Lewis Diagrams
Construct Lewis diagrams from an electron budget; check connectivity, local duets/octets and net charge.
AP Chemistry Topic 2.5 · CED printed p. 49 / PDF p. 56. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Valence-electron counts
- Covalent sharing
Lewis diagrams are electron bookkeeping
Start by counting valence electrons from all atoms. Add one electron for each negative charge and subtract one for each positive charge. Choose a plausible skeleton: H is terminal, and the less electronegative atom is often central, with appropriate known structural information taking priority. Connect atoms with single bonds, spending two electrons per bond. Fill terminal non-H octets, then place remaining electrons on the central atom. If the central atom lacks an octet, convert an adjacent lone pair into a multiple bond; that moves electrons but does not create new ones.
Check two things separately: the total number of electrons matches the original count, and each atom has an appropriate local arrangement. H has a duet. Second-period C, N, O and F do not exceed an octet in ordinary Lewis models. Boron compounds can be electron deficient. Larger central atoms may be shown with expanded valence shells in this model. Do not force an octet on every atom without checking the species and electron total.
What the lines and dots mean
Each line is a shared electron pair. Each pair of dots is a nonbonding pair belonging to one atom in the model. A single, double or triple bond spends two, four or six electrons in total, while allowing the shared electrons to count toward both bonded atoms’ local octets. That local counting does not double the molecule’s electron total. Bracket a polyatomic ion and place its net charge outside; the charge belongs to the whole represented species.
Lewis diagrams predict connectivity, bond order and electron domains. They are not pictures of exact orbital shapes or moving electron paths. After a valid diagram, formal charge and resonance can refine the choice, and VSEPR can turn electron-domain information into geometry.
Let's work through it
Work it through
Construct CO2 and NH4+ with a counted-electron check.
- CO2 has 4 + 2(6) = 16 valence electrons. Begin O–C–O: four electrons in bonds.
- Giving each O three lone pairs spends the remaining 12, leaving C with only four electrons around it. Move one lone pair from each O into a bond: O=C=O. Each O now has two lone pairs; C has none.
- For NH4+, count 5 + 4(1) − 1 = 8 electrons. Four N–H single bonds spend all eight. H atoms have duets; N has an octet and no lone pairs. Bracket the diagram with + charge.
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
How many valence electrons are available in NO3−?
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
How many valence electrons are available in NO3−?
Worked answer and reason
5 + 3(6) + 1 = 24.
A negative charge adds electron availability to the atom totals.
Retrieval 2
A double bond is drawn between two atoms. How many electrons does it use from the total budget?
Worked answer and reason
Four, representing two shared pairs.
Counting toward two local octets does not duplicate electrons in the total.
Catch the tempting mistake
- A positive polyatomic charge means add extra electrons.
- Positive charge means electrons have been removed, so subtract its magnitude.
- Creating a double bond adds two new electrons.
- It reallocates an existing lone pair into shared bonding. Total electron count remains fixed.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 2 problem 9 · ap application · symbolic
- Unit 2 problem 10 · ap synthesis · symbolic
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For the teacher
- Ask for electron-budget annotations alongside every diagram.
- If a diagram looks plausible, still check H duets and second-period octets.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 2.5.A
- Represent a molecule with a Lewis diagram.
- 2.5.A.1
- Lewis diagrams can be constructed according to an established set of principles.
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 · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Valence electron
Valence electrons are the electrons most directly involved in an atom's bonding and chemical behavior. For main-group atoms, they occupy the highest occupied principal shell. For example, neutral chlorine has seven valence electrons, helping explain its tendency to form a chloride ion.
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Lone pair
A lone pair is a pair of valence electrons assigned to one atom rather than to a bond in a Lewis model. Lone pairs influence geometry and can participate in interactions or new bonds. For example, oxygen's two lone pairs help explain water's bent molecular shape.
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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.
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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.
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Electron domain
An electron domain is a region of electron density around an atom in the VSEPR model. A bond of any order counts as one domain, and a lone pair counts as another. For example, two bonds and two lone pairs give water's oxygen four domains and a steric number of four.