1. Chapter 11: Chemical Bonding I: Basic Concepts
Chemistry 140 Fall 2005
General Chemistry
Principles and Modern Applications
Petrucci • Harwood • Herring
8th Edition
Chapter 11: Chemical Bonding I: Basic Concepts
Philip Dutton
University of Windsor, Canada
N9B 3P4
Prentice-Hall © 2002
(modified 2003 by Dr. Paul Root and
2005 by Dr. David Tramontozzi)
Thermochemistry branch of chemistry concerned with heat effects accompanying chemical reactions.
Direct and indirect measurement of heat.
Answer practical questions: why is natural gas a better fuel than coal, and why do fats have higher energy value than carbohydrates and protiens.
Prentice-Hall © 2002
General Chemistry: Chapter 9

2. General Chemistry: Chapter 9
Contents
11-1 Lewis Theory: An Overview
11-2 Covalent Bonding: An Introduction
11-3 Polar Covalent Bonds
11-4 Writing Lewis Structures
11-5 Resonance
11-6 Exceptions to the Octet Rule
11-7 The Shapes of Molecules
11-8 Bond Order and Bond Lengths
11-9 Bond Energies
Focus on Polymers— Macromolecular Substances
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General Chemistry: Chapter 9

3. Strategy for Writing Lewis Structures
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General Chemistry: Chapter 9

4. Apparent charges on certain atoms in a Lewis structure.
Chemistry 140 Fall 2005
Formal Charge
Apparent charges on certain atoms in a Lewis structure. 1
FC = #valence e- - #lone pair e- -
#bond pair e- 2
These can be used to help determine the most probable Lewis structure when there is more than one Lewis structure possible for a compound.
The formal charge on an atom in a Lewis structure is the number of valence e- in the free atom minus the number of e- assigned to that atom in the Lewis structure.
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General Chemistry: Chapter 9

5. General Chemistry: Chapter 9
Example 11-6
Writing a Lewis Structure for a Polyatomic Ion.
Write the Lewis structure for the nitronium ion, NO2+.
Step 1:
Total valence e- = – 1 = 16 e-
Step 2:
Plausible structure: O—N—O
Step 3:
Add e- to terminal atoms: O—N—O ••
Step 4:
Determine e- left over: 16 – 4 – 12 = 0
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General Chemistry: Chapter 9

6. General Chemistry: Chapter 9
Example 11-6
Step 5:
Use multiple bonds to satisfy octets. •• •• ••
O=N=O +
O—N—O •• •• •• ••
Step 6:
Determine formal charges:
FC(O) = – (4) = 0 2 1
FC(N) = – (8) = +1 2 1
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General Chemistry: Chapter 9

7. Alternative Lewis Structure•• •• ••
O N O + + -
O—N—O •• •• •• ••
FC(O≡) = – (6) = +1 2 1
FC(N) = – (8) = +1 2 1
FC(O—) = – (2) = -1 2 1
Prentice-Hall © 2002
General Chemistry: Chapter 9

8. Alternative Lewis Structures
Sum of FC is the overall charge.
FC should be as small as possible.
Negative FC usually on most electronegative elements.
FC of same sign on adjacent atoms is unlikely. + ••
O≡N—O -
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General Chemistry: Chapter 9

9. General Chemistry: Chapter 9
Example 11-7
Using the Formal Charge Concept in Writing Lewis Structures.
Write the most plausible Lewis structure of nitrosyl chloride, NOCl, one of the oxidizing agents present in aqua regia. 2+ 2- - +
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General Chemistry: Chapter 9

10. General Chemistry: Chapter 9
Chemistry 140 Fall 2005
11-5 Resonance
Two or more plausible Lewis structures can be written but ‘correct’ structure itself cannot be written. + - - + •• •• •• •• •• •• O O O •• •• O O O •• •• •• ••
One would expect either structure to have one bond (double bond) shorter than the other (single bond). In reality, both bonds are of equal length and true structure is a hybrid of the above structures. O •• + -½
Many Lewis structures may be written for a given structure..
Ozone has two good possibilities, but neither gives the correct structure that has two equivalent O-O bonds.
In general, the more plausible Lewis structures that can be written, the more stable the molecule.
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General Chemistry: Chapter 9

11. Resonance in the Nitrate Ion (an example)
Chemistry 140 Fall 2005
Resonance in the Nitrate Ion (an example)
Draw the Lewis structure for the nitrate ion, NO3- •• •• O
The true structure of the nitrate ion is a hybrid of the three equivalent resonance structures. •• N •• O O •• •• •• •• •• - •• •• O •• •• •• O •• - •• O ••
Many Lewis structures may be written for a given structure..
Ozone has two good possibilities, but neither gives the correct structure that has two equivalent O-O bonds. + N + N + N O •• O •• •• - •• O O •• •• •• - - •• •• •• O O •• •• - •• •• •• ••
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General Chemistry: Chapter 9

12. 11-6 Exceptions to the Octet Rule
Chemistry 140 Fall 2005
11-6 Exceptions to the Octet Rule
Odd e- species.
N=O •• •• • •• H
Free radicals are highly reactive •• •
O—H
H—C—H H • ••
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General Chemistry: Chapter 9

13. Exceptions to the Octet Rule
Incomplete octets. •• B F - + •• B F - •• + •• F •• B •• F F •• •• •• •• ••
BF3 tendency is to form a coordinate covalent bond with species capable of donating an electron pair.
These species are limited to some compounds of beryllium, boron and aluminum
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General Chemistry: Chapter 9

14. Exceptions to the Octet Rule
Expanded octets. S F •• •• P Cl •• •• Cl •• P •• •• Cl Cl •• •• •• •• ••
Normal octet
Expanded octets
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General Chemistry: Chapter 9

15. Expanded Valence Shell
Chemistry 140 Fall 2005
Expanded Valence Shell
Expanded valence shell reduces the number of formal charges.
Experimentally determined sulfur to oxygen bond length is shorter than a single bond
It is not clear which is the more correct representation.
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General Chemistry: Chapter 9

16. 11-7 The Shapes of Molecules
From the Lewis structure it appears that water should be linear in shape.
The experimentally determined shape of water is bent.
Does this matter? Why the difference?
H O H
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General Chemistry: Chapter 9

17. General Chemistry: Chapter 9
Terminology
Bond length – distance between nuclei.
Bond angle – angle 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 orientatioins about an atom to minimize repulsions.
Electron group geometry –
distribution of e- groups (single, double, triple bonds, lone pairs or single electrons).
Molecular geometry – distribution of nuclei.
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General Chemistry: Chapter 9