Chapter 4

Molecular Geometry and Bonding Theories

Examples

Dr.Harbi

Valence Shell Electron Pair Repulsion (VSEPR) Theory

• based on idea that regions of electron density in valence shell of central atom will be distributed in space such that electrostatic repulsions are minimized

• places regions of electron density as far apart as possible

• produces molecular geometry

Steps in Predicting Molecular Geometry

• draw Lewis structure of substance• count regions of electron density on

central atom• draw electron pair shape• derive and draw molecular geometry

Regions of Electron Density

• single covalent bond• double covalent bond• triple covalent bond• lone pair• unpaired electron

# Regions Shape

2 linear

180°

# Regions Shape

2

3

linear

trigonal planar

180°

120°

# Regions Shape

2

3

linear

trigonal planar

4 tetrahedral

180°

120°

109.5°

5 trigonal bypyramidal90°

120°

5 trigonal bypyramidal

6 octahedral

90°

90°

120°

Central Atoms Having Less than an Octet• Relatively rare.• Molecules with less than an octet are typical for

compounds of Groups 1A, 2A, and 3A.• Most typical example is BF3.

• Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.

Exceptions to the Octet Rule

Summary of VSEPR Molecular ShapesSummary of VSEPR Molecular Shapes

e-pairs Notation Name of VSEPR shape Examples2 AX2 Linear HgCl2 , ZnI2 , CS2 , CO2

3 AX3 Trigonal planar BF3 , GaI3

AX2E Non-linear (Bent) SO2 , SnCl2

4 AX4 Tetrahedral CCl4 , CH4 , BF4-

AX3E (Trigonal) Pyramidal NH3 , OH3-

AX2E2 Non-Linear (Bent) H2O , SeCl2

5 AX5 Trigonal bipyramidal PCl5 , PF5

AX4E Distorted tetrahedral (see-sawed)

TeCl4 , SF4

AX3E2 T-Shaped ClF3 , BrF3

AX2E3 Linear I3- , ICl2

-

6 AX6 Octahedral SF6 , PF6-

AX5E Square Pyramidal IF5 , BrF5

AX4E2 Square Planar ICl4- , BrF4

-

Examples

Determine the electron-pair (Domain) and molecular geometries of each of the following. Draw and name each.

Beryllium Chloride

Beryllium Chloride

BeCl2

Beryllium Chloride

BeCl2

1. Lewis structure

Beryllium Chloride

BeCl2

1. Lewis structure

Cl Be Cl

Beryllium Chloride

BeCl2

1. Lewis structure

Cl Be Cl

2. Count regions of electron density on central atom

Beryllium Chloride

BeCl2

1. Lewis structure

Cl Be Cl

2. Count regions of electron density on central atom 2

Beryllium Chloride

BeCl2

1. Lewis structure

Cl Be Cl

2. Count regions of electron density on central atom 2

3. Draw and name electron-pair shape

Cl Be Cl

linear

Beryllium Chloride

BeCl2

3. Draw and name electron-pair shape

Cl Be Cl

linear

3. Derive and name molecular shape

Cl Be Cl

linear

Carbon Dioxide

Carbon Dioxide

CO2

Carbon Dioxide

CO2

O C O

Carbon Dioxide

CO2

O C O

2 regions

Carbon Dioxide

CO2

O C O

2 regions

Electron-pair shape, linear

O C O

Carbon Dioxide

CO2

O C O

2 regions

Electron-pair shape, linear

O C O

Molecular shape, linear

O C O

Aluminum Bromide

Aluminum BromideAlBr3

Aluminum BromideAlBr3

Al

Br

Br Br

Aluminum BromideAlBr3

Al

Br

Br Br

3 regions

Aluminum BromideAlBr3

Al

Br

Br Br

3 regionsElectron-pair shapetrigonal planar

Al

Br

Br Br

Aluminum BromideAlBr3

Al

Br

Br Br

3 regionsElectron-pair shapetrigonal planar

Al

Br

Br Br

Molecular shapetrigonal planar

Al

Br

Br Br

Nitrite Ion

Nitrite Ion

NO2–

Nitrite Ion

NO2–

O N O

–

Nitrite Ion

NO2–

O N O

–

3 regions

Nitrite Ion

NO2–

O N O

–

3 regions

Electron-pair shapetrigonal planar

NO O

–

Nitrite Ion

NO2–

O N O

–

3 regions

Electron-pair shapetrigonal planar

NO O

–

Nitrite Ion

NO2–

O N O

–

3 regions

Electron-pair shapetrigonal planar

NO O

–Molecular shape bent

NO O

–

Carbon Tetrabromide

Carbon TetrabromideCBr4

Carbon TetrabromideCBr4

C

Br

Br Br

Br

Carbon TetrabromideCBr4

C

Br

Br Br

Br

4 regions

Carbon TetrabromideCBr4

C

Br

Br Br

Br

4 regions

Electron-pair shapetetrahedral

C

Br BrBr

Br

Carbon TetrabromideCBr4

C

Br

Br Br

Br

4 regions

Electron-pair shapetetrahedral

C

Br BrBr

Br

Molecular shapetetrahedral

Arsine

ArsineAsH3

ArsineAsH3

As

H

H H

ArsineAsH3

As

H

H H

4 regionselectron-pair shape, tetrahedral

ArsineAsH3

As

H

H H

4 regionselectron-pair shape, tetrahedral

As

HH

H

ArsineAsH3

As

H

H H

4 regionselectron-pair shape, tetrahedral

As

HH

H

molecular shapetrigonal pyramidor tripod

ArsineAsH3

As

H

H H

4 regionselectron-pair shape, tetrahedral

As

HH

H

molecular shapetrigonal pyramidor tripod

AsH

HH

WaterH2O

WaterH2O

O

HH

WaterH2O

O

HH

4 regionselectron-pair shapetetrahedral

WaterH2O

O

HH

4 regionselectron-pair shapetetrahedral

O

HH

WaterH2O

O

HH

4 regionselectron-pair shapetetrahedral

O

HH

molecular shapebent

WaterH2O

O

HH

4 regionselectron-pair shapetetrahedral

O

HH

molecular shapebent

OHH

Phosphorus Pentafluoride

Phosphorus PentafluoridePF5

PF

FF

FF

Phosphorus PentafluoridePF5

PF

FF

FF

5 regionselectron-pair shapetrigonal bipyramidal

F

F

F

F

F

P

Phosphorus PentafluoridePF5

PF

FF

FF

5 regionselectron-pair shapetrigonal bipyramidal

F

F

F

F

F

P

molecular shapetrigonal bipyramidal

Sulfur Tetrafluoride

Sulfur TetrafluorideSF4

Sulfur TetrafluorideSF4

SF F

FF

Sulfur TetrafluorideSF4

SF F

FF

5 regionstrigonal bipyramidal

Sulfur TetrafluorideSF4

SF F

FF

5 regionstrigonal bipyramidal

F

F

FF

S

Sulfur TetrafluorideSF4

SF F

FF

5 regionstrigonal bipyramidal

F

F

FF

S

F

F

FF

S

molecular shapedistorted tetrahedral

Sulfur TetrafluorideSF4

SF F

FF

5 regionstrigonal bipyramidal

F

F

FF

S

molecular shapesee saw

S

F

F F

F

Chlorine Trifluoride

Chlorine TrifluorideClF3

Chlorine TrifluorideClF3

F

FF

Cl

Chlorine TrifluorideClF3

F

FF

Cl 5 regionselectron-pair shapetrigonal bipyramidal

Chlorine TrifluorideClF3

F

FF

Cl 5 regionselectron-pair shapetrigonal bipyramidal

Cl

F

F

F

Chlorine TrifluorideClF3

F

FF

Cl 5 regionselectron-pair shapetrigonal bipyramidal

Cl

F

F

F

Chlorine TrifluorideClF3

F

FF

Cl 5 regionselectron-pair shapetrigonal bipyramidal

Cl

F

F

F

molecular shapeT-shape

Cl

F

F

F

Sulfur Hexafluoride

Sulfur HexafluorideSF6

Sulfur HexafluorideSF6

SF

FF

FF

F

Sulfur HexafluorideSF6

SF

FF

FF

F

6 regionselectron-pair shapeoctahedral

S

F

F

F

F

F

F

Sulfur HexafluorideSF6

SF

FF

FF

F

6 regionselectron-pair shapeoctahedral

S

F

F

F

F

F

F

molecular shapeoctahedral

Bromine Pentafluoride

Bromine PentafluorideBrF5

Bromine PentafluorideBrF5

BrF F

FF

F

Bromine PentafluorideBrF5

BrF F

FF

F

6 regionselectron-pair shapeoctahedral

Bromine PentafluorideBrF5

BrF F

FF

F

6 regionselectron-pair shapeoctahedral

Br

F

F

F

F

F

Bromine PentafluorideBrF5

BrF F

FF

F

6 regionselectron-pair shapeoctahedral

Br

F

F

F

F

F

Bromine PentafluorideBrF5

BrF F

FF

F

6 regionselectron-pair shapeoctahedral

Br

F

F

F

F

F

molecular shapesquare pyramidal

Br

F

F

F

F

F

Xenon Tetrafluoride

Xenon TetrafluorideXeF4

Xenon TetrafluorideXeF4

XeF F

FF

Xenon TetrafluorideXeF4

XeF F

FF

6 regionselectron-pair shapeoctahedral

Xenon TetrafluorideXeF4

XeF F

FF

6 regionselectron-pair shapeoctahedral

Xe

F

F

F

F

Xenon TetrafluorideXeF4

XeF F

FF

6 regionselectron-pair shapeoctahedral

Xe

F

F

F

F

Xenon TetrafluorideXeF4

XeF F

FF

6 regionselectron-pair shapeoctahedral

Xe

F

F

F

F

Xe

F

F

F

F

molecular shapesquare planar

Tribromide Ion Br3

–

Tribromide Ion Br3

–

Br

BrBr

Tribromide Ion Br3

–

Br

BrBr 5 regionselectron-pair shapetrigonal bipyramidal

Tribromide Ion Br3

–

Br

BrBr 5 regionselectron-pair shapetrigonal bipyramidal

Br

Br

Br

Tribromide Ion Br3

–

Br

BrBr 5 regionselectron-pair shapetrigonal bipyramidal

Br

Br

Br

Tribromide Ion Br3

–

Br

BrBr 5 regionselectron-pair shapetrigonal bipyramidal

Br

Br

Br

molecular shapelinear

Br

Br

Br

Polarity of Molecules

• molecules in which dipole moments of the bonds do not cancel are polar molecules

• molecules that do not contain polar bonds or in which all dipole moments cancel are non-polar molecules

CO2 vs H2O

CO OO

H H

CO2 vs H2O

CO OO

H H

+–

+ +

––

CO2 vs H2O

CO OO

H H

+–

+ +

––

CO2 vs H2O

CO OO

H H

+–

+ +

––

0

CO2 vs H2O

CO OO

H H

+–

+ +

––

0

CO2 vs H2O

CO O

O

H H

+

–

+ +

––

0

y

x

y

x

CO2 vs H2O

CO O

O

H H

+

–

+ +

––

y

x

y

x

CO2 vs H2O

CO OO

H H

+ –

+ +

––

nonpolar polar

Study and Know

9.2 Polarity of Molecules

VSEPR Theory only explains molecular shapes

says nothing about bonding in molecules

Enter Valence Bond (VB) Theory

atoms share electron pairs by allowing their atomic orbitals to overlap

+

H H

+

H H

bond

+

H H

bond

1sE

H

+

H H

bond

1s

E

H

H

+

F F

F2

+

F F

F2

bond

1s

2s

2pE

F

1s

2s

2p

F

E

F

Methane

CH4

1s

2s

2p

E

C

Methane

CH4

1s

2s

2p

E

C

H H

Methane

CH4

1s

2s

2p

E

C

H H

H+

Methane

CH4

1s

2s

2p

E

C

H H

H+

H–

Methane

CH4

1s

2s

2p

E

C

H H

H+

H–

C

H

H

H

H

90°

90°

Methane

CH4

C

H

HH

H

109.5°

Tetrahedral Geometry 4 Identical Bonds

Problem and Solution

C must have 4 identical orbitals in valence shell for bonding

solution: hybridization

Methane

CH4

1s

2s

2p

E

Methane

CH4

1s

2s

2p

E

1s

2s 2p

E

Methane

CH4

1s

2s

2p

E

1s

2s 2p

E

Methane

CH4

1s

2s

2p

E

1s

2s 2p

E

Methane

CH4

1s

2s

2p

E

1s

Esp3

– ++

+

2p2s

– ++

+

=2p2s

an sp3 hybrid orbital

4 identical sp3 hybrid orbitals

4 identical sp3 hybrid orbitals

tetrahedral geometry

4 identical sp3 hybrid orbitals

tetrahedral geometry

4 identical sp3 hybrid orbitals

tetrahedral geometry

Methane

CH4

1s

2s

2p

E

1s

Esp3

H H H H

Hybridization vs Shape (e– pair)

• sp linear• sp2 trigonal planar• sp3 tetrahedral• sp3d trigonal bipyramidal• sp3d2 octahedral

Predict the Hybridization of the Central Atom in tribromide ion

Predict the Hybridization of the Central Atom in tribromide ion

Br3–

Predict the Hybridization of the Central Atom in tribromide ion

Br3–

Br

BrBr 5 regionselectron-pair shapetrigonal bypyramidal

Predict the Hybridization of the Central Atom in tribromide ion

Br3–

Br

BrBr 5 regionselectron-pair shapetrigonal bypyramidal

sp3d

Predict the Hybridization of the Central Atom in carbon dioxide

CO2

Predict the Hybridization of the Central Atom in carbon dioxide

CO2

O C O

2 regions

Electron-pair shape, linear

Predict the Hybridization of the Central Atom in carbon dioxide

CO2

O C O

2 regions

Electron-pair shape, linear

sp

Predict the Hybridization of the Central Atom in aluminum bromide

Predict the Hybridization of the Central Atom in aluminum bromide

Al

Br

Br Br

3 regionsElectron-pair shapetrigonal planar

Predict the Hybridization of the Central Atom in aluminum bromide

Al

Br

Br Br

3 regionsElectron-pair shapetrigonal planar

sp2

Predict the Hybridization of the Central Atom in xenon tetrafluoride

Predict the Hybridization of the Central Atom in xenon tetrafluoride

XeF F

FF

6 regionselectron-pair shapeoctahedral

Predict the Hybridization of the Central Atom in xenon tetrafluoride

XeF F

FF

6 regionselectron-pair shapeoctahedral

sp3d2

Consider Ethylene, C2H4

Consider Ethylene, C2H4

C CH H

HH

Consider Ethylene, C2H4

C CH H

HH

3 regionstrigonal planar

Consider Ethylene, C2H4

C CH H

HH

3 regionstrigonal planar

sp2

Consider Ethylene, C2H4

C CH H

HH

3 regionstrigonal planar

sp2

1s

2s

2p

E

1s

2s

2p

E

1s

2s 2p

E

1s

2s

2p

E

1s

sp22p

E

sp2

sp2

sp2 2p

sp2

sp2

sp2

2p

bond framework

bond

bond

Consider Acetylene, C2H2

C C HH

Consider Acetylene, C2H2

C C HH

2 regionslinear

Consider Acetylene, C2H2

C C HH

2 regionslinearsp

Consider Acetylene, C2H2

C C HH

2 regionslinearsp

1s

2s

2p

E

1s

2s 2p

E

1s

2s

2p

E

1s

sp 2p

E

sp sp

2p

2p

bond framework

bonds

bonds

Generally

• single bond is a bond• double bond consists of 1 and 1

bond• triple bond consists of 1 and 2

bonds

Molecular Orbital (MO) Theory

when atoms combine to form molecules, atomic orbitals overlap and are then combined to form molecular orbitals

orbitals are conserved

a molecular orbital is an orbital associated with more than 1 nucleus

like any other orbital, an MO can hold 2 electrons

consider hydrogen atoms bonding to form H2

+

H H

add

subtract

add

subtract

bonding

antibonding

add

subtract

bonding

antibonding

*1s

1s

1s 1s

1s

*1s

H HH2

E E

1s 1s

1s

*1s

H HH2

E E

1s 1s

1s

*1s

H HH2

E E

1s 1s

1s

*1s

H HH2

E E

1s 1s

1s

*1s

H HH2

E E

(1s ) 2

1s 1s

1s

*1s

H HH2

E E

(1s ) 2

total spin = 0

• Diamagnetic: slightly repelled by a magnetic field

total spin = 0• paramagnetic: attracted to a

magnetic fiels

total spin not 0• Bond Order = 1/2 (bonding e– –

antibonding e–)

1s 1s

1s

*1s

H HH2

E E

(1s ) 2

total spin = 0diamagnetic

1s 1s

1s

*1s

H HH2

E E

BO = 1/2 ( 2 – 0) = 1

Consider He2

1s 1s

1s

*1s

He HeHe2

E E

1s 1s

1s

*1s

He HeHe2

E E

1s 1s

1s

*1s

He HeHe2

E E

(1s ) 2 ( *1s ) 2

1s 1s

1s

*1s

He HeHe2

E E

diamagnetic

1s 1s

1s

*1s

He HeHe2

E E

BO = 1/2 ( 2 – 2 ) = 0

Combination of p Atomic Orbitals

2p 2p

subtract

add

bonding MO

antibonding MOsubtract

add

bonding MO

antibonding MO *2p

2p

subtract

add

2p 2p

subtract

add

antibonding MO

bonding MO

subtract

add

2p

*2p

subtract

add

2p

*2p

subtract

add

Consider Li2

2s 2s

2s

*2s

Li LiLi2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

Li LiLi2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

Be BeBe2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

Be BeBe2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

B BB2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

B BB2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

C CC2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

N NN2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

O OO2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

F FF2

E E

2p 2p

*2p

2p

2p

*2p

2s 2s

2s

*2s

Ne NeNe2

E E

2p 2p

*2p

2p

2p

*2p