Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory...
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Transcript of Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory...
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
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
?
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
VSEPR ModelOne 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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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!
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Figure of shapes (GROUPS)
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Figure of shapes (2 to 4 GROUPS)
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Figure of shapes (5 GROUPS)
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Figure of shapes (6 GROUPS)
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Comment
With experience, we tend to start
drawing Lewis dot structures
with molecular geometry
information included…
instead of
instead of
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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
3.Use the number of lone pairs and the arrangement to determine the molecular
geometry
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Tetrahedral arrangement (advanced)
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Trigonal planar arrangement (advanced)
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Problem
What is the arrangement of electron groups, and geometry around the central
atom for the following molecules?
SF2 XeO4
H3O+ AsF5
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
:
:
Cl-H
Cl-H
δδ
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Lewis structure Adding vectorsPermanent
dipole moment
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Geometry and dipole moments
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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 Cm
:
Cl-H
Dipole moment for
HCl is 1.08 D
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Problem
a) 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?
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Valence bond theory
Bonds form between atoms when:
1.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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Oxygen atom orbital diagram
We would expect water to have a 90 angle between its bonds, based on the atomic orbitals on oxygen.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Oxygen hybrid orbitals
plus gives
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Oxygen hybrid orbitals
plus givesOne s and
three p orbitals
combine to
give four sp3
hybrid orbitals
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
In general:A total of n atomic orbitals combine to give n hybrid
orbitals of a given kind.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Hybrid orbitals
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Oxygen hybrid orbital diagram
We would expect water to have a ~109.5 angle based on the hybrid sp3 orbitals on oxygen.
Chapter 10 Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Determining hybrid orbitals diagrams
1. Draw the Lewis dot structure
2. Use VSEPR theory to predict electron group arrangement
3. 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
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Problem
Describe the bonding of I3-
in terms of valence bond theory.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
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 () bond, which occurs from orbital overlap on the axis between the atoms.
The second and third bonds are pi () bonds that occur from orbital overlap both above and below the axis between the two atoms.
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Ethene has a double bond
Notice we’ve chosen to create sp2 hybrid orbitals
No orbital overlap between these p orbitals
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey
Ethyne has a triple bond
Notice we’ve chosen to create sp hybrid orbitals and
not sp3 or sp2
Chapter 10 Chemical Bonding II
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Molecular Geometry and Chemical Bonding, Paul G. Mezey