Chapter_10.pptx

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Chemical Bonding I : Basic Concepts

Copyright 2011 Pearson Canada Inc.

ONTENTS

10-1 Lewis Theory: An Overview

10-2 Covalent Bonding: An

Introduction

10-3 Polar Covalent Bonds andElectrostatic Potential Maps

10-4 Writing Lewis Structures

10-5 Resonance

10-6 Exceptions to the Octet Rule

10-7 Shapes of Molecules

10-8 Bond Order and Bond Length

10-9 Bond Energies


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Lewis Theory: An Overview

Valence e-

play afundamental role in chemical

bonding.

e-transfer leads to

ionic bonds.

Sharing of e-leads to

covalent bonds.

e-

are transferred or shared togive each atom a noble gas

configuration

the octet.


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Lewis Symbols

A chemical symbol represents the nucleus

and the coree-.

Dots around the symbol represent valence e-.

Si

N

P

As

Sb

Bi

Al

Se

Ar

I


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Ba O

O

Ba2+ 2-BaO

Writing Lewis Structures of Ionic Compounds. Write Lewisstructures for the following compounds: (a) BaO; (b) MgCl2;

(c) aluminum oxide.

Note the use of the fishhook arrow to denote a

single electron movement. A double headed

arrow means that two electrons move.

Example


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Mg

Cl

Cl

Cl

Mg

2+ -2MgCl2

Example Continued


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Coordinate Covalent Bonds

HN

H

HH

N

H

HH

H

+

Cl

Cl

-

Note the double headed arrow

showing that two electrons move.


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Multiple Covalent Bonds

C

O

O

CO O

CO O

CO O


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Multiple Covalent Bonds

N

N N

N

N N

N N


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Paramagnetism of Oxygen


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Polar Covalent Bonds and Electrostatic

Potential Maps


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Polar Molecules


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Electronegativity


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Percent I onic Character


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Writing Lewis Structures

Allthe valence e-of atoms must appear.

Usually,the e- are paired.

Usually, each atom requires an octet. H only requires 2e-.

Multiple bonds may be needed.

Readily formed by C, N, O, S, and P.


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Skeletal Structure

Identify centraland terminalatoms.

C

H

HH

HC

H

HO


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Skeletal Structure

Hydrogen atoms are always terminal atoms.

Central atoms are generally those with the

lowest electronegativity.

Carbon atoms are always central atoms.

Generally structures are compact and

symmetrical.


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Strategy for

Writing LewisStructures


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Writing a Lewis Structure for a Polyatomic Ion. Write theLewis structure for the nitronium ion, NO2+.

Step 1: Total valence e-= 5 + 6 + 61 = 16 e-

Step 3: Plausible structure: ONO

Step 4: Add e-to terminal atoms: ONO

Step 2: Identify the central and terminal atoms

Polyatomic I on


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Step 6: Use multiple bonds to satisfy octets.

ONO

O=N=O

Step 5: Determine e-left over: 16412 = 0

+


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Formal Charge

FC = #valence e-- #lone pair e-- #bond pair e-21

FC(O) = 6 - 4 (4) = 02

1

FC(N) = 5 - 0 (8) = +12

1

O=N=O+


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Alternative Lewis Structure

ONO

+ -+

FC(O) = 6 - 2 (6) = +12

1

FC(N) = 5 - 0 (8) = +12

1

FC(O) = 6 - 6 (2) = -12

1

O N O


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Alternative Lewis Structures

Sum of FC is the overall charge.

FC should be as small as possible.

Negative FC is usually on the most electronegative

elements.

FC of same sign on adjacent atoms is unlikely.

+

ONO

-+


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Using the Formal Charge in Writing Lewis Structures. Write

the most plausible Lewis structure of nitrosyl chloride, NOCl.

2+ 2- - 2+ - +-

Example


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Resonance

O O O O O O

+ +- -

O O O

+ --


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Exceptions to the Octet Rule

Odd e- species (Radicals)

N=O

HCH

H

OH


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Incomplete octets.

B

F

FF

B

F

FF

-

+

B

F

FF

-

+


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Expanded valence shells.

P

Cl

ClCl

P

Cl

Cl

Cl

Cl

ClS

F

F

F

F

F

F


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Expanded Valence Shell


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The Shapes of Molecules

H O H


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Terminology

Bond lengthdistance between nuclei.

Bond angleangle between adjacent bonds.

VSEPR Theory

Electron pairs repel each other whether they are in

chemical bonds (bond pairs) or unshared (lone pairs).

Electron pairs assume orientations about an atom to

minimize repulsions.

Electron group geometrydistribution of e-pairs.

Molecular geometrydistribution of nuclei.


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Balloon Analogy


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Methane, Ammonia and Water

Charge clouds of the lone pair electrons spread out.


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Molecular Geometry as a Function of

Electron Group Geometry


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Applying VSEPR Theory

Drawa plausible Lewis structure. Determinethe number of e-groups and identify them

as bondor lonepairs.

Establish the e-

group geometry. Determine the molecular geometry.

Multiple bonds count as one group of electrons.

More than one central atom can be handled

individually.

Di l M t


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Dipole Moments

The extent of the charge displacement in a polar covalent bond is

given by dipole moment.


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Molecular Shape and Dipole Moments

B d O d d B d L th


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Bond Order and Bond Length

Bond Order

Single bond, order = 1

Double bond, order = 2

Bond Length Distance between two nuclei

Higher bond order

Shorter bond

Stronger bond


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Bond Energies

Bond-dissociation energy : The

quantity of energy required to break

one mole of covalent bonds in a

gaseous species.


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The reaction of methane (CH4) and chlorine produces a mixture

of products called chloromethanes. One of these ismonochloromethane, CH3Cl, used in the preparation of silicones.

Calculate H for the reaction.

Hrxn = H(bond breakage) - H(bond formation)

= BE (reactants)- BE (products)

= 657 kJ/mol770 kJ/mol = -113 kJ/mol

Calculating an Enthalpy of Reaction

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