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.

?


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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.


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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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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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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


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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!


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Figure of shapes (GROUPS)


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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.


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Figure of shapes (2 to 4 GROUPS)


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Figure of shapes (5 GROUPS)


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Figure of shapes (6 GROUPS)


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Comment

With experience, we tend to start

drawing Lewis dot structures

with molecular geometry

information included…

instead of

instead of


.


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


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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.


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Tetrahedral arrangement (advanced)


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Trigonal planar arrangement (advanced)


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Problem

What is the arrangement of electron groups, and geometry around the central

atom for the following molecules?

SF2 XeO4

H3O+ AsF5


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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.


.


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.


.


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.


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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


.


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


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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.


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Geometry and dipole moments


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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.


.


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


.


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?


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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.


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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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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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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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Oxygen atom orbital diagram

We would expect water to have a 90 angle between its bonds, based on the atomic orbitals on oxygen.


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Oxygen hybrid orbitals

plus gives


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Oxygen hybrid orbitals

plus givesOne s and

three p orbitals

combine to

give four sp3

hybrid orbitals


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In general:A total of n atomic orbitals combine to give n hybrid

orbitals of a given kind.


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Hybrid orbitals


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Oxygen hybrid orbital diagram

We would expect water to have a ~109.5 angle based on the hybrid sp3 orbitals on oxygen.


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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


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Problem

Describe the bonding of I3-

in terms of valence bond theory.


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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.


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Ethene has a double bond

Notice we’ve chosen to create sp2 hybrid orbitals

No orbital overlap between these p orbitals


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Ethyne has a triple bond

Notice we’ve chosen to create sp hybrid orbitals and

not sp3 or sp2


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Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory...

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