Resonance and Formal Charge
Use formal-charge sums and equivalent resonance contributors without implying alternating molecules.
AP Chemistry Topic 2.6 · CED printed p. 50 / PDF p. 57. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Lewis electron budgets
- Bond order
One drawing can miss delocalization
Sometimes the same atomic skeleton supports several equivalent Lewis drawings that differ only in electron placement. Draw them joined by a resonance arrow. The real species is one resonance hybrid; it does not take turns switching between separate molecules. In nitrate, a double bond can be drawn to any of three O atoms. The equivalent N–O positions are therefore equivalent in the actual ion, with bonding intermediate between a localized single and double bond.
Keep atomic positions fixed when comparing resonance contributors. Moving an H atom changes connectivity, not merely resonance. Resonance matters because a single localized drawing can falsely predict different equivalent bond lengths or a charge locked onto one site. An average bond order is a useful qualitative description of delocalization, not a claim that one can see fractional bond lines flickering.
Formal charge evaluates an accounting model
Formal charge assigns half of bonding electrons to each atom: . Equivalently subtract lone-pair electrons and the number of bond lines from the neutral-atom valence count. The sum of all formal charges must equal the species’ net charge. Formal charge is bookkeeping; it is not a directly measured partial charge.
Among valid alternatives, favor appropriate octets, small formal-charge magnitudes and less unnecessary charge separation; when comparable, negative charge is usually better placed on the more electronegative atom. Apply these criteria together rather than blindly minimizing one number. Odd-electron species such as NO cannot give every atom an ordinary paired-electron octet, so the Lewis model has limits. Hypervalent models likewise need care and do not justify a literal d-orbital hybridization story.
Let's work through it
Work it through
Use one nitrate contributor with N double-bonded to Oa and single-bonded to Ob and Oc.
- Count 24 valence electrons. N has four bond lines in total and no lone pairs. Double-bonded O has two lone pairs; each single-bonded O has three.
- N formal charge = 5 − 0 − 4 = +1. Oa formal charge = 6 − 4 − 2 = 0. Each singly bonded O has 6 − 6 − 1 = −1. The sum is −1.
- Draw the other two equivalent contributors by moving the double-bond location; no atoms move.
- Across the equivalent contributors, each N–O bond has average order (2 + 1 + 1)/3 = 4/3. The resonance hybrid predicts three equal intermediate N–O bonds, not one permanently short bond and two permanently long ones.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Do resonance structures represent rapidly interconverting molecules?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Do resonance structures represent rapidly interconverting molecules?
Worked answer and reason
No. They are alternative electron-accounting drawings for one delocalized species.
The hybrid is the actual model of the species, not a sequence of isolated drawings.
Retrieval 2
What must the formal charges of SO42− add to?
Worked answer and reason
−2, regardless of which valid contributor is being evaluated.
The atom bookkeeping must reproduce the species’ net charge.
Retrieval 3
Why can an ordinary all-paired octet model fail for NO?
Worked answer and reason
NO has 11 valence electrons, an odd total; it necessarily has an unpaired electron in a simple Lewis representation.
The total budget rules out paired octets for every atom.
Catch the tempting mistake
- Formal charge equals the measured charge located on an atom.
- It comes from equal splitting of bonding electrons and need not equal physical partial charge.
- An equivalent resonance bond is either single or double at any instant.
- The species has delocalized electronic structure; contributors are models, not snapshots of switching localized bonds.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 2 problem 11 · ap application · symbolic
- Unit 2 problem 12 · ap synthesis · symbolic
- Unit 2 problem 13 · ap application · verbal
Scientific calculator workspace
Use this workspace for arithmetic, logarithms and elementary functions. Keep intermediate precision and report final values with appropriate units and significant figures. Graphing and general equation solving need a suitable approved tool.
Scientific calculator
This calculator requires JavaScript. Use the physics formula sheets and show your working manually.
+ − * / ^ ! · pi · e · sqrt · sin, cos, tan · asin, acos, atan · atan2(y,x) · ln, log · abs · exp · hypot(x,y). Use * between factors. Keep units in your written working.
For the teacher
- Check charge sums before debating “best” contributor.
- Ask students to predict equivalent bond lengths from their resonance set.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 2.6.A
- Represent a molecule with a Lewis diagram that accounts for resonance between equivalent structures or that uses formal charge to select between nonequivalent structures.
- 2.6.A.1
- In cases where more than one equivalent Lewis structure can be constructed, resonance must be included as a refinement to the Lewis structure. In many such cases, this refinement is needed to provide qualitatively accurate predictions of molecular structure and properties.
- 2.6.A.2
- The octet rule and formal charge can be used as criteria for determining which of several possible valid Lewis diagrams provides the best model for predicting molecular structure and properties.
- 2.6.A.3
- As with any model, there are limitations to the use of the Lewis structure model, particularly in cases with an odd number of valence electrons.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
-
Bond order
Bond order describes the degree of bonding between two atoms in a specified bonding model. Localized single, double, and triple bonds have orders one, two, and three, while delocalized models can give fractional values. For example, equivalent bonds in a resonance description can have bonding intermediate between a single and double bond.
-
Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
-
Electron delocalization
Electron delocalization means that an electronic description extends across multiple atoms rather than being confined to one localized bond or atom. It can affect bond equivalence and charge distribution. For example, nitrate's bonding electrons are described across several nitrogen–oxygen positions.
-
Formal charge
Formal charge is electron bookkeeping that assigns half of each bonding pair to each bonded atom. It compares that assigned electron count with the neutral atom's valence-electron count and is not a direct measurement of actual charge distribution. For example, all formal charges in an ion's Lewis structure must sum to the ion's total charge.
-
Partial charge
A partial charge describes an uneven distribution of electron density within a bond or molecule. It is marked by a delta symbol and does not necessarily represent a full ionic charge. For example, oxygen carries a partial negative charge in water while its hydrogens carry partial positive charges.
-
Orbital hybridization
Hybridization is a localized bonding model that combines atomic-orbital descriptions on an atom. It can help describe bond directions but is not a statement that isolated atomic orbitals physically mix as a timed step. For example, an sp³ model describes four tetrahedrally directed orbitals around carbon in methane.