Resonance and Formal Charge

Formal charge is an accounting convention

Assign every nonbonding electron to its atom and split each bonding pair equally. Compare that assigned total with the atom's neutral valence count.

Here V is the neutral valence count, N the number of nonbonding electrons, and B the number of bonding electrons. The sum of all formal charges must equal the overall charge. Formal charge is not an experimentally measured partial charge.

What may change between contributors

Resonance contributors have identical atom positions and connectivity but different placements of electrons. They must preserve the total electron count and overall charge. For second-row atoms, retain valid octets where applicable. Moving a hydrogen nucleus from one atom to another changes the species' bonding arrangement and is not a resonance operation.

Worked example: the nitrate ion

NO₃⁻ has 5 + 3(6) + 1 = 24 valence electrons. A valid contributor has one N=O bond and two N–O single bonds. Nitrogen has no lone pairs and eight bonding electrons, so its formal charge is 5 − 0 − 4 = +1. Each single-bonded oxygen has charge −1, while the double-bonded oxygen has charge zero. The total is −1.

Track the nitrate charge audit

Three equivalent contributors place the double bond at three different oxygen positions. Their equal importance reflects the equivalent oxygen environments, not a molecule rotating a localized double bond around.

What the hybrid explains

The real nitrate ion has delocalized bonding, so its three N–O bonds are equivalent in the ideal free ion. A simple resonance bond-order average is (2 + 1 + 1)/3 = 4/3. This is a useful description of intermediate bonding, not a claim that one third of a physical line is attached to each atom. Bond lengths should lie between comparable single- and double-bond values.

Not every contributor has equal importance

Contributors that satisfy appropriate octets and avoid unnecessary charge separation are often more important. If otherwise comparable, placing negative formal charge on a more electronegative atom is generally favored. These are evaluation guidelines, not permission to violate electron count. An aesthetically charge-free drawing with ten electrons around second-row nitrogen is not an acceptable improvement.

Use structural evidence to choose the model

If measured bond lengths are equal despite unequal bonds in one drawing, a delocalized description is supported. If the surrounding environment makes positions inequivalent, the bonds need not remain exactly equal. Explain the observed equivalence through the electronic structure instead of writing that the measurement is merely too slow to catch bonds switching.