Transcript Molecular Geometry

Molecular Geometry
VSEPR
Molecules in 3D
We saw in our last class that to properly address covalent bonding we
need to use a pictorial approach, so we turn to Lewis Structures.
These are excellent because they give us a great perspective on the
electronic arrangement in a molecule.
There is a drawback to a basic Lewis diagram and that is we do not
get a sense for how the molecule exits in 3D.
VSEPR Theory
Valence Shell Electron Pair Repulsion Theory . . . Huh?!?!
 Electrons take up space!
Electrons are all negatively charged, therefore they don’t want to be
near other electrons.
Electron pairs will repel other electron pairs.
Molecules will orient themselves in 3 dimensions to minimize the
interactions of all the electron pairs.
A lone pair takes up more space than a bonding pair (ie they push
bonding pairs further away).
Possible 3D Shapes
# of
groups
connected
to the
central
atom
# of lone Shape
pairs on
the
central
atom
Bond
Angles
Example
-
-
Linear Diatomic
2
0
Linear Triatomic
180 º
CO2
2
2
Bent (angular)
104.5 º
H2O
3
0
Trigonal Planar
120 º
BF3
3
1
Trigonal Pyramidal
107 º
NH3
4
0
Tetrahedral
109.5 º
CH4, CCl4
5
0
Trigonal Bi-pyramidal 120 º, 90 º PCl5
6
0
Octahedral
HCl
90 º
PCl6-
Linear
Bond angle of 180 º minimizes all e- - einteractions, giving maximum separation between
all substituents (things connected to) on the
central atom.
Tetrahedral
Bond angle of 109.5 º minimizes all e- - e- interactions, giving
maximum separation between all substituents (things connected to)
on the central atom.
The tetrahedral geometry is very stable and very strong.
Diamond
Tetrahedral Lattice of Carbon Atoms
A Diamond is the among the “hardest” substances in the known
universe. It gains this strength from its molecular geometry, each
carbon has a maximum spacing in the tetrahedral lattice, minimizing
any destabilizing interactions.
What about Silicon Dioxide?
Quartz (crystalline SiO2)
We know it’s NOT linear
triatomic, so what’s the deal?
Sand (crushed crystalline SiO2)
Structure and Bonding in Silicon Dioxide
Every Si centre is
tetrahedral
Every O centre is bent
Remarkably strong when a
crystalline solid (quartz)
Structure and Bonding in Silicon Dioxide
SiO2
Amorphous and
Everywhere
SiO2 is also common glass, it has a structure much different than
Quartz. All of the bonds are the same; however typical glass “looks”
like a liquid that has been flash frozen. It’s arrangement is irregular
and fluid.
Tetrahedral
Bond angle of 109.5 º minimizes all e- - e- interactions, giving
maximum separation between all substituents (things connected to)
on the central atom.
The tetrahedral geometry is very stable and very strong.
Trigonal Planar and Pyramidal
120 º bond angle
between each F atom,
maximum separation
107 º bond angle
between each F atom,
maximum separation
Derived from a
tetrahedral arrangement,
with the “top” atom
replaced by a lone pair.
BENT
Only 90 º between H and lone pair.
104.5 º between the 2 H atoms
Water  V shaped or BENT
Derived from a
tetrahedral arrangement,
with 2 atoms replaced by
lone pairs.
Practice Time
What are the shapes and bond angles in each of the molecules from
yesterday?
HCN, N2H4, CO2, CO, NI3, SCl2, AsCl3, PCl3, CH4, NH4+, HCl, BH3, O3,
CCl4, SiCl4, SiO2, CH2Cl2, IF, AlCl3, CH2O, CH2S
# of groups # of lone
Shape
connected to pairs on the
the central
central atom
atom
Bond
Angles
Example
-
-
Linear Diatomic
HCl
2
0
Linear Triatomic
180 º
CO2
2
2
Bent (angular)
104.5 º
H2O
3
0
Trigonal Planar
120 º
BF3
3
1
Trigonal Pyramidal 107 º
NH3
4
0
Tetrahedral
CH4, CCl4
109.5 º