1 chapter 8 molecular structure & covalent bonding theories

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CHAPTER 8

Molecular Structure & Covalent Bonding Theories

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Chapter Goals

1. A Preview of the Chapter

2. Valence Shell Electron Pair Repulsion (VSEPR) Theory

3. Polar Molecules:The Influence of Molecular Geometry

4. Valence Bond (VB) Theory

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Chapter Goals

5. Linear Electronic Geometry: AB2 Species

6. Trigonal Planar Electronic Geometry: AB3 Species

7. Tetrahedral Electronic Geometry: AB4 Species

8. Tetrahedral Electronic Geometry: AB3U Species

9. Tetrahedral Electronic Geometry: AB2U2 Species

10. Tetrahedral Electronic Geometry – ABU3 Species11. Trigonal Bipyramidal Geometry12. Octahedral Geometry 13. Compounds Containing Double Bonds14. Compounds Containing Triple Bonds15. A Summary of Electronic and Molecular Geometries

Molecular Shapes and BondingMolecular Shapes and Bonding

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Stereochemistry Stereochemistry is the study of the

three dimensional shapes of molecules. Some questions to examine in this

chapter are:1. Why are we interested in shapes?

2. What role does molecular shape play in life?

3. How do we determine molecular shapes?

4. How do we predict molecular shapes?

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Molecular Shapes

The shape of a molecule plays an important role in its reactivity.

By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

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Two Simple Theories of Covalent Bonding

Valence Shell Electron Pair Repulsion Theory

– Commonly designated as VSEPR

– Principal originator

• R. J. Gillespie in the 1950’s

Valence Bond Theory

– Involves the use of hybridized atomic orbitals

– Principal originator

• L. Pauling in the 1930’s & 40’s

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VSEPR Theory In order to attain maximum stability, each

atom in a molecule or ion arranges the

electron pairs in its valence shell in such a

way to minimize the repulsion of their

regions of high electron density:

(a) Lone (unshared or nonbonding) pairs of

electrons

(b) Single bond

(c) Double bond

(d) Triple bond

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VSEPR Theory

These four types of regions of high

electron density (where the electron are)

want to be as far apart as possible. The

electrons repel each other.

There are five basic molecular shapes

based on the number of regions of high

electron density around the central atom.

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VSEPR TheoryThese are the

regions of high

electron density

around the

central atom for

two through six

electron

densities around

a central atom.

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Electron-Density Geometries

All one must do is count the number of electron density in the Lewis structure.

The geometry will be that which corresponds to that number of electron density.

HH

H:

:

Tetrahedral

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VSEPR Theory1.1. Electronic geometryElectronic geometry is determined by the

locations of regions of high electron density around the central atom(s).

2.2. Molecular geometryMolecular geometry determined by the arrangement of atoms around the central atom(s).

Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.

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Molecular Geometries

The electron-density geometry is often not the shape of the molecule, however.

The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

electron-densityGeometry - tetrahedral

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VSEPR Theory

An example of a molecule that has the same electronic and molecular geometries is methane - CH4.

Electronic and molecular geometries are tetrahedral.

H

C

HHH

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VSEPR Theory An example of a molecule that has

different electronic and molecular geometries is water - H2O.

Electronic geometry is tetrahedral. Molecular geometry is bent or angular.

H

C

HHH

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VSEPR Theory

Lone pairs of electrons (unshared pairs)

require more volume than shared pairs.

– Consequently, there is an ordering of

repulsions of electrons around central atom.

Criteria for the ordering of the repulsions:

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VSEPR Theory1 Lone pair to lone pair is the strongest repulsion.2 Lone pair to bonding pair is intermediate

repulsion.3 Bonding pair to bonding pair is weakest

repulsion. Mnemonic for repulsion strengths

lp/lp > lp/bp > bp/bp

Lone pair to lone pair repulsion is why bond angles in water are less than 109.5o.

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VSEPR Theory

lp/bp

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Multiple Bonds and Bond Angles

Double and triple bonds place greater electron density on one side of the central atom than do single bonds.

Therefore, they also affect bond angles.bp/bp

repulsion

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Nonbonding Pairs and Bond Angle

Nonbonding pairs are physically larger than bonding pairs.

Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

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Nonbonding Pairs and Bond Angle

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Polarity

In Chapter 7 we discussed bond dipoles.

But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

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Polar Molecules: The Influence of Molecular Geometry

Molecular geometry affects molecular polarity.– Due to the effect of the bond dipoles and

how they either cancel or reinforce each other.

A B A

linear molecule nonpolar

A B A

angular molecule

polar

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Polarity

By adding the individual bond dipoles, one

can determine the overall dipole moment for

the molecule.

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Polarity

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Polar Molecules: The Influence of Molecular Geometry

Polar Molecules must meet two requirements:

1. One polar bond or one lone pair of electrons on central atom.

2. Neither bonds nor lone pairs can be symmetrically arranged that their polarities cancel.

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Polarity

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Valence Bond (VB) Theory

Covalent bonds are formed by the overlapoverlap of atomic orbitals.

Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.– Process is called hybridizationhybridization.

Hybrids are common:1. Pink flowers 2. Mules

Hybrid Orbitals have the same shapes as predicted by VSEPR.

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Valence Bond (VB) Theory

Regions of High Electron

Density

Electronic Geometry

Hybridization

2 Linear sp

3 Trigonal planar

sp2

4 Tetrahedral sp3

5 Trigonal bipyramidal

sp3d

6 Octahedral sp3d2

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Molecular Shapes and BondingMolecular Shapes and Bonding

In the next sections we will use the following terminology:A = central atom

B = bonding pairs around central atom

U = lone pairs around central atom For example:

AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom.

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Linear Electronic Geometry:AB2

Species (No Lone Pairs of Electrons on A)

Some examples of molecules with this geometry are: BeCl

2, BeBr

2, BeI

2, HgCl

2, CdCl

2

All of these examples are linear, nonpolar molecules.

Important exceptions occur when the two substituents are not the same!BeClBr or BeIBr will be linear and polar!

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Linear Electronic Geometry:AB

2 Species (No Lone Pairs of Electrons

on A)Electronic Geometry

H

C

HHH

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on A)Polarity

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on A)Valence Bond Theory (Hybridization)

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Linear Electronic Geometry:AB2 Species (No Lone Pairs of

Electrons on A)

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Linear Electronic Geometry:AB2 Species (No Lone Pairs of

Electrons on A)

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Trigonal Planar Electronic Geometry: AB3 Species (No

Lone Pairs of Electrons on A) Some examples of molecules with this

geometry are: BF3, BCl3

All of these examples are trigonal planar, nonpolar molecules.

Important exceptions occur when the three substituents are not the same!BF2Cl or BCI2Br will be trigonal planar and

polar!

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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons

on A)

Dot Formula Electronic Geometry

H

C

HHH

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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of

Electrons on A)

Polarity

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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of

Electrons on A)Valence Bond Theory (Hybridization)

3s 3p

Cl [Ne]

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Trigonal Planar Electronic Geometry:

AB3 Species (No Lone Pairs of

Electrons on A)

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Trigonal Planar Electronic Geometry: AB3 Species (No Lone

Pairs of Electrons on A)

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Tetrahedral Electronic Geometry: AB

4 Species (No Lone Pairs of

Electrons on A) Some examples of molecules with this

geometry are: CH

4, CF

4, CCl

4,

SiH

4,

SiF

4

All of these examples are tetrahedral, nonpolar molecules.

Important exceptions occur when the four substituents are not the same!CF3Cl or CH2CI2 will be tetrahedral and polar!

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on A)

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Electrons on A)

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Electrons on A)

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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of

Electrons on A) Some examples of molecules with this geometry

are:

NH3, NF3, PH3, PCl3, AsH3

These molecules are our first examples of central atoms with lone pairs of electrons.Thus, the electronic and molecular geometries

are different.All three substituents are the same but

molecule is polarpolar. NH3 and NF3 are trigonal pyramidal, polar

molecules.

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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)

Valence Bond Theory

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Tetrahedral Electronic Geometry:

AB3U Species (One Lone Pair of Electrons on A)

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Electronic Geometry

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Molecular Geometry

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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)

Polarity

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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of

Electrons on A) Some examples of molecules with this geometry

are: H2O, OF2, OCl2, H2S

These molecules are our first examples of central atoms with two lone pairs of electrons.Thus, the electronic and molecular geometries are

different.Both substituents are the same but molecule is polarpolar.

Molecules are angular, bent, or V-shaped and polar.

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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of

Electrons on A)Valence Bond Theory (Hybridization)

2s 2pO [He]

four sp3 hybrids

H

C

HHH

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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs

of Electrons on A) Some examples of molecules with this

geometry are: HF, HCl, HBr, HI, FCl, IBr

These molecules are examples of central atoms with three lone pairs of electrons.Again, the electronic and molecular geometries are

different.

Molecules are linear and polar when the two atoms are different.Cl2, Br2, I2 are nonpolarnonpolar.

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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)Dot Formula

H F··

····

··

Electronic Geometry

F

H :

:

:

tetrahedral

Molecular Geometry

F

H :

:

:

3 lone pairs

linear

PolarityHF is a polar molecule.

H

C

HHH

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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs

of Electrons on A)Valence Bond Theory (Hybridization)

2s 2pF [He]

four sp3 hybrids

F

H :

:

:

tetrahedral

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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3

Some examples of molecules with this geometry

are: PF5, AsF5, PCl5, etc.

These molecules are examples of central atoms

with five bonding pairs of electrons.

The electronic and molecular geometries are the same.

Molecules are trigonal bipyramidal and nonpolar

when all five substituents are the same.

If the five substituents are not the same polar polar molecules

can result, AsF4Cl is an example.

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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and

AB2U3

Valence Bond Theory

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AB2U3

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Molecular Geometry

Trigonal Bipyramidal

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If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes.

1. One lone pair - Seesaw shape

2. Two lone pairs - T-shape

3. Three lone pairs – linear

The lone pairs occupy equatorial positions because they are 120o from two bonding pairs and 90o from the other two bonding pairs.

– Results in decreased repulsions compared to lone pair in axial position.

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AB2U3

AB4U molecules have:

1. trigonal bipyramid electronic geometry

2. seesaw shaped molecular geometry

3. and are polar

One example of an AB4U molecule is

SF4

Hybridization of S atom is sp3d.

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Molecular Geometry

H

C

HHH

Lewis Dot

Electronic Geometry

seesaw

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AB2U3

AB3U2 molecules have:

1. trigonal bipyramid electronic geometry

2. T-shaped molecular geometry

3. and are polar

One example of an AB3U2 molecule is

IF3

Hybridization of I atom is sp3d.

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AB2U3

Molecular Geometry

H

C

HHH

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AB2U3


1.trigonal bipyramid electronic geometry

2.linear molecular geometry

3.and are nonpolar


XeF2

Hybridization of Xe atom is sp3d.

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AB2U3

Molecular Geometry

H

C

HHH

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Trigonal Bipyramidal ElectronicGeometry: AB5, AB4U, AB3U2,

and AB2U3

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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2

Some examples of molecules with this geometry are: SF6, SeF6, SCl6, etc.

These molecules are examples of central atoms with six bonding pairs of electrons.

Molecules are octahedraloctahedral and nonpolar nonpolar when all six substituents are the same.If the six substituents are not the same polarpolar

molecules can result, SF5Cl is an example.

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Nonpolar


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If lone pairs are incorporated into the octahedral

structure, there are two possible new shapes.

1. One lone pair - square pyramidal

2. Two lone pairs - square planar

The lone pairs occupy axial positions because

they are 90o from four bonding pairs.

– Results in decreased repulsions compared to

lone pairs in equatorial positions.

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AB5U molecules have:

1.octahedral electronic geometry

2.Square pyramidal molecular geometry

3.and are polar.

One example of an AB4U molecule is

IF5

Hybridization of I atom is sp3d2.

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Molecular Geometry

H

C

HHH

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1.octahedral electronic geometry

2.square planar molecular geometry

3.and are nonpolar.


XeF4

Hybridization of Xe atom is sp3d2.

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Molecular Geometry Polarity

H

C

HHH

nonpolar

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Sigma () Bonds

Sigma bonds are characterized by– Head-to-head overlap.– Cylindrical symmetry of electron density about the

internuclear axis.

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Pi () Bonds

Pi bonds are characterized by– Side-to-side overlap.– Electron density above

and below the internuclear axis.

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Single Bonds

Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.

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Multiple Bonds

In a multiple bond one of the bonds is a bond and the rest are bonds.

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Compounds Containing Double Bonds

Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.

Lewis dot formula

N = 2(8) + 4(2) = 24

A = 2(4) + 4(1) = 12

S = 12 Compound must have a double bond to obey

octet rule.

CC

H

HH

H

C CH

H

H

H····

·· ·· ··

··or

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Valence Bond Theory (Hybridization)

C atom has four electrons.Three electrons from each C

atom are in sp2 hybrids. One electron in each C atom

remains in an unhybridized p orbital

VSEPR Theory suggests that the C atoms are at center of trigonal planes.

C C

H

HH

H

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An sp2 hybridized C atom has this shape.

Remember there will be one electron

in each of the three lobes.

Top view of an sp2 hybrid

The single 2p orbital is perpendicular tothe trigonal planar sp2 lobes.The fourth electron is in the p orbital.

Side view of sp2 hybrid with p orbital included.

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Two sp2 hybridized C atoms plus p orbitals in proper orientation to form C=C double bond.

The head-on overlap of the sp2 hybrids is designated

as a bond.

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The other portion of the double bond, resulting from the side-on overlap of the p orbitals, is designated as a bond.

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Thus a C=C bond looks like this and is made of two parts, one and one bond.

H

C

HHH

H

C

HHH

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Multiple Bonds In a molecule like

formaldehyde (shown at left) an sp2 orbital on carbon overlaps in fashion with the corresponding orbital on the oxygen.

The unhybridized p orbitals overlap in fashion.

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Compounds Containing Triple Bonds

Ethyne or acetylene, C2H2, is the simplest triple bond containing organic compound.

Lewis Dot FormulaN = 2(8) + 2(2) = 20

A = 2(4) + 2(1) =10

S = 10

Compound must have a triple bond to obey octet rule.

C C HHCH HC·· ·· ···· ·· or

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Valence Bond Theory (Hybridization)Carbon has 4 electrons. Two of the electrons are in sp

hybrids. Two electrons remain in unhybridized p orbitals.

VSEPR Theory suggests regions of high electron

density are 180o apart.

H C C H

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The head-on overlap of the sp2 hybrids is designated

as a bond.

The two 2p orbital are perpendicular to the sp lobes.The third and fourth electrons are in the p orbitals.

An sp hybridized C atom has this shape. Remember there will be one electron in each of the two lobes.


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A bond results from the head-on overlap of two sp hybrid orbitals.

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Compounds Containing Triple Bonds The unhybridized p orbitals form two bonds.

Note that a triple bond consists of one and

two bonds.

H

C

HHH

The final result is a bond

that looks like this.

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Larger MoleculesIn larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

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Larger Molecules

This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

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N

CN

C

CC

NC

NCH

HH

CH H

H

H

CH

HH

O

O

Here is the structure for most students’ friend: CAFFEINE

1- Assign hybridization on C, N, and O. Beware I did not put

the lone pairs of electrons into the chemical drawing.

2- How many sigma bonds are present?

3- How many pi bonds are present?

4- How many lone pairs of electrons are present? (You have to look for them)

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N

CN

C

CC

NC

NH

CH H

H

H

CH

HH

O

O

Here is the structure Theobromine, one of the components of TEA

1- Assign hybridization on C, N, and O. Beware I did not put

the lone pairs of electrons into the chemical drawing.

2- How many sigma bonds are present?

3- How many pi bonds are present?

4- How many lone pairs of electrons are present? (You have to look for them)

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End of Chapter 8

This is a difficult chapter.

Essential to your understanding of chemistry!

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s slide

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molecular polarity

electrondensity geometries