CHEMISTRY 161
Chapter 9
Chemical Bonding I
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REVISION
LEWIS SYMBOLS
IONIC COMPOUDS
COVALENT COMPOUNDS
ELECTRONEGATIVITY
2
Lewis considers only valence electrons
H2O
H
bonding pair of
electrons
O
H
non-bonding, or lone pair
of electrons
single – double – triple - quadrupole
3
LEWIS STRUCTURES
1. formal charge
2. concept of resonances
3. exceptions to the octet rule
4. bond energies
4
1. formal charge
indication of the extent to which atoms
have gained or lost electrons in
formal
#valence _
charge = electrons
{
#unshared
electrons
_
} {
}
1/2#shared
electrons
structures with the lowest formal charges are likely to
have the lowest energy
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all possible Lewis structures with stable
electronic configurations for HCN and HNC
H
C
Formal
#valence _
charge = electrons
N
{
H
#unshared
electrons
N
_
} {
Calculate formal charge for this one H
FC on C
=4 -0
- 1/2 (8)
C
}
1/2#shared
electrons
C
N
=0
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All possible Lewis structures with stable
electronic configurations for HCN and HNC.
H
C
Formal
#valence _
charge = electrons
N
{
H
#unshared
electrons
N
_
} {
Calculate formal charge for this one H
FC on C
=4 -0
- 1/2 (8)
=0
FC on N
=5 -2
- 1/2 (6)
=0
C
}
1/2#shared
electrons
C
N
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All possible Lewis structures with stable
electronic configurations for HCN and HNC.
H
C
Formal
#valence _
charge = electrons
N
{
H
#unshared
electrons
N
_
} {
Calculate formal charge for this one H
FC on C
=4 -2
- 1/2 (6)
C
}
1/2#shared
electrons
N
C
= -1
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All possible Lewis structures with stable
electronic configurations for HCN and HNC.
H
C
Formal
#valence _
charge = electrons
N
{
H
#unshared
electrons
N
_
} {
C
}
1/2#shared
electrons
Calculate formal charge for this one H
FC on C
=4 -2
- 1/2 (6)
= -1
FC on N
=5 -0
- 1/2 (8)
= +1
N
C
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H
C
N
H
N
0
0
0
0
+1
C
-1
FORMAL CHARGES
THIS IS THE MOST LIKELY
H
C
N
OZONE
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2. RESONANCES
O O
O
O
O O
NO3
N:
1s22s22p3
O: 1s22s22p4
plus one extra electron for negative charge
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O
O
+
N
O
-
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experiment shows all three bonds are the same
O
N
128 pm
O
bond angles 120 0
O
any one of the structures suggests one is different!
O
O
N
O
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O
N
128 pm
bond angles 120 0
O
O
modify the description by blending the structures
blending of structures is called resonance
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RESONANCE
use a double headed arrow between the
structures
O
O
O
O
N
N
N
O
O
O
O
O
electrons involved are said to be
DELOCALIZED over the structure.
blended structure is a
RESONANCE HYBRID
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RESONANCE
We use a double headed arrow between the
structures..
O
O
O
O
N
N
N
O
O
O
O
O
O
N
O
O
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2CO3
NO2
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3. Exceptions to the octet rule
1. more than 8 electrons around central atom
2. less than an octet around central atom
3. molecules with unpaired electrons
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1. more than 8 electrons around central atom
elements in rows 3 and following can exceed octet rule
SF6
F
F
F
F
F
S
F
F
F
F
S
F
F
F
participation of d electrons
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Lewis structure for SF6
1s22s22p5
F has seven
S has six
1s22s22p63s22p4
SF2
SF4
PF3 PF5
SF6
NF3 NF5
ClO4-
SO42I3-
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2. less than an octet around central atom
BeH2
AlF3
resonances
BF3
NH3
(dative bond)
Lewis acids
Lewis base
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3. molecules with unpaired electrons
FREE RADICALS
NO
but not
NO
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4. BOND ENERGIES
HCl
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SUMMARY
1. formal charge
2. concept of resonances
3. exceptions to the octet rule
3.1. excess of electrons
3.2. missing electrons
3.3. radicals
4. bond energies
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Homework
Chapter 9
pages 347-359, problem sets
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