by
Lewis model
• offers only qualitative,
approximate predictions for non-symmetric
molecules
• the simple premise is
an oversimplification of
the underlying physical
principles
How to Determine a VSEPR Structure
1) Draw the Lewis Structure
2) Count the bonds and lone pairs about the central atom
(note that multiple bonds are one region of electron density)
3) Determine the electron pair arrangement (both bond pairs and lone pairs)
4) Position the atoms and lone pairs so as to minimize lone pair repulsions
5) Name the molecular geometry based on the position of the atoms
Examples
180º
BeF2
AX2: linear arrangement
linear geometry
F Be F
F
AX3: trigonal planar arrangement
trigonal planar geometry
120º
BF3
B
F
F
H
109.5º
CH4
AX4: tetrahedral arrangement
tetrahedral geometry
H C H
H
90º
F
PF5
F
P
120º
F
F
AX5: trigonal bipyramidal arrangement
trigonal bipyramidal geometry
F
F
F
F
S
F
F
90º
F
SF6
AX6: octahedral arrangement
octahedral geometry
O
O
NO3–
N
O
O
N
AX3, regardless of resonance:
trigonal planar
O
O
O
O
N
• multiple bonds count as a single density region!
• resonance forms do not affect VSEPR!
O
All AXn molecules have identical electron pair arrangement and molecular geometry!
More Examples
H
109.5º
CH4
NH3
H2O
H C H
H
H N H
H
P
120º
Two “corners” are lone pairs!
F
F
F
S
F
F
F
F
Cl
F
F
XeF2
AX5: trigonal bipyramidal arrangement
trigonal bipyramidal geometry
F
F
ClF3
One “corner” is a lone pair!
F
F
SF4
AX3E: tetrahedral arrangement
trigonal pyramidal geometry
AX2E2: tetrahedral arrangement
bent geometry
H O
H
90º
PF5
AX4: tetrahedral arrangement
tetrahedral geometry
Xe
F
AX4E: trigonal bipyramidal arrangement
“see-saw” geometry
First LP: equatorial
AX3E2: trigonal bipyramidal arrangement
“T-shape” geometry
Second LP: also equatorial
AX2E3: trigonal bipyramidal arrangement
linear geometry
All LP equatorial in trigonal bipyramidal!
Which corners do the lone pairs occupy?
1) tighter angle = more repulsion: 90º > 120º > 180º
⎯⎯⎯⎯⎯⎯⎯⎯⎯→ less repulsion
2) lone pairs need more room than bond pairs:
interaction: LP-LP > LP-BP > BP-BP
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ less repulsion
LPs occupy positions that give greatest LP-LP and LP-BP angles