Formal Charge
Draw Lewis Dot structure for SCN-1
Three possibilities:
-1
S
C
-1
S
N
C
-1
S
N
C
N
Which one is better? Do they resonate between them? No, the terminal atoms are not
identical, so one of them must be better than the others.
There must be some way to analyze the structures for important differences.
Formal Charge- covalent idea
Since covalent molecules share electrons, let’s assume that the bonds are shared equally.
Formal charge = # e-1 atom should have - # e-1 atom actually has
-1
S
C
N
Have 5 4 7
Shd 6
4 5
Chg +1 0 -2 = -1
-1
-1
S
C
N
S
C
6
6
0
4
4
0
6
5
-1 = -1
7
6
-1
4 5
4 5
0 0 = -1
N
Now we get some differences!
What makes the most sense? Forcing a +1 charge on a sulfur that likes to be a -2? No so
the first one is out. Then we have to decide which would rather have the -1 charge, the
sulfur in the third case or the nitrogen in the second case. Who is more electronegative?
The nitrogen is so the nitrogen gets the -1 charge so the middle one is the best.
-1
S
C
N
-1
S
C
N
-1
S
C
N
Example:
Decide which is better for the cyanate ion, NCO-1
-1
N
Have 5
Shd 5
Chg 0
C
-1
O
N
4 7
4 6
0 -1 = -1
6
5
-1
C
4
4
0
O
6
6
0 = -1
-1
N
7
5
-2
C
O
4 5
4 6
0 +1 = -1
The first is the best because the formal charges are all mostly reduced to zero and the
oxygen (the most electronegative atom) gets the formal charge of -1.
-1
-1
-1
N
C
O
N
C
O
N
C
O