Chem 1011 – Intersession 2011
Class #9
18-May-11
1
Class 9: Molecular Geometry, Shape
and Polarity
•
Sec 10.4 – VSEPR Theory: Predicting Molecular
Geometries
▫
•
Predicting the Shapes of Larger Molecules
Sec 10.5 – Molecular Shape and Polarity
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Turning 2D into 3D
• One of the problems with drawing molecules is trying to
show their dimensionality.
• By convention, the central atom is put in the plane of the
paper.
▫ Put as many other atoms as possible in the same plane and
indicate with a straight line
▫ For atoms in front of the plane, use a solid wedge
▫ For atoms behind the plane, use a hashed wedge
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3D Representation of Molecular Shapes
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
4
5
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Predicting Molecular Geometries
• Example: Draw the polyatomic ion SiF5-, with
the proper shape.
1. Draw a Lewis Structure for the molecule
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
7
Predicting Molecular Geometries
2. Determine the number of electron groups, and
classify as being either
▫
▫
A bonding group
A lone pair
5 electron groups
5 bonding groups
0 lone pairs
Ion has the form of AX5
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Predicting Molecular Geometries
3. Based on the form the ion takes, determine the
electronic geometry and molecular geometry
• AX5
▫ Electronic Geometry – Trigonal Bipyramidal
▫ Molecular Geometry – Trigonal Bipyramidal
Si
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Problems
•
Draw the Lewis structures and predict the
geometry of:
(a) ICl4(b) ClO4-
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
10
Predicting the Shapes of Larger
Molecules
• Many molecules have larger structures with many
interior atoms.
• We can think of them as having multiple central atoms.
• When this occurs, we describe the shape around each
central atom in sequence.
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Predicting the Shapes of Larger
Molecules
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Predicting the Shapes of Larger
Molecules
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
13
Molecular Shape and Polarity
•
•
•
In order for a molecule to be polar, it must
1) have polar bonds
2) be unsymmetric
Polarity affects the intermolecular forces of attraction.
Nonbonding pairs affect molecular polarity
Molecule
oriented with d+
towards
negative plate
and d- towards
positive plate.
Molecules
oriented
randomly.
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Molecular Shape and Polarity
• The H–Cl bond is polar. The bonding electrons
are pulled toward the Cl end of the molecule.
The net result is a polar molecule.
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Note: In all cases
where the dipoles of
two or more polar
bonds cancel, the
bonds are assumed
to be identical. If
one or more of the
bonds are different
from the other(s),
the dipoles will not
cancel and the
molecule will be
polar.
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
16
Molecular Shape and Polarity
•
To determine if a molecule is polar:
1.
Draw a Lewis structure for the molecule and determine
the molecular geometry
2.
Determine whether the molecule contains polar bonds
▫
3.
Do the bonded atoms have different electronegativities?
Determine if the polar bonds add together to form a
net dipole.
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Molecular Shape and Polarity
• The O–C bond is polar. The bonding electrons
are pulled equally toward both O ends of the
molecule. The net result is a nonpolar molecule.
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Molecular Shape and Polarity
• The H–O bond is polar. Both sets of bonding
electrons are pulled toward the O end of the
molecule. The net result is a polar molecule.
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
19
Molecular Shape and Polarity
• The H–N bond is polar. All the sets of bonding
electrons are pulled toward the N end of the
molecule. The net result is a polar molecule.
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Molecular Shape and Polarity
a) The resultant of two of the C-Cl bond
dipoles is shown as a red arrow; that
of the other two as a blue arrow
▫
The red and blue arrows point in
opposite directions, the dipoles cancel,
and the molecule is non polar.
b) The individual bond dipoles do not
cancel; they combine to yield a
resultant dipole moment as indicated
by the red arrow.
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Problem
•
For the given molecules / ions, draw the Lewis
structure, predict the geometry, sketch the molecule
and predict whether or not it is a polar molecule.
▫
▫
▫
▫
▫
▫
OCl2
SiCl4
CHI3
C2H4
CH3OH
ICl4–
Molecular Geometry, Shape and Polarity
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Chem 1011 – Intersession 2011
Class #9
18-May-11
22
Later Today
•
Sec 10.6 – Valence Bond Theory: Orbital Overlap
as a Chemical Bond
•
Sec 10.7 – Valence Bond Theory: Hybridization
of Atomic Orbitals
▫
▫
▫
▫
▫
sp3 Hybridization
sp2 Hybridization and Double Bonds
sp Hybridization and Triple Bonds
sp3d and sp3d2 Hybridization
Writing Hybridization and Bond Schemes
Molecular Geometry, Shape and Polarity
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