Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory 10.1 Artificial Sweeteners: Fooled by Molecular Shape (Suggested Reading) 10.2 VSEPR Theory: The Five Basic Shapes [10.1] 10.3 VSEPR Theory: The

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Chapter 10 Chemical Bonding II Molecular
Shapes, Valence Bond Theory, and Molecular
Orbital Theory 10.1 Artificial Sweeteners
Fooled by Molecular Shape (Suggested Reading)
10.2 VSEPR Theory The Five Basic Shapes 10.1
10.3 VSEPR Theory The Effect of Lone Pairs
10.1 10.4 VSEPR Theory Predicting Molecular
Geometries 10.1 10.5 Molecular Shape and
Polarity 10.2 10.6 Valence Bond Theory
Orbital Overlap as a Chemical Bond 10.3 10.4
10.7 Valence Bond Theory Hybridization of
Atomic Orbitals 10.3 10.4
Chemistry 1011 Y8Y,U Paul G. Mezey
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Chapter 10 Chemical Bonding II
Lewis dot structures

• .

Lewis dot structures only give an idea of the
electron distribution in the species. There is
NO INFORMATION about the molecular geometry,
which depends on the relative position of nuclei
around the central atom.
?
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
VSEPR Model

• .

One may connect the information of electron
distribution in a Lewis dot structure to
molecular geometry by using the Valence-Shell
Electron-Pair Repulsion (VSEPR) theory. The
essence of the VSEPR theory GROUPS of electrons
repel each other, ending up as far from each
other as possible.
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Some textbooks talk about repulsion of ELECTRON
PAIRS. The term repulsion of ELECTRON GROUPS
is perhaps better, because multiple bonds are
treated the same way as ONE PAIR of electrons in
VSEPR theory even though in a multiple bond there
are more than one pair of electrons present.

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Chapter 10 Chemical Bonding II

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Considering directions around a central atom, A
lone pair is ONE GROUP of electrons A single bond
is ONE GROUP of electrons A double bond is ONE
GROUP of electrons A triple bond is ONE GROUP of
electrons
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Paul G. Mezey
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Chapter 10 Chemical Bonding II
Electron distribution vs. geometry

• .

Electron distribution Molecular geometry
The shape of electron group distribution The shape of nuclear positions around the central atom
The shape of electron distribution INCLUDES all lone pairs Lone pairs influence molecular geometry, but they are not part of this shape, since there are no terminal nuclei on lone pairs
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Electron distribution vs. geometry

• .

For simple molecules with a central atom If the
central atom has NO lone pairs on it, then the
electron group distribution and the molecular
geometry ARE THE SAME!
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Figure of shapes (GROUPS)

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Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
AXn notation

• .

The central atom A is bonded to n atoms or
functional groups, denoted as X. This notation
ignores lone pairs, so it is suited for
categorizing molecular geometries, which also
ignore lone pairs.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Figure of shapes (2 to 4 GROUPS)

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Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Figure of shapes (5 GROUPS)

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Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Figure of shapes (6 GROUPS)

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Paul G. Mezey
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Chapter 10 Chemical Bonding II
Comment

• .

With experience, we tend to start drawing Lewis
dot structures with molecular geometry
information included
instead of
instead of
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Getting geometry information

• .

• 1. Draw the Lewis dot structure
• 2. Determine the number of electron groups on
  the central atom to get electron group
  arrangement
• Use the number of lone pairs and the arrangement
  to determine the molecular
• geometry

Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Advanced geometry considerations

• .

Lone pairs are the biggest electron groups
(best at repelling other electron
groups). Triple bonds are the next biggest
groups. Double bonds are smaller. Single
bonds are the smallest electron groups.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Tetrahedral arrangement (advanced)

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Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Trigonal planar arrangement (advanced)

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Chapter 10 Chemical Bonding II
Problem

• .

What is the arrangement of electron groups, and
geometry around the central atom for the
following molecules? SF2 XeO4 H3O AsF5
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Molecular dipole moments

• .

If there are polar covalent bonds (partial charge
separations) in a molecule, the molecule MAY OR
MAY NOT have a permanent dipole moment. A
permanent dipole moment means there are regions
of the entire molecule that are permanently
partially negative and permanently partially
positive.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Permanent dipole moments

• .

To determine if a molecule has a permanent dipole
moment, we add together the vectors
that describe the charge separation of
polar covalent bonds.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Permanent dipole moments

• .

To add vectors, we chain vectors by putting the
tail of the next vector on the head of the
previous vector. The resultant vector is then
drawn from the tail of the first vector to the
head of the last vector in the chain. This
resultant vector is the permanent dipole moment.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Recall HCl

• .

We saw earlier that the diatomic molecule HCl has
a polar covalent bond. Since there is only one
bond, this one vector of charge separation ALSO
describes the permanent dipole moment of HCl.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Water

• .

The permanent dipole moment in water can be seen
by adding together the charge separation vectors
of the two polar covalent O-H bonds.
Permanent dipole moment
Lewis structure
Adding vectors
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Symmetry and dipole moments

• .

A molecule with more than one polar bond MIGHT
NOT have a permanent dipole moment when the
charge separations are symmetrically distributed
so that the resultant vector sums up to to zero.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Geometry and dipole moments

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Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Permanent dipole moments and molecular properties

• .

Ionic bonds are generally strong because of the
strong electrostatic attraction between positive
and negative charges. Molecules with permanent
dipole moments have regions with partial positive
and negative charges that attract the opposite
regions on other molecules of the same type.
Such intermolecular forces affect the bulk
properties of collections of molecules.
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Quantifying dipole moments

• .

Dipole moments measure the amount of charge
separation (in Coulombs) that occurs over the
bond length (in meters) in a derived unit called
a debye (D) 1 D 3.34 x 10-30 C?m
Dipole moment for HCl is 1.08 D
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Problem

• .

• The molecule BrF3 has a dipole moment of 1.19 D.
  Which of the following geometries are possible
  trigonal planar, trigonal pyramidal, or T-shaped?
• b) The molecule TeCl4 has a dipole moment of 2.54
  D. Is the geometry tetrahedral, seesaw, or
  square planar?

Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Valence bond theory

• .

• Bonds form between atoms when
• Orbitals (the allowed electron distributions)
  in the atoms overlap to create molecular bonding
  orbitals.
• 2. Each molecular bonding orbital has NO MORE
  THAN 2 electrons in it.

Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Bond strength

• .

Covalent bonds are strongest when there is
maximum orbital overlap between atomic orbitals.
This maximum overlap occurs in the same direction
as the atomic orbitals point.
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Chapter 10 Chemical Bonding II
Hybrids

• .

Hybrids occur when we mix two or more different
types of things from the same class. The
resultant hybrid shows similarities to the
original things, but is distinctly different from
them.
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Chapter 10 Chemical Bonding II
Hybrid orbitals

• .

The atomic orbitals of atoms can be mixed
together (WHEN REQUIRED!) to form hybrid
atomic orbitals that are different from the
source orbitals. Such hybrid orbitals are
used to better explain molecular geometry and
bonding.
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Chapter 10 Chemical Bonding II
Oxygen atom orbital diagram

• .

We would expect water to have a 90? angle between
its bonds, based on the atomic orbitals on oxygen.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals

• .

plus
gives
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals

• .

plus
gives
One s and three p orbitals combine to give
four sp3 hybrid orbitals
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
In general

• .

A total of n atomic orbitals combine to give n
hybrid orbitals of a given kind.
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Paul G. Mezey
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Chapter 10 Chemical Bonding II
Hybrid orbitals

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Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Oxygen hybrid orbital diagram

• .

We would expect water to have a 109.5? angle
based on the hybrid sp3 orbitals on oxygen.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Determining hybrid orbitals diagrams

• .

• Draw the Lewis dot structure
• Use VSEPR theory to predict electron group
  arrangement
• Use Table 10.2 to determine what hybrid orbitals
  have the same arrangement
• 4. Create the hybrid orbital diagram based on
  changing the ground state diagram

Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Problem

• .

Describe the bonding of I3- in terms of
valence bond theory.
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Multiple bonding

• .

Multiple bonds (double or triple bonds) are
possible when more than one set of orbitals can
overlap between two atoms. The first bond is the
sigma (s) bond, which occurs from orbital overlap
on the axis between the atoms. The second and
third bonds are pi (p) bonds that occur from
orbital overlap both above and below the axis
between the two atoms.
Molecular Geometry and Chemical Bonding,
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Chapter 10 Chemical Bonding II
Ethene has a double bond

• .

No orbital overlap between these p orbitals
Notice weve chosen to create sp2 hybrid orbitals
Molecular Geometry and Chemical Bonding,
Paul G. Mezey
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Chapter 10 Chemical Bonding II
Ethyne has a triple bond

• .

Notice weve chosen to create sp hybrid orbitals
and not sp3 or sp2
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Chapter 10 Chemical Bonding II

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Molecular Geometry and Chemical Bonding,
Paul G. Mezey