Read Resonance as Averaged Bond Length, Not a Real Structure
A resonance-stabilised ion has ONE real structure — an average of its contributing forms — which is exactly why its bond lengths are all equal (or fall into distinct equal-length sets) rather than matching any single Lewis structure drawn on paper.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
More equivalent resonance structures means more real bond-length averaging
SO4²⁻ has the most resonance structures among common oxoanions of this size because sulfur can form S=O double bonds to any of its four oxygens interchangeably — the true structure isn't any one drawn form but the average across all of them, giving all four S-O bonds identical, intermediate bond length and order.
Not every oxoanion has fully equivalent bonds
Some species (HCO₂⁻, Cr₂O7²⁻) have more than one TYPE of X-O bond length because not every oxygen is chemically equivalent by symmetry — a terminal, doubly-bonded-character oxygen differs from a bridging or singly-bonded oxygen, so resonance only averages bonds WITHIN each equivalent set, not across the whole molecule.
Bond order and formal charge from resonance-averaged structures
For HCO₂⁻ (formate), the true C-O bond order is 1.5 (averaging one C=O and one C-O across two equivalent resonance forms), and each oxygen's formal charge is -0.5 (averaging 0 and -1) — compute both the SAME way, by averaging across the equivalent contributing structures, not by picking just one Lewis structure and reading values off it.
Octet exceptions in Lewis structures
Species with an odd electron count (like NO) or expanded octets on period-3+ central atoms are genuine exceptions to the simple octet rule — recognise these before trying to force a Lewis structure that satisfies 8 electrons everywhere, since some stable, real species simply don't.
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- ✓More equivalent resonance structures means more complete bond-length averaging across those positions.
- ✓Not every atom of the same element in an oxoanion is chemically equivalent — check molecular symmetry before assuming all X-O bonds average together.
- ✓Bond order and formal charge in a resonance-stabilised ion are both computed by averaging across the equivalent contributing structures.
- ✓Odd-electron species and expanded-octet species are genuine, real exceptions to the octet rule, not errors to fix.
Don't assume every oxygen in a polyatomic oxoanion has the same bond length just because resonance is present — check the molecule's actual symmetry first; asymmetric species like HCO₂⁻ or Cr₂O7²⁻ genuinely have more than one distinct X-O bond length.
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