Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
January 25, 2010 – Lecture 8
Lecture 8
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10.4 VSEPR Theory: Predicting Molecular
Geometries
– Predicting the Shapes of Larger Molecules
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10.5 Molecular Shape and Polarity
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Chem 1011
Dr. L. Dawe
Winter 2010
Representing Three-Dimensional Shapes on a
Two-Dimensional Surface
• 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.
Chem 1011
Dr. L. Dawe
Winter 2010
Representing Three-Dimensional Shapes on a
Two-Dimensional Surface
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Chem 1011
Dr. L. Dawe
Lecture 8
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
VSEPR Theory: Predicting Molecular Geometries
Lecture 8
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Example: Draw the Lewis structure for the molecule
SiF5-.
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1. Draw a Lewis
structure for the
molecule.
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2. Determine the Total number of electron groups around Si = 5
total number of (each single bond counts as one electron group)
electron groups
around the
central atom.
Chem 1011
Dr. L. Dawe
Winter 2010
VSEPR Theory: Predicting Molecular Geometries
3. Determine the 5 Bonding Pairs
number of
0 Lone Pairs
bonding groups AX
5
and the number
of lone pairs
around the
central atom.
Dr. L. Dawe
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4. Determine the AX5
electron
Electron geometry: Trigonal bipyramidal
geometry and
Molecular geometry: Trigonal bipyramidal
the molecular
geometry.
Chem 1011
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Winter 2010
VSEPR Theory: Predicting Molecular Geometries
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Problem: Draw the Lewis structures and predict the geometry of:
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(a) ICl4(b) ClO4-
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Chem 1011
Dr. L. Dawe
Problem: Draw the Lewis structures and predict the geometry of:
(a) ICl4(b) ClO4-
Lecture 8
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
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.
Lecture 8
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Chem 1011
Dr. L. Dawe
Winter 2010
Predicting the Shapes of Larger Molecules
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Chem 1011
Dr. L. Dawe
Winter 2010
Predicting the Shapes of Larger Molecules
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
Lecture 8
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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
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Molecule
oriented with δ+
towards
negative plate
and δ- towards
positive plate.
Molecules
oriented
randomly.
Chem 1011
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
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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.
Chem 1011
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
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.
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
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To determine if a molecule is polar:
1.
2.
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Draw a Lewis structure for the molecule and determine the
molecular geometry
Determine whether the molecule contains polar bonds
Do the bonded atoms have different electronegativities?
3.
Lecture 8
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Determine if the polar bonds add together to form a net
dipole.
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Chem 1011
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
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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.
Chem 1011
Dr. L. Dawe
Molecular Shape and Polarity
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Winter 2010
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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.
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
Lecture 8
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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.
Chem 1011
Dr. L. Dawe
___________________________________
Winter 2010
Molecular Shape and Polarity
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(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.
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(b) The individual bond dipoles do not cancel;
they combine to yield a resultant dipole
moment as indicated by the red arrow.
Chem 1011
Dr. L. Dawe
Winter 2010
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Chem 1011
Dr. L. Dawe
Winter 2010
Molecular Shape and Polarity
Looking Ahead: January 27, 2010 – Lecture 9
Problem: For the given table, draw the Lewis structure, predict
the geometry, sketch the molecule and predict whether or
not it is a polar molecule.
10.6 Valence Bond Theory: Orbital Overlap as a
Chemical Bond
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
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Chem 1011
Dr. L. Dawe
Lecture 8
Problem: For the given table, draw the Lewis structure, predict the geometry, sketch the molecule and
predict whether or not it is a polar molecule.
Molecular
Formula
Lewis Diagram
Shape Around
Central Atom(s)
Shape Diagram and
Bond Dipoles
Polarity of
Molecule
OCl2
SiCl4
CHI3
C2H4
CH3OH
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Chem 1011
ICl4-
Dr. L. Dawe
Lecture 8
10