AP Chemistry · Unit 2 · Topic 2.5
Lewis Diagrams
A Lewis diagram is an electron-accounting model before it is a drawing exercise. If the total electron count is wrong, an attractive-looking picture is still chemically wrong. Work systematically, then use the structure to make predictions about bonding and shape.
Count the available valence electrons
Add each atom's valence electrons. Add electrons for an overall negative charge and subtract them for a positive charge. A neutral oxygen atom contributes six valence electrons; that is not the same as saying every oxygen must end with six electrons around it in a molecule. Shared bonding electrons count toward the valence shell of both bonded atoms.
Build and complete the framework
Choose a reasonable skeleton, usually with the less electronegative nonhydrogen atom central. Hydrogen forms one bond and is terminal. Connect atoms with single bonds, complete terminal octets, and place remaining electrons on the central atom. If a second-row central atom still lacks an octet, convert suitable lone pairs into multiple bonds without changing the total number of electrons.
Worked example: formaldehyde
For CH2O, the electron budget is 4 + 2(1) + 6 = 12. Put carbon in the center with two C–H single bonds and one C–O single bond, using six electrons. Placing the remaining six on oxygen leaves carbon with only six electrons around it. Move one oxygen lone pair into a C=O bond. Carbon then has an octet, oxygen has an octet, and each hydrogen has a duet.
Audit the finished structure
| Location in CH₂O | Electrons assigned to that feature | Structural role |
|---|---|---|
| Two C–H bonds | 4 | Two shared pairs |
| One C=O bond | 4 | Two shared pairs |
| Two oxygen lone pairs | 4 | Nonbonding pairs |
| Total | 12 | Matches initial budget |
The table counts each electron once. Counting every atom's completed shell instead would count bonding electrons twice and would not be a valid budget audit.
Recognize exceptions without abandoning accounting
Hydrogen follows a duet, not an octet. Boron can appear in electron-deficient compounds such as BF3. Species with an odd total electron count cannot give every atom a conventional paired-electron octet. Some third-period and heavier atoms are represented with expanded valence shells in Lewis models; second-period carbon, nitrogen, oxygen, and fluorine do not expand their octets.
A structure should explain evidence
A C=O bond is generally shorter and stronger than a comparable C–O single bond. Spectroscopy and bond-length measurements can therefore help test whether a proposed structure is reasonable. If all measured bonds of a given kind are equal but one Lewis drawing makes them different, resonance may be needed. The drawing is a model to evaluate, not the final authority over observations.
Keep geometry and electron accounting separate
A flat Lewis sketch does not establish the actual bond angles. Use electron-domain reasoning after the electron structure is correct. Also show brackets and overall charge for a polyatomic ion, and preserve the atom connectivity when comparing resonance contributors. Moving an atom produces a different structural arrangement, not merely another electron depiction.
Practice, then check your reasoning
How many valence electrons must be shown for NH₄⁺?
Answer and reasoning: 5 + 4(1) − 1 = 8 electrons, enough for four N–H bonds.
Why can carbon in CH₂O not keep three single bonds and a lone pair while oxygen keeps three lone pairs?
Answer and reasoning: That proposal uses 14 electrons instead of the available 12; the electron budget rules it out.
Scientific references: OpenStax: Lewis structures; OpenStax: evaluating electron structures. OpenStax Chemistry 2e authors: Paul Flowers, Klaus Theopold, Richard Langley, and William R. Robinson. Consult source pages for their license terms. Accessed September 13, 2026. Explanations, comparison tables, and practice tasks were independently written for this site; numerical teaching datasets are illustrative unless explicitly identified otherwise.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.