Chemical Bonding:
Molecular Shapes
VSEPR Theory
• From a correct Lewis structure, we can get to
the 3-D shape using this theory.
• VSEPR stands for valence shell electron pair
repulsion.
• The theory is based on the idea that e- pairs
want to get as far away from each other as
possible!
VSEPR Categories
• There are 5 categories from which all molecular
shapes derive.
Drawing w/ Perspective:
Dash Diagrams
• We use the conventions below to depict a 3-D
object on a 2-D surface.
Determining 3-D Shape
• The 5 categories are a starting point.
• To determine the 3-D shape of a molecule, we
consider the # of atoms and the # of e- pairs that
are associated w/ the central atom.
• All the possibilities for molecular geometry can be
listed in a classification chart.
Steps to Determine Molecular Shape
1) Draw Lewis structure.
2) Count # of bonds and lone pair e-’s on
the central atom.
3) Select geometric category.
4) Place e-’s and atoms that lead to most
stable arrangement (minimize erepulsions).
5) Determine 3-D shape.
Linear/Trigonal Planar Geometries
• First, we have the linear and trigonal planar
categories.
Category
Bonds
Lone Pairs
Geometry
Linear
2
0
linear
Trigonal
planar
3
0
2
1
trigonal
planar
bent
Like Charges Repel
• These clouds of electrons, areas of negative charge
repel each other in such a way as to get as far apart
from each other as possible.
• When there are only two electron clouds, the
farthest they can get away from each other is 180o, a
straight line.
Three Clouds
• With three electron clouds, the farthest the
electron clouds can get away from each other is
120o, the corners of an equilateral triangle. This
shape is known as trigonal planer.
Tetrahedral Geometries
Category
Bonds
Lone Pairs Geometry
Tetrahedral
4
0
tetrahedral
3
1
pyramidal
2
2
bent
1
3
linear
Four clouds
• When four clouds are attached to a central atom,
the farthest they can get away from each other is
the four corners of a tetrahedron. The
tetrahedral angle is 109.5o.
Non-bonding clouds also repel both bonding
and non-bonding clouds.
• NH3, ammonia, has a non-bonding pair of electrons. The
central atom, N, is surrounded by 3 bonding and 1 nonbonding pair.
• These four clouds form roughly a tetrahedron, but because
non-bonding pairs “spread out” more than bonding pairs, the
angle between atoms is 107o instead of 109.5o.
• The atoms form a pyramidal shape.
H2O, water, has 2 bonding and 2 nonbonding pairs of electrons.
• These four clouds form roughly a tetrahedron, but
because non-bonding pairs “spread out” more
than bonding pairs, the angle between atoms is
105o instead of 109.5o.
• The atoms form a bent shape.
Distortion of Angles
• Lone pair e-’s take up a lot of room, and they distort the
angles seen in the geometric categories.
Trigonal Bipyramidal Geometries
Category
Trigonal
Bipyramidal
Bonds Lone Pairs
Geometry
5
0
4
3
1
2
trigonal
bipyramidal
see-saw
T-shaped
2
1
3
4
linear
linear
Trig Bipy is Special
• In other categories, all
positions are
equivalent.
• In trig bipy, lone pairs
always choose to go
equatorial first.
• Why?
Octahedral Geometries
Category
Octahedral
Bonds Lone Pairs
Geometry
6
0
octahedral
5
1
4
2
square
pyramidal
square planar
3
3
T-shaped
2
4
linear
1
5
linear