Formal Charge
Formal Charge: number of valence electrons for an atom - (number of lone pair electrons
+ number electrons in bonds/2)
FC=Nvalence−
(
Nlonepairs+Nelectrons 2
)
In chemistry, a formal charge (FC) on an atom in a molecule is defined as:
FC = number of valence electrons of the atom - ( number of lone pair electrons on this
atom + total number of electrons participating in covalent bonds with this atom / 2).
FC=Nvalence−
(
Nlonepairs+Nparticipatingelectrons 2
)
When determining the correct Lewis structure (or predominant resonance structure) for
a molecule, the structure is chosen such that the formal charge on each of the atoms is
minimized.
9.2.1 Examples
carbon in methane
FC= 4−
(
0+^82
)
= 0
Nitrogen inNO− 2
FC= 5−
(
2+^62
)
= 0
double bonded oxygen inNO 2 −
FC= 6−
(
4+^42
)
= 0
single bonded oxygen inNO− 2
FC= 6−
(
6+^22
)
=− 1
Figure 12 Methane (CH 4 ): black is carbon, white is hydrogen
Figure 13Nitrogen dioxide (NO 2 ): blue is nitrogen, red is oxygen