Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Chapter 9
Chemical
Bonding I:
Lewis Theory
Question
Complete the following sentence…
Properties of substances can be explained in terms
of differences in chemical __________
e.g. -salt dissolves in water better than oil
-certain substances are electrolytes
-alcohol evaporates quicker than water
-wax melts at a lower temperature than salt
Bonding Theories
•
•
•
•
•
explain how and why atoms attach together
one of the simplest bonding theories is called Lewis Theory
Lewis Theory uses valence electrons to explain bonding
explains why some combinations of atoms are stable and others
are not
using Lewis Theory, we can draw models – called Lewis
structures – that allow us to predict many properties of
molecules
 such as molecular shape, size, polarity
Tro, Chemistry: A Molecular Approach
3
Types of Bonds
Types of Atoms
metals to
nonmetals
nonmetals to
nonmetals
metal to
metal
Tro, Chemistry: A Molecular Approach
Type of Bond
Ionic
Covalent
Metallic
Bond
Characteristic
electrons
transferred
electrons
shared
electrons
pooled
9
Types of Bonding
10
Determining the Number of Valence
Electrons in an Atom
• the column number on the Periodic Table tells us the
no. valence e1A
2A
3A
4A
5A
6A
7A
8A
Li
Be
B
C
N
O
F
Ne
1 e-
2 e-
3 e-
4 e-
5 e-
6 e-
7 e-
8 e-
Tro, Chemistry: A Molecular Approach
13
Lewis Symbols of Atoms
• use symbol of element to represent nucleus and
•
inner electrons
use dots around the symbol to represent valence
electrons
 pair first two electrons for the s orbital
 put one electron on each open side for p electrons
 then pair rest of the p electrons

Li

Be


B

Tro, Chemistry: A Molecular Approach



C




N





O






F






Ne



14
Lewis Symbols of Ions
• Cations have Lewis symbols without valence ee.g. lithium
Li•
Li+
e- loss
• Anions have Lewis symbols with 8 valence electrons
e.g. flourine



F

Tro, Chemistry: A Molecular Approach







F






e- gain
15
Question
Draw Lewis dot structures of elemental magnesium and magneisum
ion
Draw Lewis dot structures of elemental nitrogen and the nitride ion
Stable Electron Arrangements
And Ion Charge
• Metals form cations by losing
e- to become isoelectric to the
previous noble gas
Atom
• Nonmetals form anions by
gaining enough e- to become
Na
isoelectric to the previous
Mg
noble gas
[Ne] = 1s22s22p6
Tro, Chemistry: A Molecular Approach
Atom’s
Electron
Config
[Ne]3s1
Na+
Ion’s
Electron
Config
[Ne]
[Ne]3s2
Mg+2
[Ne]
Al
[Ne]3s23p1
Al+3
[Ne]
O
[He]2s22p4
O-2
[Ne]
F
[He]2s22p5
F-
[Ne]
Ion
18
Lewis Theory
• the basis of Lewis Theory is that there are
certain electron arrangements in the atom that
are more stable
octet rule
• bonding occurs so atoms attain a more stable
electron configuration
Tro, Chemistry: A Molecular Approach
19
Octet Rule
• when atoms bond, they tend to gain, lose, or share e- to result in
•
8 valence ens2np6
 noble gas configuration
• many exceptions
 H, Li, Be, B attain an electron configuration like He
 He = 2 valence e Li loses its one valence e H shares or gains one e though it commonly loses its one electron to become H+
 Be loses 2 electrons to become Be2+
 though it commonly shares its two electrons in covalent bonds, resulting in 4
valence electrons
 B loses 3 electrons to become B3+
 though it commonly shares its three electrons in covalent bonds, resulting in 6
valence electrons
 expanded octets for elements in Period 3 or below
 using empty valence d orbitals
Tro, Chemistry: A Molecular Approach
20
Lewis Theory and Ionic Bonding
• Transfer of e- from metal atom to nonmetal atom,
resulting in ions that are attracted to each other and
therefore bond, e.g. NaCl

Na

+


Cl

Tro, Chemistry: A Molecular Approach


Na +





Cl  




NaCl
22
Tro, Chemistry: A Molecular Approach
23
Predicting Ionic Formulas
Using Lewis Symbols
• e- are transferred until the metal loses all its valence eand the nonmetal obtains an octet
Li



Li

O

Tro, Chemistry: A Molecular Approach


 O 



  



2 Li +
2
Li2O
24
Crystal Lattice
• Ionic substances exist as crystal lattices of repeating
unit cells
Model of NaCl
Tro, Chemistry: A Molecular Approach
25
Ionic Bonding
Model vs. Reality
• ionic compounds have high melting points and boiling
points
 MP generally > 300°C
 all ionic compounds are solids at room temperature
• because the attractions between ions are strong,
breaking down the crystal requires a lot of energy
 the stronger the attraction (larger the lattice energy), the
higher the melting point
Tro, Chemistry: A Molecular Approach
37
Properties
• Describe the general properties of ionic compounds
• Metals react with non-metals:
 Crystalline solids
 3-D units extended
 high mp/bp (all solids)
 brittle
 aqueous solutions conduct electricity
Ionic Bonding
• When ionic compounds are dissolved in water, they
dissociate to form aqueous ions:
NaCl(s)
→
Na+(aq) + Cl-(aq)
• The resulting solution conducts electricity and is called
an electrolyte
Tro, Chemistry: A Molecular Approach
40
Conductivity of NaCl
in NaCl(s), the
ions are stuck in
position and not
allowed to move
to the charged
rods
Tro, Chemistry: A Molecular Approach
in NaCl(aq), the
ions are
separated and
allowed to move
to the charged
rods
41
Question
Use Lewis dot structures to represent the formation of aluminum
bromide
Use Lewis dot structures to represent the formation of lithium
hydride
Tro, Chemistry: A Molecular Approach
43
Types of Bonds
Types of Atoms
metals to nonmetals
Type of Bond
Ionic
Bond Characteristic
e- transferred
nonmetals to nonmetals
Covalent
e- shared
metal to metal
Metallic
e- pooled
Tro, Chemistry: A Molecular Approach
44
Types of Bonding
45
Single Covalent Bonds
• two atoms share a pair of electrons
e.g. fluorine
F
Tro, Chemistry: A Molecular Approach
••
••
F
••
••
••
• F
••
••
••
••
F •
••
••
••
F
••
F
47
Single Covalent Bonds
H•
••
•O
••
•
e.g. water
•H
duet
duet
••
••
••
H O H
••
octet
2 bonding pairs
2 lone pairs
Tro, Chemistry: A Molecular Approach
48
Double Covalent Bond
• two atoms sharing two pairs of electrons
••
•O
••
•
••
•O
••
•
e.g. oxygen
O •• O
••
··
··
··O O ··
Tro, Chemistry: A Molecular Approach
49
Triple Covalent Bond
• two atoms sharing 3 pairs of electrons
••
•N
•
•
••
•N
•
•
e.g. nitrogen
N •• N
••
••
··N
Tro, Chemistry: A Molecular Approach
N ··
50
Covalent Bonding
Predictions from Lewis Theory
• Lewis theory allows us to predict the formulas of molecules
• Lewis theory predicts that some combinations should be stable, while others
should not
 because the stable combinations result in “octets”
• Lewis theory also shows that covalent bonds are highly directional
 the shared electrons are most stable between the bonding atoms
 resulting in molecules rather than an array
Tro, Chemistry: A Molecular Approach
51
Ionic Bonding
Model vs. Reality
• molecular compounds do not conduct electricity in the liquid state
• molecular acids conduct electricity when dissolved in water, but not in the
solid state
• in molecular solids, there are no charged particles around to allow the
material to conduct
• when dissolved in water, molecular acids are ionized, and have the ability to
move through the structure and therefore conduct electricity
Tro, Chemistry: A Molecular Approach
55
Bond Polarity
• covalent bonding between unlike atoms results in unequal sharing
of the e one atom pulls the electrons in the bond closer to its side
 one end of the bond has larger electron density than the other
• the result is a polar covalent bond
 bond polarity
 the end with the larger electron density gets a partial negative
charge
 the end that is electron deficient gets a partial positive charge
Tro, Chemistry: A Molecular Approach
56
HF

EN
2.1
ENd+
2.1
H
F


d4.0
EN

Tro, Chemistry: A Molecular Approach
57
Electronegativity
• Ability of an atom to attract e- to itself in a chemical bond
• increases across period (left to right) and
• decreases down group (top to bottom)
Tro, Chemistry: A Molecular Approach
58
Electronegativity and Bond Polarity
• If ΔE.N. between bonded atoms is 0, the bond is pure covalent
 equal sharing
• If ΔE.N. between bonded atoms is 0.1 - 0.4, the bond is nonpolar covalent
• If ΔE.N. between bonded atoms 0.5 - 1.9, the bond is polar covalent
• If ΔE.N. between bonded atoms ≥ 2.0, the bond is ionic
4%
NP
0
0.4
Percent Ionic Character
51%
PC
IONIC
2.0
Electronegativity Difference
“100%”
4.0
59
Bond Polarity
ENCl = 3.0
ΔEN = 3.0 - 3.0 = 0
Pure Covalent
Tro, Chemistry: A Molecular Approach
ENCl = 3.0
ENH = 2.1
ΔEN = 3.0 – 2.1 = 0.9
Polar Covalent
ENCl = 3.0
ENNa = 1.0
ΔEN = 3.0 – 0.9 = 2.1
Ionic
60
Lewis Structures
of Molecules
•
•
•
•
shows pattern of valence electron distribution in the molecule
useful for understanding the bonding in many compounds
allows us to predict shapes of molecules
allows us to predict properties of molecules and how they will
interact together
Tro, Chemistry: A Molecular Approach
66
Writing Lewis Structures of Molecules
HNO3
O
1) Write skeletal structure
 H always terminal

in oxyacid, H outside attached to O’s
H
O
N
O
 make least electronegative atom central

N is central
2) Count valence e sum the valence electrons for each
atom
 add 1 e- for each −ve charge
 subtract 1 e- for each +ve charge
Tro, Chemistry: A Molecular Approach
N=5
H=1
O3 = 3(6) = 18
Total = 24 e-
68
Writing Lewis Structures of Molecules
HNO3
3) Attach central atom to the surrounding atoms with
pairs of e- and subtract from the total
O

H — O — N — O
Tro, Chemistry: A Molecular Approach
eStart 24
Used 8
Left 16
69
Writing Lewis Structures of Molecules
HNO3

4) Complete octets, outside-in
: O :
 H is already complete with 2


H — O — N — O
 1 bond
and re-count
Tro, Chemistry: A Molecular Approach

e-
N=5
H=1
O3 = 3(6) = 18
Total = 24 e-

eStart 24
Used 8
Left 16

:
eStart 16
Used 16 (8 pairs)
Left 0
70
Writing Lewis Structures of Molecules
HNO3
5) If all octets complete, give extra
electrons to central atom.
 elements with d orbitals can have
more than 8 electrons

Period 3 and below
:

O

|
octet, bring in electrons from
H — O — N
outside atoms to share
6) If central atom does not have
 follow common bonding patterns
if possible
Tro, Chemistry: A Molecular Approach

:

O:
71
Practice - Lewis Structures
Draw Lewis structures for the following:
• CO2
• NH3
• NO2-
Tro, Chemistry: A Molecular Approach
72
Writing Lewis Formulas of
Molecules (cont’d)
7)
Assign formal charges to the atoms
a)
formal charge = valence e- - lone pair e- - ½ bonding eb)
follow the common bonding patterns




O

0


S
H

+1 

-1 
O


O


|
||

H  C  C  O  H
|
H

all 0
sum of all the formal charges in a molecule = 0
in an ion, total equals the charge
Tro, Chemistry: A Molecular ApproacH
74
Practice - Assign Formal Charges
• CO2
• NH3
• NO2••
•O
•
••
N
Tro, Chemistry: A Molecular Approach
••
O ••
••
-
76
Practice - Assign Formal Charges
• CO2
• NH3
all 0
all 0
• NO2••
•O
•
••
N
Tro, Chemistry: A Molecular Approach
-1
••
O ••
••
77
Resonance
• when there is more than one Lewis structure for a molecule that
•
•
differ only in the position of the electrons, they are called
resonance structures
the actual molecule is a combination of the resonance forms – a
resonance hybrid
 it does not resonate between the two forms, though we often
draw it that way
look for multiple bonds or lone pairs
••
••
••
•• O ••
••
•• S •• O
••
Tro, Chemistry: A Molecular Approach
••
•• O
••
••
••
••S ••
•• O ••
78
Rules of Resonance Structures
• Resonance structures must have the same connectivity
 only electron positions can change
• Resonance structures must have the same number of
•
•
•
•
•
electrons
Second row elements have a maximum of 8 electrons
 bonding and nonbonding
 third row can have expanded octet
Formal charges must total same
Better structures have fewer formal charges
Better structures have smaller formal charges
Better structures have − formal charge on more
electronegative atom
Tro, Chemistry: A Molecular Approach
81
Drawing Resonance Structures
1. draw first Lewis structure that
maximizes octets
2. assign formal charges
3. move electron pairs from atoms
with (-) formal charge toward
atoms with (+) formal charge
4. if (+) fc atom 2nd row, only move
in electrons if you can move out
electron pairs from multiple
bond
5. if (+) fc atom 3rd row or below,
keep bringing in electron pairs to
reduce the formal charge, even if
get expanded octet.
Tro, Chemistry: A Molecular Approach
··
··O ··
-
-1
0
·· O
··
N
+1
··
·· O ··
+1
··
O ··
··
-1
-
-1
··
··O
··
-1
N
··
O
··
82
Exceptions to the Octet Rule
• expanded octets
elements with empty d orbitals can have more
than 8 electrons
• odd number electron species e.g., NO
will have 1 unpaired electron
free-radical
very reactive
• incomplete octets
B, Al
Tro, Chemistry: A Molecular Approach
83
Drawing Resonance Structures
1. draw first Lewis structure that
maximizes octets
2. assign formal charges
3. move electron pairs from atoms
with (-) formal charge toward
atoms with (+) formal charge
4. if (+) fc atom 2nd row, only move
in electrons if you can move out
electron pairs from multiple bond
5. if (+) fc atom 3rd row or below,
keep bringing in electron pairs to
reduce the formal charge, even if
get expanded octet.
Tro, Chemistry: A Molecular Approach
H
H
··
O
··
··
O
··
-1
··
··O ··
+2
S
·
O ··
·
-1 ··
0
··
O ··
S
0
·· O 0
··
··
O
··
··
O
··
H
H
84
Question
Draw Lewis structures with assigned formal charges of
HCl, H2O2 and SF6
Metallic Bonds
• low ionization energy of metals allows them to lose electrons
easily
• the simplest theory of metallic bonding involves the metals
atoms releasing their valence electrons to be shared by all to
atoms/ions in the metal
 an organization of metal cation islands in a sea of electrons
 electrons delocalized throughout the metal structure
• bonding results from attraction of cation for the delocalized
electrons
Tro, Chemistry: A Molecular Approach
97
Metallic Bonding
Tro, Chemistry: A Molecular Approach
98
Metallic Bonding
Model vs. Reality
• metallic solids conduct electricity
• because the free electrons are mobile, it allows the electrons
•
•
to move through the metallic crystal and conduct electricity
as temperature increases, electrical conductivity decreases
heating causes the metal ions to vibrate faster, making it
harder for electrons to make their way through the crystal
Tro, Chemistry: A Molecular Approach
99
Metallic Bonding
Model vs. Reality
• metallic solids conduct heat
• the movement of the small, light electrons through the solid
•
•
can transfer kinetic energy quicker than larger particles
metallic solids reflect light
the mobile electrons on the surface absorb the outside light
and then emit it at the same frequency
Tro, Chemistry: A Molecular Approach
100
Metallic Bonding
Model vs. Reality
• metallic solids are malleable and ductile
• because the free electrons are mobile, the direction of the
•
attractive force between the metal cation and free electrons
is adjustable
this allows the position of the metal cation islands to move
around in the sea of electrons without breaking the
attractions and the crystal structure
Tro, Chemistry: A Molecular Approach
101
Metallic Bonding
Model vs. Reality
• metals generally have high melting points and boiling points
•
•
•
•
•
 all but Hg are solids at room temperature
the attractions of the metal cations for the free electrons is strong
and hard to overcome
melting points generally increase to right across period
the charge on the metal cation increases across the period,
causing stronger attractions
melting points generally decrease down column
the cations get larger down the column, resulting in a larger
distance from the nucleus to the free electrons
Tro, Chemistry: A Molecular Approach
102