PROBLEM: Show all possible resonance structures for each of the following molecules
or ions:
(a) SO2
(b) HNO2
(c) SCN–
Step 1: Count the number of valence electrons to figure out the number of electron pairs.
(a) SO2
(b) HNO2
(c) SCN–
S=6
O=6x2
Total = 18 e– or
9 pairs
H =1
N=5
O=6x2
Total = 18 e– or
9 pairs
S=6
C=4
N=5
–1=1
Total = 16 e– or 8 pairs
Step 2: Set up basic structure of each with one shared pair between each atom
(a) SO2
(b) HNO2
(c) SCN–
O – S – O
H – N – O
|
O
S – C – N
Step 2: Add unshared pairs to all elements. Count pairs and see if you are done.
(a) SO2
_
_
_
|O – S – O|
10 pairs
Too many
Not done
(b) HNO2
_
(c) SCN–
_
H – N – O|
|
|O|
10 pairs
Too many
Not done
_
_
_
|S – C – N|
10 pairs
Too many
Not done
Step 3: Remove extra pairs of electrons, and create double and triple bonds where needed
(c) SCN–
(a) SO2
(b) HNO2
|O – S = O|
or
_
|O = S – O|
H – N = O|
|
|O|
_
or
_
_
H – O – N = O|
or
or
_
H – N – O|
|S – C Ξ N| –
or
|S = C = N| –
_
H – O = N – O|
or
_
|S Ξ C – N| –
||
|O
9 pairs – Done
9 pairs – Done
8 pairs – Done