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Lewis structures – formal charge, resonance and dative bonds

ChemistryChemical BondingAges 15–16

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Build Lewis structures for 22 molecules and ions (H₂O, NH₃, CO₂, CO, O₃, NO₃⁻, SO₄²⁻, ClO₃⁻, PCl₅, SF₆, XeF₄…): tap a bond to change its order and tap an atom to add lone pairs. The simulation counts valence electrons, checks the octet rule (allowing the incomplete octet of boron and expanded octets from period 3) and shows the formal charge FC = V − N − B/2 on every atom so you can choose the best structure. Resonance mode draws the contributors of CO₃²⁻, NO₃⁻, NO₂⁻, O₃, SCN⁻ and benzene with their hybrid; dative bond mode forms NH₄⁺, H₃O⁺, H₃N→BF₃ and Al₂Cl₆, with dot-and-cross diagrams.

Lesson: Covalent bonding: Lewis structures, the octet rule, formal charge, resonance and dative (coordinate) bonds

What it shows

A Lewis structure shows how the valence electrons of a molecule or ion are arranged as bonding pairs and lone pairs. Count the valence electrons, join the atoms with single bonds, complete the octets of the outer atoms and put the rest on the central atom; turn lone pairs into multiple bonds if the central atom still lacks an octet. Period 3 atoms can exceed eight electrons. When several structures are valid, the one with formal charges closest to zero, and negative charge on the most electronegative atom, is preferred. Equivalent structures are resonance contributors of one delocalised hybrid.

How to use

In Build mode choose a Molecule, select + lone pair or − lone pair, then tap atoms; tap a bond to cycle single, double and triple. Watch the electron count, the coloured octet rings and the formal charges, and read the table. Show answer cycles through the best structures. Switch Drawing to Dot-and-cross, try Resonance mode, or use Dative bond mode and press Play.

Parameters you can change

  • Mode Build Lewis structures, Resonance, Dative bonds
  • Molecule (Build mode) Water (H₂O), Ammonia (NH₃), Methane (CH₄), Hydrogen cyanide (HCN), Methanal (H₂CO), Carbon dioxide (CO₂), Carbon monoxide (CO), Nitrogen (N₂), Ozone (O₃), Sulfur dioxide (SO₂), Nitrite ion (NO₂⁻), Nitrate ion (NO₃⁻), Carbonate ion (CO₃²⁻), Sulfate ion (SO₄²⁻), Chlorate ion (ClO₃⁻), Thiocyanate ion (SCN⁻), Boron trifluoride (BF₃), Phosphorus pentachloride (PCl₅), Sulfur hexafluoride (SF₆), Xenon tetrafluoride (XeF₄), Ammonium ion (NH₄⁺), Oxonium ion (H₃O⁺)
  • Species (Resonance mode) Carbonate ion (CO₃²⁻), Nitrate ion (NO₃⁻), Nitrite ion (NO₂⁻), Ozone (O₃), Thiocyanate ion (SCN⁻), Benzene (C₆H₆)
  • Reaction (Dative bond mode) NH₃ + H⁺ → NH₄⁺, H₂O + H⁺ → H₃O⁺, NH₃ + BF₃ → H₃N→BF₃, 2 AlCl₃ → Al₂Cl₆
  • Drawing style Lewis structure (bond lines), Dot-and-cross diagram
  • Show formal charges
  • Octet check and hints

Questions to explore

  1. Why is O=C=O preferred to O≡C–O even though both give every atom an octet?
  2. How many equivalent structures does the nitrate ion have, and what is its N–O bond order?
  3. Why can sulfur have more than eight electrons around it but oxygen cannot?

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