Chapter 9: Models of Chemical
Bonding
1.
2.
3.
4.
5.
6.
Lewis Dot Symbols
Ionic Bonding
Metallic Bonding
Covalent Bonding
Polar Bonds
Electronegativity
• Lewis symbols
Examples: Na  and : C :

Practice
• Give Lewis dot symbols for:
magnesium
Oxide ion
nitrogen
Sodium ion
fluorine
argon
boron
Types of Bonding and Properties
Ionic
Metallic
Covalent
Ionic Bonding
• Ionic compounds form
• The attraction
• Forming ions takes energy
Metallic Bonding
• Outer electrons
• Large numbers of metal atoms
• The electrons are ‘delocalized’
Covalent Bonds
Network Covalent Solids
http://www.hull.ac.uk/chemistry/intro_inorganic/images/diamond.jpg
Polar Covalent Bonds
• When two identical atoms form a covalent
bond
• When different kinds of atoms combine,
(a)
(b)
• Electronegativity
• In general, electronegativity increases
http://www.chem.ufl.edu/~itl/2045/change/C9F16.GIF
• The difference in electronegativity
• There is no sharp dividing line between
ionic and covalent bonding:
• A bond is mostly ionic when
• The degree of polarity, or ionic
character, varies continuously with the
electronegativity difference
Each atom in a
bond has a partial
charge of about
+0.5 or –0.5 units
when the
electronegativity
difference is 1.7.
Electronegativity diff.
Type of bonding
>1.7
Mostly ionic
0.4-1.7
Polar covalent
<0.4
Mostly covalent
0
Nonpolar covalent
Practice Questions
•
A.
B.
C.
D.
Identify the following bonds as
Mostly ionic
Polar Covalent
Mostly Covalent
Nonpolar Covalent
C-Cl
Re-H
Li-Cl
P-H
Sn-Br
Sr-O
• Lewis symbols can be used to
represent the covalent or electron
pair bond
HHH:H

 Formulas drawn with Lewis symbols are
called Lewis formulas or Lewis structures
•
The term structural formula
•
Many molecules obey the octet rule:
•
The number of bonds an atom (second-row)
forms is determined by:
Number of Bonds
C
N
O
F
B
Has 4 e-
Needs 4 e-
Forms 4 bonds
• single bond
• Double and triple bonds
• The bond order
•
•
•
•
A single bond has bond order of
a double bond a bond order of
a triple bond a bond order of
Bond length depends on bond order:
Drawing Lewis Structures
•
•
•
The least electronegative atom is usually in the
middle. (Or the atom which can form the most
bonds.)
Count total number of valence electrons in
molecule/ion.
Place them around the atoms to satisfy the octet
rule:
Phosphorus Trichloride
Carbon Dioxide
• Not all structures obey the octet rule
Sulfur tetrafluoride
Bromine pentafluoride
• The preferred Lewis structure is the
one that best fits the experimental
data
The structure of sulfuric acid in
the vapor state. There are two
different sulfur-oxygen bond
lengths. The preferred Lewis
structure needs different bond
orders for these atoms.

:O:
|

:O:


||

HO
 S O
H


HO
 S O
H


:O
:

:O:
StructureI
StructureII
|
• Structure I
• Structure II
||
• formal charge
• The formal charge on an atom is calculated
• Consider the sulfur atoms in the two structures
for sulfuric acid:

:O:

|
:O:


||

HO
 S O
H


HO
 S O
H


:O
:

:O:
|
||
StructureI
StructureII
Structure I: formal charge on S =
Structure II: formal charge on S =
 When several Lewis structures are possible,

Nitrate ion
Iodate ion
Formal charge on O =
Formal charge on I =
Formal charge on O =
Formal charge on I =
• Some molecules and ions are not well
represented by a single Lewis structure
• Consider the case of the formate ion, HCO2-
 Experiment gives
These are called resonance structures
Draw the resonance forms for nitrate.
Draw resonance forms for thiosulfate.
Draw the resonance forms for dinitrogen monoxide.