VESPR
“Valence Shell Shared Pair Repulsion”
Electrons repel one another.
Pairs of electrons in Lewis dot models will be
found as far apart as possible.
This repulsion determines the geometry of a
molecule.
Basic Shapes
Looking at compounds made of hydrogen plus
each of the elements in period 2 of the Periodic
Table, we can illustrate all the basic shapes you
will need to know.
These shapes are all based on a maximum of 8
valence electrons. Other shapes are possible
with elements that “break” that octet rule: you’ll
see examples in the text, but not be responsible
for these other shapes on a test.
LiH & BeH2
Lewis
Model
Li H
Be H
Structural
Model
Molecular
Shape
Li H
Linear
H Be H
Linear
The farthest away you can keep 2 pairs
of electrons is 180°.
H
BH3
Lewis
Model
B H
H
Structural
Model
H
B
H
Molecular
Shape
H
Trigonal
Planar
The farthest away you can keep 3
pairs of electrons is 120°: in a flat
triangle.
H
CH4
Lewis
Model
H
H C H
H
Structural
Model
H
H
C
Molecular
Shape
H
Tetrahedral
H
The farthest away you can keep 4
pairs of electrons is NOT a flat square
(only 90°) but rather a
TETRAHEDRON
at 109°.
QuickTime™ and a
decompressor
NH3
Structural
Model
Lewis
Model
N H
H
H
N
Molecular
Shape
H
H
Trigonal
Pyramidal
NH3 has FOUR pairs of electrons around the
central atom: 3 bonding pairs and one lone
pair. So its geometry is based on a
tetrahedron. But with only three H attached,
QuickTime™ and a
decompressor
you SEE is a pyramid.
are needed to seewhat
this picture.
H
H2O
Lewis
Model
O H
H
QuickTime™ and a
decompressor
are needed to see this picture.
Structural
Model
Molecular
Shape
O
Bent or
Angular
H
H
H2O has FOUR pairs of electrons around
the central atom: 2 bonding pairs and 2 lone
pairs. So its geometry is based on a
tetrahedron. But with only two H attached,
what you SEE is an angle.
HF
Lewis
Model
H F
Structural
Model
H F
Molecular
Shape
Linear
HF, like LiF, is what mathematicians call a “trivial”
solution. That doesn’t mean not important, but rather
“simple”. The ONLY way you can connect two atoms
is in a line.