Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.6Compound Structure and Properties

Resonance and Formal Charge — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 13 1 quiz

📚 AP CHEMISTRY Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

2.6 Resonance and Formal Charge — Notes — AP CHEMISTRY notes: Use resonance and formal charge to decide which Lewis diagram best models a species.Resonance = two or more valid Lewis diagrams that differ only in the positions of electrons, not atoms.The real structure is a resonan…

  • Bond order
  • Delocalised electrons
  • Resonance hybrid
  • Best structure
  • Electrons are delocalised, and equivalent bonds have identical lengths and energies between single and double bonds
  • Delocalisation lowers the energy, making the species more stable

Section 1 of 8Resonance

Resonance
Resonance two or more valid Lewis diagrams that differ only in the positions of electrons, not atoms.
The real structure is a resonance hybrid of these diagrams.
Electrons are delocalised, and equivalent bonds have identical lengths and energies between single and double bonds.
Resonance is needed for qualitatively accurate predictions of structure and properties, as in benzene, which has six equal C–C bonds.
Delocalisation lowers the energy, making the species more stable.

Section 2 of 8Bond order in a hybrid

Bond order in a hybrid
Bond order
Bond order total bonds shared between the atoms ÷ number of bonding locations.
Nitrate, NO3−, has three resonance structures, so each N–O bond has bond order 4/3.
Ozone has bond order 3 ÷ 2
1.5.

Section 3 of 8Formal charge

Formal charge
V − N − B/2, using valence electrons of the free atom, nonbonding electrons and bonding electrons.
The sum of the formal charges equals the overall charge of the molecule or ion.
Formal charge is a bookkeeping tool, not the real charge, which depends on electronegativity.
SpeciesAtomValence e−Nonbonding e−B/2Formal charge
CO2C4040
CO2O (double bond)6420
NO3−N504+1
NO3−O (single bond)661−1
O3Central O623+1

Section 4 of 8Choosing the best structure

1
Draw all valid Lewis diagrams for the species.
2
Calculate the formal charge on every atom of each diagram.
3
Check that the charges sum to the overall charge.
4
Prefer the structure with formal charges closest to zero.
5
Among the rest, prefer the one with negative formal charge on the most electronegative atom.
  • Formal charge can justify expanded octets or choose between atom arrangements such as SCN− and CNS−.

Section 5 of 8Limits of the model

  • Where the octet rule cannot be applied, as with odd numbers of valence electrons, Lewis structures have limitations.

Section 6 of 8More from the specification

Valence shell
  • Outermost electron shell.
Bonding pair
  • A shared pair of electrons forming a covalent bond.
Triple bond
  • Three shared pairs of electrons.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.

Section 7 of 8Key terms

TermMeaning
ResonanceSeveral valid Lewis structures differing only in electron positions.
Delocalised electronsElectrons spread over several atoms.
Formal chargeValence electrons minus nonbonding electrons minus half the bonding electrons.
Resonance hybridActual structure, a mixture of the resonance structures.
Bond orderAverage number of shared pairs between two atoms.
Best structureOne with formal charges nearest zero and negative charge on the most electronegative atom.

Section 8 of 8Exam technique

1
Treating resonance structures as rapidly interconverting — the real structure is a single hybrid
2
Moving atoms between resonance structures — only electrons move
3
Forgetting that formal charges must sum to the overall charge
4
Treating formal charge as the actual charge on an atom

Last updated: