Chemistry
Notes
Bonding and Molecular Shapes
Molecular Shapes
 Recall: Properties of elements are
consequences of their atoms
 Properties of molecules are
consequences of their structure
(bonding arrangement)
 Shapes of molecules give them
certain properties
Molecular Shapes
 Ex: NH3, H2O and C6H14
 Different molecular Shapes:
 http://dwb4.unl.edu/ChemAnime/mole_struct
ure.htm
Molecular Shapes
Two explanations for how shapes of
molecules arise
 VSEPR theory – deals with the repulsive
forces of electrons on each other
 Atomic orbital overlap (hybridization and
bonding)
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
 Recall Lewis Structures. There are two types
of electrons pairs to consider.
..
Unshared pair of electrons
H– N–H
Share pair of bonded electrons
H
 Each pair of shared and unshared electrons
forms a negatively charged clouds that repel
each other.
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
 Two reasons for this repulsion:
 Electrons have same charges (electrostatic
forces)
 Pauli Exclusion Principle: two electrons of
the same spin can not be in the same orbital
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
 The repulsion between charge clouds in
the outer level of an atom determines the
arrangement of the orbital
 Electron clouds spread out as far as
possible to minimize repulsion forces
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
 Shared and Unshared pairs of electrons
within a molecule determine the shape of the
molecule.
 Unshared electrons pairs are acted on by only one
nucleus therefore occupy more space
 Shared (bonded) electron pairs are acted on by two
nuclei and therefore occupy less space
 Because unshared electron pairs occupy
more space they also create more of a
repulsive force than shared pairs of
electrons.
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
 Unshared electrons pairs are acted on by only
one nucleus therefore occupy more space
 Shared (bonded) electron pairs are acted on by
two nuclei and therefore occupy less space
VSEPR Theory
( Valance Shell Electron Pair Repulsion
theory)
Comparison of electron pair repulsion
Repulsion
between two
unshared pairs
of electrons
>
Repulsion
between a
pair of
unshared
electrons
and a pair of
shared
electrons
>
Repulsion
between two
unshared
pairs of
electrons
Examples of Molecular
Shapes
CH4
All electron pairs are equal in
repulsion
All angles = 109.5o
CH4 is tetrahedral
Examples of Molecular
Shapes
 NH3
 One unshared pair of
electrons put a greater
force on bond
electrons
 Bond angles = 107.3o
 NH3 is trigonal planar
Examples of Molecular
Shapes
 Two unshared pair of
electrons put a greater
force on bond electrons
 Bond angles = 104.5o
 H2O is bent
Other Types of Geometric
Molecular Shapes




T – planer
Octahedral bi-pyramidal
Linear
Trigonal bi-pyramidal
Electron Pair and Molecular Geometry
Bond
Angle
Electron
Molecular
Pair
Geome Example
Geome
try
try
0
180
linear
linear
BeF2
3
0
120
trigonal
planar
trigonal
planar
BF3
3
2
1
120
trigonal
planar
bent
NO2-
4
4
0
109.5
tetrahedral tetrahedral CCl4
NH3
H2O
Total # of
electro
n pairs
Shared
2
2
3
Unshared
4
3
1
109.5
trigonal
tetrahedral
pyrami
dal
4
2
2
109.5
tetrahedral bent
Bonding and Molecular
Shape
 Key Point: the differences in molecular
shape are due to the unequal space
occupied by unshared and shared
electrons.