Class 33

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CHEMISTRY 161
Chapter 9
Chemical Bonding I
www.chem.hawaii.edu/Bil301/welcome.html
1
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
5
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
6
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
7
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
8
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
9
H
C
N
H
N
0
0
0
0
+1
C
-1
FORMAL CHARGES
THIS IS THE MOST LIKELY
H
C
N
OZONE
10
2. RESONANCES
O O
O
O
O O
NO3
N:
1s22s22p3
O: 1s22s22p4
plus one extra electron for negative charge
11
O
O
+
N
O
-
12
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
13
O
N
128 pm
bond angles 120 0
O
O
modify the description by blending the structures
blending of structures is called resonance
14
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
15
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
16
2CO3
NO2
17
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
18
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
19
Lewis structure for SF6
1s22s22p5
F has seven
S has six
1s22s22p63s22p4
SF2
SF4
PF3 PF5
SF6
NF3 NF5
ClO4-
SO42I3-
20
2. less than an octet around central atom
BeH2
AlF3
resonances
BF3
NH3
(dative bond)
Lewis acids
Lewis base
21
3. molecules with unpaired electrons
FREE RADICALS
NO
but not
NO
22
4. BOND ENERGIES
HCl
23
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
24
Homework
Chapter 9
pages 347-359, problem sets
25
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