Chapter 10 Molecular Geometry and Chemical Bonding Theory Dr. Peter Warburton [email protected]

Chapter 10Molecular Geometry and Chemical Bonding Theory

Dr. Peter [email protected]://www.chem.mun.ca/zcourses/1011.php

© Peter Warburton 2008All media copyright of their respective

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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 Model

We try and connect the information of electron distribution in a Lewis dot structure to molecular geometry by using the Valence-Shell Electron-Pair

Repulsion (VSEPR) theory.

GROUPS of electrons repel each other, ending up as far from each other as

physically possible.


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The textbook talks about repulsion of ELECTRON PAIRS. I prefer the term repulsion of ELECTRON GROUPS

because multiple bonds are treated as ONE PAIR of electrons in VSEPR

theory even though there are more than one pair of electrons present.


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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

Looks at “shape” of electron group

distribution

Looks at “shape” of nuclear positions

around the central atom

INCLUDES lone pairs

No terminal nuclei on lone pairs means we IGNORE (“can’t see”)

all lone pairs.


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Electron distribution vs. geometry

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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Modified Figure 10.2 (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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Modified Fig 10.4 (2 to 4 GROUPS)


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Modified Fig 10.7 (5 GROUPS)


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Modified Fig 10.7 (6 GROUPS)


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Notice!

With experience, we tend to start

drawing Lewis dot structures with

molecular geometry

information included…

instead of

instead of


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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 (Figure 10.2)

3. Use the number of lone pairs and the arrangement (Figures 10.5 and 10.7) to determine molecular geometry


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Advanced geometry considerations

Lone pairs are the “biggest” electron group (better at repelling other electron

groups).

Triple bonds are the next “biggest” group.

Double bonds are “smaller”.

Single bonds are the “smallest” electron group.


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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.


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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.


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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


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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.


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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


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Problem 10.24

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 the

same thing.The resultant hybrid has

similarities to the original things, but is distinctly different

from both.


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Labradoodle

A Labradoodle is a hybrid dog breed that mixes Labrador retrievers with poodles.

+


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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 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 structure2. Use VSEPR theory to predict electron

group arrangement3. Use Table 10.2 to determine what hybrid orbitals have the same arrangement4. Create the hybrid orbital diagram based on changing the ground state diagram


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Problem 10.32

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

There is 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

There is orbital overlap between two different sets of p orbitals

Documents

Chapter 10 Molecular Geometry and Chemical Bonding Theory Dr. Peter Warburton [email protected]