Chapter 9
The Chemical Bond
Setting the Stage – Salt and Water


Earth is a complex world of chemicals
We will focus on two basic types of
compounds:




Salt (sodium chloride) is a typical ionic
compound
Water is a typical molecular compound
These substances are formed from other
substances with very different properties.
We will explore these differences and
reactions.
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Concepts of Chemistry 9e
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Setting a Goal – Part A
Chemical Bonds and the Nature of
Ionic Compounds

You will learn how the octet rule is
used to determine the charge on ions
and the formulas of ionic
compounds.
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Concepts of Chemistry 9e
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Objectives for Section 9-1


Explain the significance of the octet
rule.
Write the Lewis dot structure of the
atoms of any representative element.
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Concepts of Chemistry 9e
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Professor’s Little Jokes
What do two dipoles say to each other?
“Have you got a moment?”
 Why did the white bear not get along with the
other bears?
He was polar.
 My name is Bond – Ionic Bond

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Concepts of Chemistry 9e
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9-1 Bond Formation and the
Representative Elements

The s block and p block elements
(often termed the representative
elements) will often form bonds such
that there are eight outer electrons
surrounding each atom (the octet
rule).
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Concepts of Chemistry 9e
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Bond Formation and the
Representative Elements….Cont’d


Obtaining this configuration is the driving force
for bond formation for many compounds formed
by the representative elements.
The exceptions are: H and Li (which tend to
follow a “duet” rule - filling the ns subshell);
some compounds of group IIA and IIIA elements
(e.g. BF3); odd-electron molecules (‘radicals’, e.g.
NO); and hypervalent compounds of period 3+
elements (e.g. SF6).
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Concepts of Chemistry 9e
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Bond Formation and Noble Gases





Noble gases rarely form compounds.
They have filled s and p outer subshells.
This is a total of eight electrons, referred
to as an octet.
Eight electrons in the outer s and p
orbitals is a particularly stable
configuration.
The energy required to remove an electron
from these full subshells is particularly
high.
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Concepts of Chemistry 9e
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Chemical Bonds




A chemical bond is the force that holds
two or more atoms together.
Chemical bonds involve the valence
electrons.
The valence electrons are the electrons in
the outer s and p subshells.
A bond results if a more stable electron
configuration results.
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Concepts of Chemistry 9e
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How Atoms Achieve an Octet



Metals can lose one to three electrons to
form a cation with the electron
configuration of the previous noble gas.
Nonmetals can gain one to three electrons
to form an anion with the electron
configuration of the next noble gas.
Atoms can share electrons.
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Concepts of Chemistry 9e
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Lewis Dot Symbols for Elements

G.N. Lewis – originator of dot structures
and the octet rule
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Concepts of Chemistry 9e
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Lewis Dot Symbols for Elements
. B:
Since only the valence



electrons are involved
in bonding we can
concentrate on those.
Lewis dot symbols are
used to represent the
valence electrons of
an atom.
See Table 9-1.
Malone and Dolter - Basic
Concepts of Chemistry 9e
:C:
..
.F:
..
..
. Cl :
..
12
Objective for Section 9-2

Using the Lewis dot structure and
the octet rule, predict the charges on
the ions of representative elements
and the formulas of binary ionic
compounds.
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Concepts of Chemistry 9e
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9-2 Formation of Ions and Ionic
Compounds

Metals can lose electrons to form ions



Nonmetals can gain electrons to form ions


Na ([Ne]3s1)  Na+ ([Ne]) + eIf a metal loses all of its outer electrons, it
acquires the octet of the previous noble gas
Cl ([Ne]3s23p5 + e-  Cl- ([Ne]3s23p6)
Lewis dot structures of the atoms can be
very helpful here.
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Concepts of Chemistry 9e
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Forming Ionic Compounds
_
Na
+
Cl
Na+ Cl
(Formula NaCl)




represents
a single
electron
transfer
Reaction of Na with Cl
Na donates an electron to Cl
Na+ has the previous noble gas structure
(Ne)
Cl- has the next noble gas structure (Ar)
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Concepts of Chemistry 9e
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Binary Ionic Compounds



In NaCl, each Na+ is surrounded by six Cl-, and
each Cl- is surrounded by six Na+.
Ionic lattice is a three-dimensional array of ions,
held together by electrostatic attractions.
These electrostatic attractions are called ionic
bonds.
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Concepts of Chemistry 9e
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Setting a Goal – Part B
Chemical Bonds and the Nature of
Molecular Compounds

You will learn how to apply the octet
rule to draw structures that form the
basis of our understanding of
bonding in most molecular
compounds.
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Concepts of Chemistry 9e
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Objective for Section 9-3

Describe the covalent bond and how
the octet rule determines the number
of such bonds for simple
compounds.
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Concepts of Chemistry 9e
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9-3 The Covalent Bond


Covalent bonds result from electron sharing
between two atoms.
We use Lewis dot structures to show the order
and arrangement of the atoms in a molecule and
all of the valence electrons.
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Concepts of Chemistry 9e
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Lewis Structure of F2
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Types of Covalent Bonds



Two nonmetals can share one, two or
three electron pairs.
The bonds resulting from this sharing are
referred to as single, double, or triple
bonds respectively.
Multiple bonds are frequently observed in
compounds of 2nd period elements.
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Concepts of Chemistry 9e
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Types of Covalent Bonds:
Examples
H
H
C
. H
H..
. . .
. C. . C
.
..
H
H
C
H
H
Ethylene
..
..
O
C
O
..
..
Carbon dioxide
H
C
.
N.
..
..
.. .. .. ..
O
C
O
..
..
. .. .. .
H . C . . N.
Hydrogen
cyanide
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Concepts of Chemistry 9e
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Objective for Section 9-4

Draw Lewis structures of a number
of molecular compounds and
polyatomic ions.
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Concepts of Chemistry 9e
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9-4 Writing Lewis Structures


The octet rule and Lewis dot structures
allow us to justify the formulas that we
know.
We can also predict the formulas of new
compounds.
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Concepts of Chemistry 9e
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Lewis Dot Structures
1.
2.
3.
Count the number of valence electrons
for the atoms in the molecule.
For ions, add one electron for each
negative charge or subtract one electron
for each positive charge.
Place the most electropositive atom in
the center (the inner atom). Array the
more electronegative atoms around this
atom as outer atoms.
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Concepts of Chemistry 9e
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Lewis Dot Structures…Cont’d
4.
5.
6.
Connect the outer atoms to the inner
atoms with single bonds.
Subtract two electrons from the total
number of valence electrons for each
bond.
Array the remaining electrons around
the outer atoms in pairs to complete
their octet.
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Concepts of Chemistry 9e
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Lewis Dot Structures…Cont’d
7.
Check the octets of all atoms.
8.
Use lone pairs on the outer atoms to
form multiple bonds to the inner atom if
needed to complete the inner atom octet.
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Concepts of Chemistry 9e
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Exceptions to the Octet Rule



Some molecules do not follow the octet
rule: one type has an odd number of
electrons, such as NO and NO2.
These two molecules have an odd number
of electrons (11 for NO; 17 for NO2). They
are known as (free) radicals
Another exception involves central atom
(period 3 and beyond) valence shell
extension in high-valence compounds,
like SF6
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Concepts of Chemistry 9e
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Exceptions to the Octet Rule



A second type has a correct Lewis
structure, but other evidence is
inconsistent with the Lewis structure,
such as O2.
Experiments show that O2 is also a free
radical with two unpaired electrons.
More complex theories are needed to
explain such substances.
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Concepts of Chemistry 9e
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Example: Write a Lewis structure for each
of sulfur tetrafuoride and the triiodide ion
: .. :
F
F
I3-
_
I
F
F
: ..F :
F
I
S
I
These are both examples
where the central atom
disobeys the octet rule
(expanded valence shell)
..
:..I
..
:..I
xx
S
: .. :
F
:.. :
S
F
: .. :
F
F
:.. :
SF4
: .. :
F
: ..F :
I
..
..I :
..
xx
I
I:
xx xx ..
Malone and Dolter - Basic
Concepts of Chemistry 9e
_
_
30
Objectives for Section 9-5


Discuss the significance of
resonance.
Write resonance structures of
appropriate molecules or ions.
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Concepts of Chemistry 9e
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9-5 Resonance Structures



In compounds with multiple bonds,
sometimes you can draw structures which
vary only by placement of the double
bonds.
The structures are called resonance
structures, and are an approximation of
the true structure of the molecule.
Actually, the molecule is a superposition
of all of the resonance structures.
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Concepts of Chemistry 9e
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Example of Resonance: Nitrate Ion



Nitrate has three resonance structures.
Each is identical except for the placement of the
double bond and associated lone pairs.
Experimentally, all N-O bonds are identical.
_
_
_
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Concepts of Chemistry 9e
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Objective for Section 9-6

Determine the validity of a Lewis
structure based on formal charge
considerations.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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9-6 Formal Charge


Formal charge is the charge that each
atom in a molecule would have if the
electrons in the bonds were divided
equally between the two atoms.
This approach essentially treats all bonds
as nonpolar (complete electron sharing).
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Concepts of Chemistry 9e
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Calculation of Formal Charge

Formal charge is calculated by subtracting
the number of non-bonded electrons on
the atom in question and half the shared
electrons from the group number.
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Concepts of Chemistry 9e
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Other Applications of Formal
Charge



Formal charge can help decide which is the best
ordering of bonds.
Formal charge can also help decide whether a
specific Lewis structure is legitimate (i.e. just
because we can write a Lewis structure that
follows the octet rule does not necessarily mean
that the structure actually represents the bonding
in the molecule).
The best Lewis structure is generally the one with
all zero formal charges (where this is possible).
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Concepts of Chemistry 9e
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Choosing Best Lewis Structure -1
Nitrosyl chloride
0.
0
.
..
... .
.
O
N
..Cl .
+1
0.
.
..
... .
..O N ..Cl
No formal
charge (0)
Non-zero
formal
charges
0
-1
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Choosing Best Lewis Structure -2
Nitric oxide
.
..
.N O ..
+1
.
.
.
.N O ..
No formal
charges (0)
Non-zero
formal
charges
0
0
-1
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Choosing Best Lewis Structure -3

NOTE: some molecules can only be represented
by Lewis structures with non-zero formal
charges. A classic example is ozone O3.
...... ..+ ...
..+
..
_...
... ..
_
O
O
O
O
AX2E type
O
O
..
..
O
.+.
O
... _
...
O
bent molecule
Partial double bond character
and dipole moment in real
molecule - agrees with
experiment.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Flowchart for Treating Molecules
Malone and Dolter - Basic
Concepts of Chemistry 9e
41
Setting a Goal – Part C
The Distribution of Charge in
Chemical Bonds

You will learn how the correct Lewis
structure of a compound allows
chemists a thorough understanding
of its properties.
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Concepts of Chemistry 9e
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Objective for Section 9-7

Classify a bond as being polar,
nonpolar, or ionic.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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9-7 Electronegativity and Polarity
of Bonds


Electronegativity - the ability of an atom to
attract electrons in a bond to itself.
Difference in electronegativities of atoms
that are bonded together results in a
partial transfer of electron charge to the
more electronegative atom.
electronegativity
3.9
electronegativity
2.1
H
F
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Concepts of Chemistry 9e
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Electronegativity and Polarity of
Bonds


The bond is therefore a polar covalent
bond.
The polar bond has a negative end and a
positive end (a so-called dipole; which we
indicate with a  with the appropriate sign
added).

H
F
_

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Concepts of Chemistry 9e
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Electronegativity
Cs 0.79
Na 0.93
H 2.20
C 2.55
N 3.04
O 3.44
Cl 3.16
F 3.99

L. Pauling – joint originator (with R. Mulliken) of
electronegativity scale
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Concepts of Chemistry 9e
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Polarity of Bonds


Bonds that involve atoms of differing
electronegativities have a concentration of
negative charge at the more
electronegative atom, and a deficiency of
charge at the less electronegative atom.
This unequal distribution of negative
charge creates a dipole, where one end of
the bond is slightly negative and the other
is slightly positive.
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Concepts of Chemistry 9e
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Bond and Molecular Dipoles
- see later for more details
Bond Dipole (illustrating two conventions)

H
F


Older Convention
H
F

IUPAC Convention
Molecular Dipole as the Vector Sum of Bond Dipoles

O


C
O
No molecular
dipole

Linear
nonpolar
O
S
Bent
O  polar
Molecular dipole
moment
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Predicting the Polarity of Bonds



When two atoms compete for a pair of electrons
in a bond, three possibilities exist:
1. Both atoms share the electrons equally,
forming a nonpolar bond (the electronegativity
difference between the the atoms is less than
0.4).
Examples: X-X (where X is same element: C-C, HH etc), C-H, Si-P, P-S, S-Se.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Bond Polarity….Contd.




2. The two atoms share electrons unequally,
forming a polar bond (the electronegativity
difference between the two atoms is less than 1.8,
but greater than 0.4).
Examples: H+-Hal-, Cl-F, >C=O, C-Si,
O-H
3. The electron pair is not shared, since one atom
acquires the electrons (the electronegativity
difference between the two atoms is greater than
1.8).
Examples: Cs+Cl-, Na+F-, Al3+(F-)3
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Nonpolar, Polar and Ionic bonds
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Objective for Section 9-8

Determine the bond angles and
geometries present in simple
molecules or ions from the Lewis
structure.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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9-8 Geometry of Simple Molecules



Electron pairs will repel each other, and
will govern the structure of the molecule,
all other things being equal.
Electron pairs will arrange themselves to
be as far apart as possible.
Note that molecular geometry is described
by the bonded atoms and does not include
the lone pairs.
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Concepts of Chemistry 9e
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Valence Shell Electron Pair
Repulsion - VSEPR

Degree of repulsion depends on the
electron pair types; in order of decreasing
repulsion




lone pair-lone pair
lone pair-bonding pair
bonding pair-bonding pair
We will also treat all of the electrons that
bond together two atoms as one electron
group regardless of whether the bond is
single, double or triple.
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Concepts of Chemistry 9e
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Parent Structures



We will consider three parent structures
(there are more, but will be covered later).
The central or inner atom is designated A,
outer atoms are designated X, and lone
pairs are designated E.
The parent structures are based on the
number of electron pairs that surround the
central atom.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Parent Structures
Parent
structure
Name
Bond angles
AX4
tetrahedron
109.5°
AX3
trigonal
planar
120°
AX2
linear
180°
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Concepts of Chemistry 9e
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Parent Structure Derivatives


Each parent structure can give rise to a
family of derivatives, simply by replacing
bonding pairs with lone pairs.
For AX4, there are two derivatives:



AX3E - the trigonal pyramid (NH3)
AX2E2 - bent or V-shape (H2O)
Replacement of the bonding pairs with the
lone pair(s) compresses the bond angles.
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Concepts of Chemistry 9e
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Linear Examples - AX2


The bonding pairs in the following
molecules arrange themselves to be as far
apart as possible.
These examples have two electron pairs.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Trigonal Examples - AX3



The examples shown here have three electron
pairs around the central atom.
Note that when one of the bonding pairs is
replaced by a lone pair, the bond angle is smaller.
Structure is designated by AX2E, where E is a
lone pair.
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Concepts of Chemistry 9e
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Tetrahedral Examples - AX4


Each molecule has four electron pairs
around the central atom.
Replacement of a bonding pair with a lone
pair yields the AX3E (NH3), AX2E2 (H2O)
Tetrahedral
Pyramidal
Malone and Dolter - Basic
Concepts of Chemistry 9e
Bent or V-shape
60
Objective for Section 9-9

Classify a molecule as polar or
nonpolar based on geometry and
electronegativity.
Malone and Dolter - Basic
Concepts of Chemistry 9e
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9-9 Polarity of Molecules


If the forces are equal but opposite, the
polarity of the bonds cancel.
Such molecules have polar bonds, but are
themselves nonpolar (e. g. CO2).
Malone and Dolter - Basic
Concepts of Chemistry 9e
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Nonpolar Molecules
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Concepts of Chemistry 9e
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Polar Molecules



Polar molecules result from an
arrangement of polar bonds such that the
entire molecule has a dipole.
The polar bonds can be arranged to
cancel the polarity (as in CO2, SO3, etc).
The best way to predict this in complex
cases is vector algebra, but it is important
to learn to recognize this in clear cut
structures.
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Concepts of Chemistry 9e
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Polar Molecules


(a) If the two bonds are opposite but not equally
polar, then a net dipole exists
(b) If the two bonds are equally or not equally
polar, yet are not opposite, a net molecular dipole
exists.
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Concepts of Chemistry 9e
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Linear Example: BeClH


The Be-Cl and Be-H bonds are both polar, but not
to the same extent.
Even though BeClH is linear, the polarity of the
bonds is not equal so the molecule is polar.
(Cl Be H)
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Concepts of Chemistry 9e
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Noncanceling Bond Polarities

Water is bent, so the dipolar O-H bonds
cannot cancel each other.
O
H
Malone and Dolter - Basic
Concepts of Chemistry 9e
H
67