CH101 Notes on the VSEPR Model of Molecular Geometry
The valence shell electron pair repulsion (VSEPR) model is a tool of the valence bond
method; it is based on Lewis structures and it relates molecular geometry to the atomic
hybridization mode of the central atom.
Basic Rules for Molecules ABn (A = central atom, B = outer atom)
1. Bonding (single and multiple) electron pairs and nonbonding electron pairs
(= “lone pairs”) repel each other and therefore avoid each other, as far as
possible, in the geometry of the molecule.
2. The repulsive force of a single bond is less than that of a lone pair or a
multiple bond. This is because lone pairs and double bonds occupy more space
around the central atom (A). The more space a lone pair occupies, the greater
the repulsive effect on bond electrons and the greater the distortion.
3. As the electronegativity difference between the central (A) and outer atoms
(B) increases (where B is the more electronegative partner), the BAB bond
angle decreases. This is because the bond electrons are localized more and
more upon L, hence reducing their repulsive effect on other electron pairs around
A.
4. As the size (radius) of A decreases or the size of B increases (where B is
the more electronegative partner), the BAB bond angle increases. This is
because the electronic and steric repulsions between B increase.
Application of rules 1 and 2 to ABnEm (where m is the number of lone pairs of electrons
on A) leads to the following arrangements of electron pairs around A (the sum n + m is
called the steric number):
n+m
2
Idealized Linear
shape of (sp)
electron
pairs
around
A
3
4
5
Trigonal
planar
(sp2)
Tetrahedral Trigonal
(sp3)
bipyramidal
(sp3d)
6
7
Octahedral Pentagonal
(sp3d2)
bipyramidal
8
Anticubic
The structural names of molecules of the type AB2 through AB6 are given, with
examples, in Table 1.
Distortion of Bond Angles
Note that molecules whose central atom A has lone pairs of electrons or double bonds,
as well as single bonds, may possess distorted bond angles, as governed by rule 2.
Similarly, the operation of the factors that give rise to rules 3 and 4 may also result in
distorted bond angles.
This is summarized in Table 2 for some common compounds.
1
Table 1. Structures of Molecules of Type ABn (: = lone pair)
n + m Arrangement
Type
Shape
Examples
2
Linear
AB2
Linear
CO2, HCN, NO2+, BeCl2 (g)
3
Trigonal
planar
AB3
Trigonal planar
:AB2
Bent (V-shape)
BCl3, CO32-, COCl2, NO3SO3
O3, SO2, NO2-, SnCl2 (g),
PbCl2
AB4
Tetrahedral
BF4-, CH4, NH4+, SO42-,
SO2Cl2, ClO4-, XeO4, PF3O
:AB3
Pyramidal
NH3, SOCl2, SO32-, XeO3
::AB2
Bent (V-shape)
AB5
Trigonal bipyramidal
:AB4
Pivot
or
tetrahedron)
::AB3
T-shape
ClF3, XeOF2
:::AB2
Linear
ICl2-, I3-, XeF2
AB6
Octahedral
:AB5
Square-pyramidal
::AB4
Square planar
SF6, AlF63-, SiF62-, PF6-,
IOF5, XeO2F4
TeCl5-, BrF5, XeOF4, SF5-,
SbF52-, ClOF4-, XeF5+
BrF4-, XeF4
4
5
6
Tetrahedral
Trigonal
bipyramidal
Octahedral
see-saw
H2O, NH2-, SCl2, Cl2O, I3+,
XeO2, H2F+
PF5, SbCl5, SOF4, XeO3F2,
IO2F3, IO3F2(distorted SbF4-,
SeF4,
ClOF3,
XeO2F2, IO2F2-, SbF4-
Table 2. Bond Angles of Tetrahedron-Based Molecules (AB4, :AB3, ::AB2)
[increasing in direction shown by arrows]
Hydrogen compounds
Halogen compounds
CH4 (109.5o)  NH3 (106.8o)  H2O (104.5o)


SiH4 (109.5o)  PH3 (93.5o)  H2S (92.3o)


GeH4 (109.5o)  AsH3 (92.0o)  H2Se (91.0o)


SnH4 (109.5o)  SbH3 (91.5o)  H2Te (89.5o)
OF2 (103.7o)  OCl2 (111.2o)


SF2 (98.2o)  SCl2 (102.7o)


SeF2 (95.8o)  SeCl2 (99.6o)


TeF2 (93.3o)  TeCl2 (97.0o)
NF3 (102.4o)  NCl3 (107.4o)


PF3 (97.4o)  PCl3 (100.1o)


AsF3 (96.0o)  AsCl3 (98.6o)


SbF3 (95.0o) 
SbCl3 (95.5o)
2
Trigonal Bipyramid- and Octahedron-Based Structures
Unlike the idealized tetrahedron or octahedron, in which all the vertices are equivalent,
idealized trigonal pyramids have two distinct types of vertices: equatorial (eq) and axial
(ax), as shown below.
F (ax)
F (eq)
(eq) F
Sb
F (eq)
F (ax)
As a general rule, lone pairs (stronger repellers than single bond pairs) preferentially
occupy equatorial positions, because the number of lone pair-bond pair repulsions at 90o
is fewer (2, as against 3). The presence of lone pairs has a distorting effect on the bond
angles, as illustrated by the examples below.
SF4
Pivot or see-saw
(distorted tetrahedron)
BrF3
Distorted T-shape
F(ax)
F(eq))
:
S
F(eq))
F(ax)
:
F(ax)
Br
F(eq)
F(ax)
angle F(ax)S F(ax) = 173o
angle F(eq)S F(eq) = 101o
:
angle F(ax)Br F(eq) = 86o
angle F(ax)Br F(ax) = 172o
The more symmetrical molecules PF5, PCl5 and SbF5 have regular trigonal bipyramidal
shapes.
Six electron pairs around the central atom give rise to octahedral, square pyramidal and
square planar shapes. The first and last of these are regular (undistorted) provided that
the outer atoms are identical (e.g. as in SF6 and XeF4, respectively), whereas the
second situation is always distorted:
BrF5
(Distorted) square
pyramid
F
F(ax)
F
Br
F
F
angle F(ax)Br F = 86o
(instead of 90o)
..
3