Lewis Structures & Resonance Cheat Sheet

A printable reference covering valence electrons, octet rules, formal charge, resonance structures, and resonance hybrids for grades 10-12.

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Lewis structures show how valence electrons are arranged in molecules and polyatomic ions. This cheat sheet helps students draw structures correctly, check electron counts, and compare possible bonding patterns. It is especially useful for predicting molecular stability before studying shape, polarity, and reactions. The core ideas are counting valence electrons, placing bonds and lone pairs, applying the octet rule, and using formal charge to choose the best structure. Resonance occurs when more than one valid Lewis structure can represent the same electron arrangement. The real molecule is a resonance hybrid, not a structure switching back and forth.

Key Facts

The total valence electron count equals the sum of valence electrons from all atoms, plus $1$ electron for each $-1$ charge and minus $1$ electron for each $+1$ charge.
A single covalent bond contains $2$ shared electrons, so each single bond uses $2$ electrons from the total count.
Most second-period atoms follow the octet rule and are most stable with $8$ valence electrons around them.
Hydrogen follows the duet rule and is stable with $2$ valence electrons in one single bond.
Formal charge is calculated with $FC = V - \left(N + \frac{B}{2}\right)$ , where $V$ is valence electrons, $N$ is nonbonding electrons, and $B$ is bonding electrons.
The best Lewis structure usually has the smallest formal charges and places negative formal charge on the more electronegative atom.
Resonance structures must have the same atom positions and the same total number of valence electrons, but different electron placements.
The bond order in a resonance hybrid can be estimated with $\text{bond order} = \frac{\text{total bond count between two atoms}}{\text{number of equivalent bonds}}$ .

Vocabulary

Valence Electron: A valence electron is an outer-shell electron that can participate in bonding or appear as a lone pair.
Lewis Structure: A Lewis structure is a diagram that shows atoms, covalent bonds, lone pairs, and sometimes formal charges.
Lone Pair: A lone pair is a pair of valence electrons that belongs to one atom and is not shared in a bond.
Formal Charge: Formal charge is the charge assigned to an atom in a Lewis structure using $FC = V - \left(N + \frac{B}{2}\right)$ .
Resonance Structure: A resonance structure is one of two or more valid Lewis structures that differ only in electron placement.
Resonance Hybrid: A resonance hybrid is the real electron distribution represented by the combined effect of all valid resonance structures.

Common Mistakes to Avoid

Forgetting to adjust for ion charge is wrong because a $-1$ ion has one extra electron and a $+1$ ion has one fewer electron.
Changing atom positions between resonance structures is wrong because resonance only moves electrons, not nuclei.
Putting more than $8$ electrons around second-period atoms like $C$ , $N$ , $O$ , or $F$ is wrong because these atoms cannot expand their octets.
Choosing a structure with large formal charges is usually wrong because the most stable Lewis structure minimizes charge separation.
Drawing resonance as rapid flipping is wrong because the real molecule is a resonance hybrid with delocalized electrons.

Practice Questions

1 Draw the Lewis structure for $NH_3$ and state how many lone pairs are on the nitrogen atom.
2 Calculate the total number of valence electrons in $CO_3^{2-}$ .
3 For an oxygen atom with $6$ valence electrons, $4$ nonbonding electrons, and $4$ bonding electrons, calculate $FC = V - \left(N + \frac{B}{2}\right)$ .
4 Explain why the three resonance structures of $NO_3^{-}$ represent one resonance hybrid rather than three separate molecules.

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