Formal Charge
MAKE SURE YOU HAVE THE CORRECT
STRUCTURE
Formal Charge
 The Formal Charge of an atom is the charge that an
atom in a molecule would have if all atoms had the
same electronegativity.
 The “Big Idea” – you want to draw a structure where
the formal charges are equal to zero, or as low as
possible if you cannot achieve zero.
 If you cannot achieve zero, the negative formal
charge should be on the most electronegative atom
Calculating Formal Charge
1.
All of the unshared (nonbonding) electrons are
assigned to the atom on which they are found.
2.
Half of the bonding electrons are assigned to each
atom in the bond.
3.
Formal Charge =
# of valence e- in the isolated atom
minus
# of electrons assigned to the atom in the Lewis
Structure.
Example
 Draw the Lewis Structure for CO2
O
VE
6
AE
6
FC
0
C
O
4
or
O
C
O
6
6
4
6
4
6
7
4
5
0
0
-1
0
1
Another Example
Sulfate Ion
2-
2-
-1
O
O
-1
O
S
+2
O -1
O
0
-1
O
S
-1
0
O -1
O
0
Try H2CO2
O
H
C
O
H
O
Preferred
H
C
O
H