FORMAL CHARGE
The number of electrons officially assigned to an atom in a Lewis structure does not always equal
the number of valence electrons of the free atom. The difference between these two numbers is the
formal charge of the atom. Formal charge can be calculated using the following formula:
where V is the number of valence electrons in the free atom, Nbonding is the number of bonding
electrons, and Nnonbonding is the number of nonbonding electrons.
The charge of an ion or molecule is equal to the sum of the formal charges of the individual atoms
comprising the ion or molecule. In other words, for a neutral molecule, the formal charges of the
individual atoms have to add up to zero.
Example: Calculate the formal charge on the central N atom of NH 4 +.
Solution: The Lewis structure of NH 4 + is
Place any extra electrons on the central atom. If the central atom has less than an octet, try to
write double or triple bonds between the central and surrounding atoms using the
nonbonding, unshared lone electron pairs.
The HCN structure above does not satisfy the octet rule for C because C possesses only four
valence electrons. Therefore, two lone electron pairs from the N atom must be moved to form
two more bonds with C, creating a triple bond between C and N. Finally, bonds are drawn as
lines rather than pairs of dots.H – C ≡ N:
Now the octet rule is satisfied for all three atoms, since C and N have eight valence electrons
and H has two valence electrons.