Audit formal charge and compare resonance contributors

A correct total is only the first check

Nitrate, NO₃⁻, has 5 + 3 × 6 + 1 = 24 valence electrons. Start with its all-single proposal. Three bonds use six distinct electrons; eighteen terminal nonbonding electrons bring the total to 24. Each oxygen has an octet, but nitrogen has only six surrounding electrons. A correct total does not establish the selected second-period Lewis model.

Switch to proposal A. One oxygen now shares a double bond; its nonbonding count decreases from six to four. The total remains 24 while nitrogen reaches eight surrounding electrons. Capture both proposals and use the per-atom ledger to identify what changed. In the too-many-multiple-bonds proposal, the total still matches, but nitrogen has ten surrounding electrons. That fails a different local constraint.

Each bond line represents two shared electrons. Count a pair once in the total electron budget; include that pair around each bonded atom when checking local octets. These two counting rules answer different questions. The existing high-school chemistry Lewis ledger supplies a return for neutral molecules; the ion budgets and contributor comparisons here add distinct work.

Calculate a formal charge at each atom

Formal charge assigns all nonbonding electrons to their atom and half of each bond’s electrons to each bonded atom. In symbols, FC = V − N − B/2, where V is the neutral atom’s valence-electron count, N is the number of nonbonding electrons drawn there, and B is the number of shared bonding electrons around that atom. These are electron counts, not measured charges in coulombs.

In nitrate proposal A, nitrogen has V = 5, N = 0 and B = 8, so FC = +1. The double-bond oxygen has 6 − 4 − 4/2 = 0. Each single-bond oxygen has 6 − 6 − 2/2 = −1. The sum is −1, matching nitrate’s entity charge.

Now choose carbonate, CO₃²⁻, with the same bond arrangement. Carbon supplies four neutral valence electrons, and the extra two electrons required by the −2 charge again give a total of 24. Carbon’s formal charge becomes zero, while the oxygen charges remain 0, −1 and −1. Their sum is −2. Reusing a drawing without checking the central element would miss this distinction.

Equivalent contributors represent one entity

Nitrate proposals A, B and C place the double bond at different labeled oxygen sites. The nuclear connectivity and total electrons remain fixed; in the ideal isolated entity the oxygen positions are symmetry-equivalent. Choosing one drawing cannot privilege one particular site as permanently different.

Treating the three equivalent contributors equally gives formal average bond order (2 + 1 + 1)/3 = 4/3 at each position. This is a representational summary, not evidence that the molecule rapidly switches between a single and a double bond. Nor does averaging the assigned formal charges turn them into measured partial charges. The real electron distribution requires more than these integer drawings.

The IUPAC resonance-form definition identifies contributors to a description of the electronic structure. It does not identify separately existing molecules whose populations the menu measures. The diagram’s fixed positions and numbered oxygens make bookkeeping inspectable; no animation or physical switching is implied.

Non-equivalent proposals need a different comparison

For CO₂, compare two double bonds with a triple bond on one side and a single bond on the other. Both have 16 valence electrons, local octets and zero total charge. But the two-double-bond proposal has zero formal charge on each atom; the triple/single proposal assigns +1 to one oxygen and −1 to the other.

These two descriptions are not equivalent under symmetry: swapping the oxygens changes which side of the triple/single proposal carries each charge, but cannot turn it into two double bonds. Do not apply the nitrate equal-weight arithmetic automatically. Within this restricted comparison, octets and reduced formal-charge separation favor the two-double-bond representation. The activity does not calculate quantum contributor weights or promise that formal-charge minimization is an infallible rule for arbitrary chemistry.

Know which charge and model you are using

IUPAC’s formal-charge convention shares bonding electrons equally regardless of electronegativity. Partial charge describes an assigned distribution of actual electron density under a specified scheme; oxidation-state bookkeeping uses another convention. Neither is supplied by the formal-charge table alone.

The finite checker assumes paired terminal electrons and these selected second-period structures. NO has 5 + 6 = 11 valence electrons, an odd total. It cannot put every electron into pairs. An odd-electron species exposes a limitation of the paired-octet exercise rather than permission to invent a missing electron. For the new carbonate practice, show the budget, local counts and charge sum separately before deciding which inference follows.