CHEM 109A
CLAS
Determining Formal Charge
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
(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
b. hydronium ion
c. borohydride ion
d. formic acid (HCOOH)
e. nitrobenzene
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 w/added examples. Most
common patterns are in bold font.
FC
on…
Carbon
Nitrogen
CH4
Oxygen
NH3
H2 O
Fluorine
1 bond
3 lp
HF
2 bonds
2 lp
3 bonds
1 lp
4 bonds
CH2CH2
N2H2
CHCH
N2
H2CO
formaldehyde
0
2 bonds
1 lone
pair (lp)
carbene
(sextet = very
unstable)
1 bond
2 lp
3 lp
nitrene
(sextet = very
unstable)
(sextet = very
unstable)
Page 1 of 2
CHEM 109A
CLAS
Determining Formal Charge
3 bonds
CH3+
methyl cation
4 bonds
(sextet = very
unstable, but can be
stabilized by
hyperconjugation)
NH4+
ammonium
ion
3 bonds
1 lp
H3 O+
hydroiunium
ion
2 bonds
2 lp
H2F+
fluoronium
ion
+1
5 bonds
CH5+
2 bonds
1 lp
(exceeds octet = very
unstable)
3 lp
NH2+
(sextet = very
unstable)
F+
fluorenium
ion
(sextet = very
unstable)
2 bonds
1 lp
+2
-1
3 bonds
1 lp
-
CH3
methyl anion
2 bonds
2 lp
NH2
-
1 bond
3 lp
OH2+2
(sextet = very
unstable)
4 lp
HOhydroxide ion
Page 2 of 2
Ffluoride ion