CHEM 109A
CLAS
Determining Formal Charge - KEY
Formal Charge (FC) is a method used to estimate the electron density at an atom in a
molecule. Using this method assumes that all bonds are covalent which means all
electrons are shared equally
(a decent assumption in organic chemistry).
Procedure
1. Draw Lewis structure(s)
2. Count the number of electrons around each atom (all lone pairs and ½ the bonding
electrons) in the structure.
3. Subtract the number of electrons around each atom in the structure from the
number of valence electrons.
For example…
1. Determine the FC on each atom in the following compounds
a. water
Hs
1
O
# valence e6
6
e- in
1
(2 lone pair + ½ the
structure
bonding e-)
FC
0
0
There is no FC on any atom in the molecule
b. hydronium ion
, so should look like
O
6
5
e- in
1
(1 lone pair + ½ the
structure
bonding e-)
FC
0
+1
FC of +1 on O atom
# valence e-
Hs
1
c. borohydride ion
, so should look like
Page 1 of 4
CHEM 109A
CLAS
Determining Formal Charge - KEY
Hs
1
B
# valence e3
e- in
4
1
structure
(½ the bonding e-)
FC
0
-1
FC of -1 on B atom
d. formic acid (HCOOH)
Structure A
Hs
C
O1
O2
# valence e1
4
6
6
e- in
1
4
6
6
structure
FC
0
0
0
0
Structure B
Hs
C
O1
O2
# valence e1
4
6
6
e- in
1
4
7
6
structure
FC
0
0
-1
+1
Structure A has no FC & therefore is the more stable resonance
contributor.
FCs of Structure B balance each other out, so there is no net charge on
the molecule. The total (net) FC must be the same for all resonance
contributors.
e. nitrobenzene
Page 2 of 4
CHEM 109A
CLAS
Determining Formal Charge - KEY
Structure A
Hs
Cs
N
O1
O2
# valence e1
4
5
6
6
e- in
1
4
4
6
7
structure
FC
0
0
+1
0
-1
Structure B
Hs
Cs
N
O1
O2
# valence e1
4
5
6
6
e- in
1
4
4
7
6
structure
FC
0
0
+1
-1
0
FCs balance each other out, so there is no net charge on the molecule.
Both resonance contributors have the same amount of FC on the same
atoms so they contribute equally to the resonance hybrid (see Drawing
Resonance Structures Handout).
OR (instead of performing steps 2 & 3 above) you can learn to identify some
common bonding patterns and the associated FC.
Bonding Patterns & FC – reproduced from an Aue handout. Most common patterns
are in bold font.
FC
on…
Carbon
Nitrogen
CH4
Oxygen
NH3
2 bonds
2 lp
3 bonds
1 lp
4 bonds
CH2CH2
N2H2
CHCH
N2
H2 O
Fluorine
1 bond
3 lp
HF
H2CO
formaldehyde
0
2 bonds
1 lone
pair (lp)
carbene
1 bond
2 lp
(sextet = very
unstable)
+1
3 bonds
CH3+
methyl cation
(sextet = very
unstable, but can be
3 lp
nitrene
(sextet = very
unstable)
(sextet = very
unstable)
4 bonds
NH4+
ammonium
3 bonds
1 lp
H3 O+
hydroiunium
2 bonds
2 lp
Page 3 of 4
H2F+
fluoronium
CHEM 109A
CLAS
Determining Formal Charge - KEY
stabilized by
hyperconjugation)
5 bonds
CH5+
ion
2 bonds
1 lp
(exceeds octet = very
unstable)
ion
ion
3 lp
NH2+
(sextet = very
unstable)
F+
fluorenium
ion
(sextet = very
unstable)
2 bonds
1 lp
+2
OH2+2
(sextet = very
unstable)
-1
3 bonds
1 lp
-
CH3
methyl anion
2 bonds
2 lp
NH2
-
1 bond
3 lp
4 lp
HOhydroxide ion
Page 4 of 4
Ffluoride ion