Formal Charges

Some molecules or polyatomic ions can be represented by multiple Lewis structures, but how to decide which one is the dominant structure?

By calculating the formal charges of the atoms, the Lewis structure closest to the actual structure of the molecule can be determined.

Each atom is assigned a hypothetical charge called the formal charge, which would be the charge on the atom if all other atoms in the molecule had the same electronegativity. It assumes that each bonding electron is equally shared by the two atoms.

Consider hydrogen chloride. To determine the formal charge on each atom, first add the number of nonbonding electrons to half the number of bonding electrons and then subtract the obtained value from the number of valence electrons.

An aggregate of all formal charges in a molecule or ion is equal to the net charge of the molecule or ion.

For example, nitrous oxide can be represented by three possible Lewis structures – one with two double bonds, one with a triple bond between the nitrogen atoms, and one with a triple bond between the nitrogen and oxygen — all that satisfy the octet. The best Lewis structure is identified by formal charge calculations.

Nitrogen has five valence electrons, and oxygen has six valence electrons. The calculation based on the number of nonbonding electrons and half the number of bonding electrons gives the formal charge for each structure. Because nitrous oxide is a neutral molecule, the sum of all formal charges needs to be zero.

Generally, the formal charges on the individual atoms in a dominant Lewis structure are closest to zero. Hence, the third structure with a higher formal charge can be ignored.

Additionally, a negative formal charge, if present, should be carried by the most electronegative atom.

Since oxygen is more electronegative than nitrogen, the second structure, with the negative formal charge on the oxygen, is identified as the dominant structure for nitrous oxide.

Formal charges are not the actual charges of molecules or atoms, but a bookkeeping convention. The actual charge of the molecule depends on several factors, including the difference in electronegativity between the constituent atoms.