Chapter 9
Bonding and Molecular Structure:
Fundamental Concepts
Dr. S. M. Condren
Chemical Bonding
Problems and questions —
How is a molecule or
polyatomic ion held
together?
Why are atoms distributed at
strange angles?
Why are molecules not flat?
Can we predict the structure?
How is structure related to
chemical and physical
properties?
Dr. S. M. Condren
Structure & Bonding
NN triple bond. Molecule is unreactive
White phosphorus
is a tetrahedron of
P atoms.
Very reactive!
Red phosphorus,
a polymer. Used
in matches.
Less reactive!
Dr. S. M. Condren
Forms of Chemical Bonds
There are 2 extreme forms of connecting or bonding atoms:
Ionic—complete transfer of
1 or more electrons from
one atom to another
Covalent—some valence
electrons shared between
atoms
Most bonds are somewhere in between.
Dr. S. M. Condren
Ionic Compounds
Metal of low IE
Nonmetal
of high EA
2 Na(s) + Cl2(g) ---> 2 Na+ + 2 ClDr. S. M. Condren
Covalent Bonding
The bond arises from the mutual
attraction of 2 nuclei for the same
electrons. Electron sharing results.
Bond is a balance of attractive and repulsive forces.
Dr. S. M. Condren
Bond Formation
A bond can result from a “head-to-head”
overlap of atomic orbitals on
neighboring atoms.
••
H
+
••
•
•
Cl
H
••
•
•
Cl
••
Note that each atom has a
single, unpaired electron.
Overlap of H (1s) and Cl (3p)
Dr. S. M. Condren
Chemical Bonding: Objectives
Objectives are to understand:
1. valence e- distribution in
molecules and ions.
2. molecular structures
3. bond properties and their
effect on molecular properties.
Dr. S. M. Condren
Electron Distribution in Molecules
• Electron distribution is
depicted with
Lewis electron
dot structures
• Valence electrons are
distributed as shared or
G. N. Lewis
1875 - 1946
BOND PAIRS
and unshared or
LONE PAIRS.
Dr. S. M. Condren
Bond and Lone Pairs
• Valence electrons are distributed as
shared or BOND PAIRS and
unshared or LONE PAIRS.
••
H
•
•
Cl
lone pairs
••
shared or
bond pair
This is called a LEWIS
ELECTRON DOT structure.
Dr. S. M. Condren
Valence Electrons
Electrons are divided between core and
valence electrons
B 1s2 2s2 2p1
Core = [He] , valence = 2s2 2p1
Br [Ar] 3d10 4s2 4p5
Core = [Ar] 3d10 , valence = 4s2 4p5
Dr. S. M. Condren
Rules of the Game
No. of valence electrons of a main group
atom = Group number
•For Groups 1A-4A, no. of bond pairs =
group number.
• For Groups 5A -7A, BP’s = 8 - Grp. No.
Group 3A
Group 5A
Dr. S. M. Condren
Rules of the Game
No. of valence electrons of an atom = Group number
For Groups 1A-4A, no. of bond pairs = group number
For Groups 5A -7A, BP’s = 8 - Grp. No.
•Except for H (and sometimes atoms of
3rd and higher periods),
BP’s + LP’s = 4
This observation is called the
OCTET RULE
Dr. S. M. Condren
Hydrophobic vs. Hydrophilic
• Hydrophobic - translation of Greek – water
fear
• Hydrophilic – translation of Greek – water
friendship
Dr. S. M. Condren
Building a Dot Structure
Ammonia, NH3
1. Decide on the central atom; never H.
Central atom is atom of lowest affinity
for electrons.
Therefore, N is central
2. Count valence electrons
H = 1 and N = 5
Total = (3 x 1) + 5
= 8 electrons / 4 pairs
Dr. S. M. Condren
Building a Dot Structure
3. Form a single bond between
the central atom and each
surrounding atom
4. Remaining electrons form LONE
PAIRS to complete octet as needed.
3 BOND PAIRS and 1 LONE PAIR.
H N H
H
••
H N H
H
Note that N has a share in 4 pairs (8 electrons),
while H shares 1 pair.
Dr. S. M. Condren
Lewis Structures
CH4
methane
Step 1. Central atom = C
Step 2. Count valence electrons
C= 4
4xH = 4x1= 4
TOTAL = 8 e- or 4 pairs
Step 3. Form bonds
Dr. S. M. Condren
Lewis Structures
C2H6
ethane
Dr. S. M. Condren
Multiple Covalent Bonds
double bond => 2 pairs shared
triple bond => 3 pairs shared
normally occurs between:
C atoms; N atoms; O atoms;
a C atom and a N, O or S atom
a N atom and a O or S atom
a S atom and an O atom
Dr. S. M. Condren
Double and even
triple bonds are
commonly
observed for C,
N, P, O, and S
H2CO
SO3
C2F4
Dr. S. M. Condren
Carbon Dioxide, CO2
C
1. Central atom = _______
4 or __
2 pairs
2. Valence electrons = __
3. Form bonds.
This leaves 6 pairs.
Dr. S. M. Condren
Carbon Dioxide, CO2
4. Place lone pairs on outer atoms.
5. So that C has an octet, we shall form
DOUBLE BONDS between C and O.
The second bonding pair forms a pi
Dr. S. M. Condren
(π) bond.
Steps to form Lewis Electron Dot Structure
1.
2.
3.
4.
5.
Central atom = _______
Valence electrons = __ or __ pairs
Form bonds.
Place lone pairs on outer atoms.
Form multiple bonds as necessary to
obey Lewis’ “Octet Rule”.
6. Remember that there are MANY
compounds that do not obey the Octet
Rule.
Dr. S. M. Condren
Lewis Structures
CO carbon monoxide
Dr. S. M. Condren
Exceptions to Octet Rule
NO nitric oxide
Dr. S. M. Condren
Exceptions to Octet Rule
NO2 nitrogen dioxide
resonance
Dr. S. M. Condren
Exceptions to Octet Rule
PF5
expanded octet
Dr. S. M. Condren
Exceptions to Octet Rule
expanded octet
SF4
••
•F
•
••
••
•• F
••
••
S
•• F ••
••
Dr. S. M. Condren
••
•
F
•
••
Exceptions to Octet Rule
SF6
expanded octet
Dr. S. M. Condren
Formal Atom Charges
• Atoms in molecules often bear a charge
(+ or -).
• The predominant resonance structure of a
molecule is the one with charges as close
to 0 as possible.
• Formal charge
= Group number
– 1/2 (no. of bonding electrons)
- (no. of LP electrons)
Dr. S. M. Condren
Carbon Dioxide, CO2
+6 - ( 1 / 2 ) ( 4 ) - 4
••
•
•
O
••
C
O
+4 - ( 1 / 2 ) ( 8 ) - 0
Dr. S. M. Condren
=
•
•
0
= 0
Thiocyanate Ion, SCN-
6 - (1/2)(2) - 6 = -1
5 - (1/2)(6) - 2 = 0
••
•
•
S
C
N
•
•
••
4 - (1/2)(8) - 0 = 0
Dr. S. M. Condren
Thiocyanate Ion,
••
••
•
•
S
SCN
C
N
•
•
•
•
••
S
C
N
•
•
••
••
•
•
S
C
N
•
•
••
Which is the most important resonance form?
Dr. S. M. Condren
MOLECULAR GEOMETRY
Dr. S. M. Condren
MOLECULAR GEOMETRY
VSEPR
Valence Shell Electron Pair Repulsion theory.
Most important factor in determining geometry is
relative repulsion between electron pairs.
Molecule adopts the shape that
minimizes the electron pair repulsions.
Dr. S. M. Condren
Electron Pair Geometries
Dr. S. M. Condren
Sulfur Dioxide, SO2
1. Central atom = S
2. Valence electrons = 18 or 9 pairs
3. Form double bond so that S has an
octet — but note that there are two ways of
doing this.
bring in
left pair
••
•
•
O
••
••
S
OR bring in
right pair
••
•
•
O
••
Dr. S. M. Condren
Sulfur Dioxide, SO2
This leads to the following structures.
These equivalent structures are called
RESONANCE STRUCTURES. The true
electronic structure is a HYBRID of the two.
Dr. S. M. Condren
Geometries for Four Electron Pairs
Dr. S. M. Condren
Structure Determination by VSEPR
Ammonia, NH3
••
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
H N H
Dr. S. M. Condren
H
Structure Determination by VSEPR
Ammonia, NH3
The electron pair geometry is tetrahedral.
lone pair of electrons
in tetrahedral position
N
H
H
H
The MOLECULAR GEOMETRY, the
positions of the atoms, is PYRAMIDAL.
Dr. S. M. Condren
Structure Determination by VSEPR
Water, H2O
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
The electron pair geometry is
TETRAHEDRAL.
The molecular
geometry is BENT.
Dr. S. M. Condren
Consequences of H2O Polarity
Dr. S. M. Condren
Structures with Central Atoms
with More Than or less Than
4 Electron Pairs
Often occurs with Group 3A elements and
with those of 3rd period and higher.
Dr. S. M. Condren
Molecular
Geometries for
Five Electron Pairs
All based on trigonal
bipyramid
Dr. S. M. Condren
Molecular
Geometries for Six
Electron Pairs
All are based on the
8-sided octahedron
Dr. S. M. Condren
Bond Properties
• What is the effect of bonding and structure
on molecular properties?
Free rotation
around C–C single
bond
Dr. S. M. Condren
No rotation
around C=C
double bond
Bond Order
# of bonds between a pair of atoms
Double bond
Single bond
Acrylonitrile
Triple
bond
Dr. S. M. Condren
Bond Order
Fractional bond orders occur in molecules with
resonance structures.
1
••
••
Consider NO2N
N
•• • •••
••
••
O
O• • O
O
••
••
••
••
1
The N—O bond order = 1.5
Bond order =
Total # of e - pairs used for a type of bond
Total # of bonds of that type
3 e - pairs in N— O bonds
Bond order =
2 N — O bonds
Dr. S. M. Condren
Bond Order
Bond order is proportional to two important
bond properties:
(a) bond strength
(b) bond length
414 kJ
123 pm
110 pm
Dr. S. M. Condren
745 kJ
Dr. S. M. Condren
Compare O-O and O=O. Is O=O expected
to be
stronger, weaker, or the same strength?
Dr. S. M. Condren
Is O=O expected to be
longer, shorter, or the same length?
Dr. S. M. Condren
Using Bond Energies
Estimate the energy of the reaction
= 436 kJ/mol
H—H + Cl—Cl ----> 2 H—Cl H—H
Cl—Cl = 242 kJ/mol
H—Cl = 432 kJ/mol
Net = ∆H = [S bondsbroken] – [S bondsformed]
Sum of H-H + Cl-Cl bond energies =
436 kJ + 242 kJ = +678 kJ
2 mol H-Cl bond energies = 864 kJ
Net = ∆H = +678 kJ - 864 kJ = -186 kJ
∆Hfo (HCl(g)) = -92.31 kJ/mol or -184 kJ
Dr. S. M. Condren
Molecular Polarity
Water
Boiling point
= 100 ˚C
Methane
Boiling point
= -161 ˚C
Why do water and
methane differ so
much in their
boiling points?
Why do ionic compounds dissolve in
water?
Dr. S. M. Condren
Bond Polarity
HCl is POLAR
because it has a
positive end and a
negative end.
+d -d
••
••
H Cl
••
Cl has a greater share
in bonding electrons
than does H.
Cl has slight negative charge (-d) and H
has slight positive charge (+ d)
Dr. S. M. Condren
Bond Polarity
• Three molecules with
polar, covalent bonds.
• Each bond has one
atom with a slight
negative charge (-d)
and another with a
slight positive charge
(+ d)
Dr. S. M. Condren
Linus Pauling, 1901-1994
The only person to receive two unshared
Nobel prizes (for Peace and Chemistry).
Chemistry areas: bonding, electronegativity,
protein structure
Dr. S. M. Condren
Electronegativity, 
 is a measure of the ability of an atom in a
molecule to attract electrons to itself.
Dr. S. M. Condren
Bond Polarity
+d -d
••
••
H Cl
••
Due to the bond polarity, the H—Cl
bond energy is GREATER than
expected for a “pure” covalent
bond.
BOND
“pure” bond
real bond
ENERGY
339 kJ/mol calc’d
432 kJ/mol measured
Difference = 92 kJ. This difference is
proportional to the difference in
ELECTRONEGATIVITY, .
Dr. S. M. Condren
Electronegativity
Pauling Scale
• relative attraction of an atom for electrons,
its own and those of other atoms
• same trends as ionization energy,
increases from lower left corner to the
upper right corner
• fluorine: E.N. = 4.0
Dr. S. M. Condren
Electronegativities of the Elements
Dr. S. M. Condren
Bond Polarity
Which bond is more polar (or DIPOLAR)?
O—H
O—F

3.5 - 2.1
3.5 - 4.0

1.4
0.5
OH is more polar than OF
-d
+d
+d
-d
F
O
H
O
and polarity is “reversed.”
Dr. S. M. Condren
Molecular Polarity
Molecules will be polar if
a) bonds are polar
AND
b) the molecule is NOT “symmetric”
All above are NOT polar
Dr. S. M. Condren
Polar or Nonpolar?
Compare CO2 and H2O. Which one is polar?
Dr. S. M. Condren
Covalent Bond Properties
electronegativity
nonpolar bonds => diff. EN = 0
polar bonds => diff. EN > 0
ionic bonds => diff. EN > 1.5
Dr. S. M. Condren
CH4 … CCl4
•
Polar or Not?
Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”
Dr. S. M. Condren