Chapter 9 Models of Chemical Bonding
Lewis structure and the Octet Rule
The Molecular Geometry
Polarity of Molecules
Atomic orbital and hybridization
Chapter highlights
•
Draw Lewis structure for molecules and polyatomic ions
•
Write the resonance forms
•
Predict the molecular geometry
•
Predict the polar molecules
•
State the hybridization of an atom in a bonded species
•
State the number of sigma and pi bonds in a species
Models of Chemical Bonding
9.1 Atomic Properties and Chemical Bonds
9.2 The Ionic Bonding Model
9.3 The Covalent Bonding Model
9.4 Bond Energy and Chemical Change
9.5 Electronegativity and Bond Polarity
9.6 An Introduction to Metallic Bonding
1
Types of Chemical Bonding
1. Metal with nonmetal:
electron transfer and ionic bonding
2. Nonmetal with nonmetal:
electron sharing and covalent bonding
3. Metal with metal:
electron pooling and metallic bonding
The three models of chemical bonding.
2
Depicting Ion Formation
PROBLEM:
Use partial orbital diagrams and Lewis symbols to depict the
formation of Na+ and O2- ions from the atoms, and determine the
formula of the compound.
Draw orbital diagrams for the atoms and then move electrons to make
filled outer levels. It can be seen that 2 sodiums are needed for each
oxygen.
PLAN:
SOLUTION:
O2-
Na
3s
2s
3p
2p
O
2s
2 Na+
2p
2Na+ + :O 2-:
+ :O :
.
3p
: :
.
Na
3s
.
Na
.
Na
Three ways to represent the formation of Li+ and F- through electron transfer.
Electron configurations
Li 1s22s1
+
F 1s22s22p5
Li+ 1s2
F- 1s22s22p6
+
Orbital diagrams
Li+
Li
2p
2s
1s
1s
+ F
1s
2s
2p
+
2p
2s
F2s
1s
2p
Lewis electron-dot symbols
.
: F :
Li+
+
: F::
+
:
Li .
:
3
Periodic Trends in Lattice Energy
Coulomb’s Law
charge A X charge B
electrostatic force a
distance2
energy = force X distance
therefore
charge A X charge B
electrostatic energy a
distance
cation charge X anion charge
electrostatic energy a
cation radius + anion radius
a DH0lattice
Trends in lattice energy.
4
Bond length and covalent radius.
Internuclear distance
(bond length)
Covalent
radius
Internuclear distance
(bond length)
72 pm
Internuclear distance
(bond length)
Covalent
radius
100 pm
Covalent
radius
114 pm
Internuclear distance
(bond length)
Covalent
radius
133 pm
5
Distribution of electron density of H2.
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6
SAMPLE PROBLEM 9.2
PROBLEM:
Comparing Bond Length and Bond Strength
Using the periodic table, but not Tables 9.2 and 9.3, rank the
bonds in each set in order of decreasing bond length and bond
strength:
(a) S - F, S - Br, S - Cl
PLAN:
(b) C = O, C - O, C
O
(a) The bond order is one for all and sulfur is bonded to halogens;
bond length should increase and bond strength should decrease
with increasing atomic radius. (b) The same two atoms are
bonded but the bond order changes; bond length decreases as
bond order increases while bond strength increases as bond order
increases.
SOLUTION:
(a) Atomic size increases going down a group. (b) Using bond orders we get
Bond length: S - Br > S - Cl > S - F
Bond length: C - O > C = O > C
Bond strength: S - F > S - Cl > S - Br
Bond strength: C
O
O>C=O>C-O
7
SAMPLE PROBLEM
PROBLEM:
Postulating Hybrid Orbitals in a Molecule
Use partial orbital diagrams to describe mixing of the atomic orbitals
of the central atom leads to hybrid orbitals in each of the following:
(a) Methanol, CH3OH
PLAN:
(b) Sulfur tetrafluoride, SF4
Use the Lewis structures to ascertain the arrangement of groups and
shape of each molecule. Postulate the hybrid orbitals. Use partial
orbital box diagrams to indicate the hybrid for the central atoms.
SOLUTION:
H
(a) CH3OH
The groups around C are
arranged as a tetrahedron.
C O
H
H H
SAMPLE PROBLEM
O also has a tetrahedral
arrangement with 2 nonbonding epairs.
Postulating Hybrid Orbitals in a Molecule
continued
2p
2s
2p
sp3
single C atom
hybridized
C atom
2s
sp3
hybridized
O atom
single O atom
(b) SF4 has a seesaw shape with 4 bonding and 1 nonbonding e- pairs.
F
F S
F
F
3d
3d
3p
sp 3d
3s
S atom
hybridized
S atom
8