Bonding – General Concepts

Shows how valence electrons are arranged among atoms in a molecule.Reflects central idea that stability of a compound relates to noble gas electron configuration.

Lewis Structures

CH

H

H

Cl

..

....

..

Completing a Lewis Structure -CH3Cl

Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14

Join peripheral atoms

to the central atom with electron pairs.

Complete octets on

atoms other than hydrogen with remaining electrons

Make carbon the central atom

..

..

..

Multiple Covalent Bonds:Double bonds

C C

H

H

H

H

Two pairs of shared electrons

C C

H

H

H

H

Ethene

Multiple Covalent Bonds:Triple bonds

C C HH

Three pairs of shared electronsEthyne

C C HH

ResonanceResonance is invoked when more than one valid Lewis structure can be written for a particular molecule.

The actual structure is an average of the resonance

structures.

H

H

H

H

HH

H

H

H

H

H

HBenzene, C6H6

The bond lengths in the ring are identical, and between those of single and double bonds.

Resonance Bond Length and Bond Energy

Resonance bonds are shorter and stronger than single bonds.

Resonance bonds are longer and weaker than double

bonds.

H

H

H

H

HH

H

H

H

H

H

H

Resonance in Ozone, O3

Neither structure is correct.

O O O

O O O

Oxygen bond lengths are identical, and intermediate to single and double bonds

Resonance in a carbonate ion:

Resonance in an acetate ion:

Resonance in Polyatomic Ions

Localized Electron ModelLewis structures are an application of the “Localized Electron Model”

L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bondingResonance points out a weakness in the Localized Electron Model.

Models Models are attempts to explain

how nature operates on the microscopic level based on experiences in the macroscopic world.Models can be physical as with this DNA modelModels can be mathematicalModels can be theoretical or philosophical

Fundamental Properties of Models

A model does not equal reality.Models are oversimplifications, and

are therefore often wrong.Models become more complicated

as they age.We must understand the underlying

assumptions in a model so that we don’t misuse it.

VSEPR • Valence Shell Electron

Pair Repulsion theory.• Most important factor in

determining geometry is relative repulsion between electron pairs.

Molecule adopts the shape that minimizes the electron pair repulsions.

MOLECULAR GEOMETRY

VSEPR – Valence Shell Electron Pair RepulsionX + E Overall Structure Forms

2 Linear AX2 AX3 Trigonal Planar AX3, AX2E4 Tetrahedral AX4, AX3E, AX2E2

5 Trigonal bipyramidal AX5, AX4E, AX3E2, AX2E3

6 Octahedral AX6, AX5E, AX4E2

A = central atomX = atoms bonded to A

E = nonbonding electron pairs on A

VSEPR: Linear

AX2 CO2

John A. SchreifelsChemistry 211

Chapter 10-16

Figure 9.2 Molecular Shapes 2,3,4 electron pairs.

John A. SchreifelsChemistry 211

Chapter 10-17

Figure 9.3 Molecular shapes 5, 6 electron pairs

• AX2 - Linear

• AX - Linear

• Draw CO2 and HF

AX3

AX2E

BF3

SnCl2

Triganol planar

Bent

VSEPR: Tetrahedral

AX4

AX3E

AX2E2

CCl4

PCl3

Cl2O

tetrahedral

Triangular Pyramidal

Bent

VSEPR: Trigonal Bi-pyramidal

AX5

AX4E

AX3E2

AX2E3

PCl5

SF4

ClF3

I3-

Triangular bipyramidal

See-saw

T-shaped

Linear

VSEPR: Octahedral

AX6

AX5E

AX4E2

SF6

ICl4-

BrF5

Octahedral

Square pyramidal

Square planar

Are the following molecules polar or nonpolar ? Which will dissolve in water?

• OF2 SO3 PCl3 SF6 NH3

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Formal charge (FC) in Lewis structures

Comparison of the formal number of valence electrons about an atom in the Lewis structure of a molecule and

comparison of FN with the number of valence electrons (VE) in the neutral atom.

Computation of FC:

FC = VE (neutral atom) - LE (atom in molecule) - 1/2BE (atom in molecule)

where

VE = the number of valence electrons in the neutral atomLE = the number of lone pair electrons on the atom in the moleculeBE = the number of bonding electrons on the atom in the molecule

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Example of formal charge computation: ozone, O3

O O O

Note: New charge for molecule = 0, sum of formal charges must = 0.

VE (atom) = 6 6 61/2 BE (molecule)= -2 -3 -1LE (molecule) = -4 -2 -6FC = 0 +1 -1

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HCN = atomic compositional structure

HCN possesses 10 VE = Lewis compositional structures

Two possible Lewis constitutional structures:

H-C-N or H-N-C

Both need to have 10 VE in their Lewis structureProblem: Try to achieve an acceptable Lewis structure (duet and octet rule followed) for

both.

Isomers: Same composition, two different constitutional Lewis structures

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HCN = atomic compositional structure

HCN: 10 VE = Lewis compositional structures

Two possible Lewis constitutional structures

H-C-N or H-N-C

Any acceptable Lewis structure for HCN needs to show 10 VE

Try to achieve an acceptable Lewis structure (duet and octet rules obeyed) for all isomeric structures.

H C N H N C

Two acceptable Lewis structures. Which is better?

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Use formal charges to decide on the stability of isomeric Lewis structures

VE (atom) 1 4 5 1 5 4

1/2 BE (molecule) -1 -4 -3 -1 -4 -3

UE (molecule) 0 0 -2 0 0 -2________________________________________________________FC on atom 0 0 0 0 +1 -1

H C N H N C

H C N H N C

Important: the net charge of composition HCN = 0, so the sum of the formal charges in any

acceptable Lewis structure must be = 0 also.