AP® Chemistry · Unit 2 · Topic 2.6
Resonance and Formal Charge
Resonance happens when one molecule needs more than one valid Lewis structure to describe it; the real molecule is a single blended hybrid, not a flickering mix, and formal charge tells you which contributing structure matters most.
Some molecules can't be drawn with just one Lewis structure. Take SO3: sulfur forms one double bond and two single bonds to three oxygens, but you could place that double bond on any of the three oxygens and still satisfy the electron count. Experimentally, all three bonds measure the same length, roughly between a single and a double bond. That is the clue that no single drawing is right. The real molecule is a resonance hybrid: one unchanging structure whose bonds are an average of all the valid Lewis structures, each oxygen carrying about a third of a double bond's worth of character.
Formal charge is the tool for judging which Lewis structures matter most when several are possible. It asks: if every bond's electrons were split evenly between the two atoms, would this atom have gained or lost electrons compared to its free neutral self? Formula: formal charge = valence electrons - 2 x lone pairs - number of bonds. The best structures keep formal charges close to zero and put any negative charge on the more electronegative atom. In thiocyanate, SCN-, comparing S=C=N versus S-C triple bond N(-) shows the charges land differently, and the structure with negative charge on the more electronegative atom is favored as the major contributor to the real hybrid.
This connects forward to carbonate, CO3(2-), where three equivalent C-O resonance structures average to a carbon-oxygen bond order of 1.33, and it connects back to bond energy: delocalizing electrons over several atoms lowers the molecule's energy, making resonance structures more stable than any single drawing suggests.
Key terms
- Resonance structures
- Two or more valid Lewis structures for the same molecule that differ only in electron placement, not atom position.
- Resonance hybrid
- The single real structure of a molecule, with bond orders and charges averaged over all its resonance structures.
- Formal charge
- The charge an atom would have if bonding electrons were shared equally, calculated as valence electrons minus lone pair electrons minus number of bonds.
- Bond order
- The effective number of bonds between two atoms, which can be a fraction once resonance is averaged in.
- Delocalization
- The spreading of electrons over several atoms rather than confining them to one bond or atom.
Watch it explained
- How it worksHow do several equivalent Lewis structures blend into one real delocalized molecule?
- Worked exampleFind the formal charge on every atom in the two candidate structures for the thiocyanate ion.
- Common mistakeWhy isn't a resonance hybrid a molecule flickering back and forth between two structures?
- Going furtherWhat happens to the carbon-oxygen bond order in carbonate once resonance is included?
Check yourself
End of topic.