Transcript Slide 1

PowerPoint to accompany
Chapter 8
Molecular
Geometry
and Bonding
Theories
Molecular Shapes

The shape of a molecule
plays an important role in
its reactivity.

By noting the number of
bonding and nonbonding
electron pairs, we can
easily predict the shape of
the molecule.
Figure 8.2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
CH4 , C , H2O
(SO4
-2
)
, (SiO4
-4
)
(PO4)-3 , CCl2F2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
What Determines the Shape
of a Molecule?

Simply put, electron pairs,
whether they are bonding
or nonbonding, repel each
other.

By assuming the electron
pairs are placed as far as
possible from each other,
we can predict the shape
of the molecule.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Valence Shell Electron Pair
Repulsion Theory (VSEPR)
“The best arrangement of a given
number of electron domains is the one
that minimizes the repulsions among
them.”
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domains
This molecule has
four electron domains.

We can refer to the
electron pairs as electron
domains.

In a double or triple bond,
all electrons shared
between those two atoms
are on the same side of
the central atom.
Therefore, they count as
one electron domain.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron-Domain Geometries
These are the
electron-domain
geometries for two
through six electron
domains around a
central atom.
Table 8.1
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron-Domain Geometries

All one must do is
count the number of
electron domains in
the Lewis structure.

The geometry will be
that which
corresponds to that
number of electron
domains.
Figure 8.6
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Geometries

The electron-domain geometry is often not the
shape of the molecule, however.

The molecular geometry is defined by the
positions of only the atoms in the molecules, not
the nonbonding pairs.
Figure 8.6
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Linear

In this domain, there is only one molecular
geometry: linear.

NOTE: If there are only two atoms in the
molecule, the molecule will be linear no matter
what the electron domain geometry is.
Table 8.2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Geometries
Within each electron
domain structure,
there might be more than
one molecular geometry.
Given these examples, try
and draw the electron
domain geometry for OH- ,
The hydroxide ion.
Once you try this,
look at the notes for more
chemical consequences of
these simple geometries.
Table 8.2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Trigonal
Planar

There are two molecular geometries:


Trigonal planar, if all the electron domains are bonding
Bent, if one of the domains is a nonbonding pair.
Table 8.2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Tetrahedral

There are three molecular geometries:



Tetrahedral, if all are bonding pairs
Trigonal pyramidal, if one is a nonbonding pair
Bent, if there are two nonbonding pairs
Table 8.2
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain:
Trigonal Bipyramidal

There are two distinct
positions in this
geometry:

Figure 8.8


Axial (Apical if
degenerate)
Equatorial
Name and draw the 4
possible degenerate
geometries.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Trigonal
Bipyramidal

Lower-energy conformations result from
having nonbonding electron pairs in
equatorial, rather than axial, positions in
this geometry.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Trigonal
Bipyramidal

There are four distinct molecular geometries in
this domain:




Trigonal bipyramidal
Seesaw
T-shaped
Linear
Table 8.3
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Domain: Octahedral

All positions are equivalent in the octahedral
domain. There are three molecular geometries:



Octahedral
Square pyramidal
Square planar
Table 8.3
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
The Effect of Nonbonding
Electrons and Multiple Bonds
on Bond Angles

Nonbonding pairs are physically
larger than bonding pairs (all charge
and virtually no mass to constrain
them).

Therefore, their repulsions are
greater; this tends to decrease bond
angles in a molecule.
Figure 8.7
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
The Effect of Nonbonding
Electrons and Multiple Bonds
on Bond Angles

Double and triple bonds
place greater electron
density on one side of
the central atom than
do single bonds.

Therefore, they also
affect bond angles.
For hazards and gas
warefare read notes.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Shapes of Larger Molecules
In larger molecules, it
makes more sense to
talk about the
geometry of a
particular atom rather
than the geometry of
the molecule as a
whole.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Shape and
Molecular Polarity
If a molecule
possesses polar
bonds, it does not
mean the molecule as
a whole will be polar.
Figure 8.11
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Shape and
Molecular Polarity
Figure 8.12
By adding the
individual bond
dipoles, one can
determine the overall
dipole moment for the
molecule.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecules Containing Polar
Bonds
Figure 8.13
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
F12
F22
Ozone-depleting gas trends
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Chapter 8 End of Part 1


Molecular Shapes
Electron Domain Shapes
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia