Chemical Bonding & Lewis Structures Cheat Sheet
Valence counting, the octet rule, drawing structures, formal charge, exceptions, and resonance.
Keep going with the free Chemical Bonding & Lewis Structures study guide → (graded practice with a subscription).
Chemical Bonding & Lewis Structures
Valence counting, the octet rule, drawing structures, formal charge, exceptions, and resonance.
Count valence electrons
CORE RULETotal = sum of each atom's valence electrons (main-group group number: H 1, B 3, C/Si 4, N/P 5, O/S 6, halogens 7) + 1 per negative charge − 1 per positive charge. The finished structure must use exactly this many.
Octet (duet for H)
CORE RULEAtoms aim for 8 valence electrons (H for 2). A bonding pair counts toward BOTH bonded atoms; a lone pair counts only for its own atom.
Pick the central atom
CORE RULECentral atom = usually the lowest-electronegativity atom. H is always terminal; F almost always terminal; C is central with H/N/O/halogens (CO₂ → C; H₂O → O; SiO₂ → Si).
Bonds & multiple bonds
CORE RULESingle = 1 shared pair, double = 2, triple = 3. If the central atom is short of an octet, convert a terminal lone pair into a shared pair. A double/triple bond is still ONE bond domain per atom.
Distribute the electrons
METHOD(1) Complete terminal octets first (terminal halogen/O gets 3 lone pairs). (2) Put leftovers on the central atom. (3) If a period-2 central atom is still short, make a multiple bond; period-3+ may keep the extra as a lone pair.
Verify the count
CORE RULEAdd it back: 2 e⁻ per single bond, 4 per double, 6 per triple, 2 per lone pair, 1 per radical. The sum must equal the valence total you started with.
Incomplete octets
WATCH OUTBe and B can be stable with fewer than 8: Be has 4 in BeCl₂, B has 6 in BF₃: not enough electrons, and no terminal lone pair worth converting.
Odd-electron radicals
WATCH OUTAn odd valence total (NO 11, NO₂ 17, ClO₂ 19) cannot fully pair, so one atom carries a lone unpaired electron, on the central/less-electronegative atom. That atom is one short of an octet, and that is correct.
Expanded octets (hypervalent)
CORE RULEPeriod-3+ central atoms (P, S, Cl, Br, I, Xe) can be drawn past an octet: PCl₅ (10), SF₆ (12), also BrF₅, XeF₄. Period-2 atoms (B, C, N, O, F) never exceed 8. Modern models credit delocalized / 3-centre-4-electron bonding, not d-orbital expansion.
Formal charge
CORE RULEFC = valence electrons − nonbonding electrons − (bonding electrons / 2). Equivalently: valence − lone-pair electrons − number of bonds. Sum of all FCs = the species charge (0 for a neutral molecule).
Best structure by formal charge
CORE RULEPrefer: fewest nonzero formal charges; then smallest magnitudes; then negative FC on the more electronegative atom; avoid like charges on adjacent atoms. It is a guideline; defer to species-specific convention when both forms are valid.
Resonance
CORE RULEResonance forms share atom positions and electron count, differing only in π-bond / lone-pair placement (σ bonds never break). Move a π bond to a lone pair on one side while promoting a lone pair to a π bond on the other; link with ↔.