The Valence Shell Electron Pair Repulsion Theory
Typical
Formula
Total
number of
electron
groups
around the
central atom
Number
of Lone
Pairs
Number
of Bonds
Electron
Geometry
Molecular
Shape
AB2
2
0
2
linear
linear
3
trigonal
planar
trigonal
planar
2
trigonal
planar
v-shaped
(bent)
AB3
AB2
3
3
0
1
Bond
Angles
Hybrid
Orbitals on
Central
Atom
180
sp
120
sp2
120
sp2
B
109.5
sp3
B
<109.5
sp3
<109.5
sp3
Structure
B
A
B
B
B
A
B
B
B
..
A
B
AB4
AB3
4
4
0
1
4
3
tetrahedral
tetrahedral
A
tetrahedral
B
..A
trigonal
pyramidal
AB2
4
2
2
tetrahedral
v-shaped
(bent)
AB
4
3
1
tetrahedral
linear
5
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
see-saw
trigonal
bipyramidal
t-shaped
trigonal
bipyramidal
linear
octahedral
octahedral
B
B
B
B
.. A
..
....A
..
B
B
B
AB5
AB4
AB3
AB2
AB6
AB5
AB4
5
5
5
5
6
6
6
0
1
2
3
0
1
2
4
3
2
6
5
4
octahedral
octahedral
B
A
B
B
.. A
B
B
B
B
B
B
B
.
...
A
not
applicabl
e
120
and
90
120
and
<90
sp3d
<90
sp3d
180
sp3d
90
sp3d2
<90
sp3d2
90
sp3d2
sp3
sp3d
B
B
square
pyramidal
square
planar
.. A...
.
B
B
B
B
B
B
B
B
A
B
B
..A
..A
..
B
B
B
B
B
B
Hints:
Always start by drawing the Lewis structure and then focusing on the central atom.
By "pair" we really mean group of electrons. So a double or triple bond (even though it really
represents 4 or 6 electrons) counts as one group or "pair."
There is a large amount of memorization associated with this concept. But, you can think logically
about the shape. You should try to visualize the arrangement of total groups of electron pairs around
the central atom. This will help you determine the electron geometry. Then, imagine that any lone
pairs are invisible. The structure that remains is the shape of the molecule.
You do need to memorize the bond angles associated with each shape. Lone pairs or non-bonded
pairs of electrons are not localized between two atoms. They are not being pulled in by two nuclei, so
they occupy a greater volume than bonded pairs of electrons, which are being pulled in by two nuclei.
For this reason, when the central atom has lone pairs as well as bonded pairs, the bond angles are
less that what they would be if the central atom had only bonded pairs of electrons. In other words,
lone pairs occupy a greater volume, push the bonded pairs of electrons away, and decrease the bond
angles between the bonded pairs of electrons. Only bond angles of less than 120 are affected by
lone pairs.
For the reasons outlined above, lone pairs will occupy equatorial positions (rather than axial
positions) in trigonal bipyramidal molecules and will take positions opposite each other in octahedral
molecules.
When drawing a two dimensional picture of a three dimensional shape:
indicates a lone or non-bonded pair of electrons.
indicates a bond headed backwards into the plane of the paper or board.
indicates a bond coming forward out of the plane of the paper or board.
Hybridization is used to explain how the s, p, and d atomic orbitals of the central atom can be used to
make molecules with bond angles other than 90. The orbitals of the central atom are said to mix
together to form an equal number of hybrid orbitals that have the shape given below and the
arrangement needed to form the proper bond angles.
The following is the minimum that must be memorized from the other side. After knowing this, you
will probably be able to "reason through" the rest.
total groups
electron geometry
bond angles
hybridization
2
linear
180
sp
3
trigonal planar
120
sp2
4
tetrahedral
109.5
sp3
5
trigonal bipyramid
6
octahedral
90 and 120
90
sp3d
sp3d2