Difficulty: Intermediate | Prerequisites: Lewis structures, VSEPR theory (see Parts 1 and 2)
TL;DR
Some molecules cannot be described by a single Lewis structure. Resonance uses multiple equivalent structures, connected by double-headed arrows, to show that the real molecule is a blend of all of them. Formal charges help you judge which resonance structure best represents the molecule. This section also covers how organic compounds (carbon-based chains and rings) differ structurally from inorganic compounds.
Resonance
The representation of a molecule using two or more equivalent Lewis structures when no single structure captures the true distribution of electrons. The actual molecule is a weighted average (resonance hybrid) of all the structures. Think of it as the molecule existing as a blend, not flipping between forms.
Resonance structure
One of the individual Lewis structures that contributes to the resonance hybrid. Each one is "correct" by Lewis rules, but none alone is the complete picture.
Resonance hybrid
The true structure of the molecule, representing the average of all resonance structures. Bond orders become fractional (for example, 1.33 instead of strictly 1 or 2).
Double-headed arrow
The symbol placed between resonance structures to indicate resonance. This is not an equilibrium arrow; the molecule does not switch between forms.
Formal charge
A bookkeeping tool that assigns hypothetical charges to atoms in a Lewis structure. Formal charge = (valence electrons) - (bonds) - (unshared electrons). Used to evaluate which Lewis structure is the best representation.
Structural isomers
Molecules with the same molecular formula but different connectivity of atoms. Ethanol (CH₃CH₂OH) and dimethyl ether (H₃COCH₃) are structural isomers: same atoms (C₂H₆O), different arrangements.
Organic compound
A compound built on a carbon skeleton, typically with hydrogen and often oxygen, nitrogen, or other elements. Carbon's ability to form four bonds and chain with itself makes an enormous variety of structures possible.
Inorganic compound
Broadly, any compound that is not primarily a carbon-chain-based organic molecule. Includes ionic salts, metal complexes, and small molecules like BeCl₂, BF₃, PCl₅, and XeF₂.
Resonance arises when a molecule has multiple valid Lewis structures that differ only in the placement of electrons (not atoms). All resonance structures are drawn, connected by double-headed arrows.
Nitrate ion (NO₃⁻) as the classic example:
Nitrogen is the central atom with three equivalent oxygen atoms.
One N=O double bond and two N-O single bonds in each structure.
The double bond can be drawn to any of the three oxygen atoms, giving three equivalent resonance structures.
The actual bond order in NO₃⁻ is 1.33 (one-third double bond character per N-O bond), not a pure single or double bond on any of them.
Benzene (C₆H₆):
Six carbon atoms in a ring, each bonded to one hydrogen.
Alternating single and double bonds can be drawn in two ways.
The resonance hybrid has all C-C bonds equivalent, with a bond order of 1.5.
Formal charge = (number of valence electrons) - (number of bonds) - (number of unshared electrons)
Rules for the "best" Lewis structure:
Minimise the formal charges on all atoms (smaller absolute values are better).
Place negative formal charges on more electronegative atoms.
Place positive formal charges on less electronegative atoms.
The sum of all formal charges must equal the overall charge of the molecule or ion.
SCN⁻ has 16 valence electrons (S: 6, C: 4, N: 5, plus 1 for the negative charge).
Two important resonance structures:
Structure 1: S=C=N with a lone pair on S and two lone pairs on N. Formal charges: S = 0, C = 0, N = -1. This is the better structure because the negative charge sits on nitrogen, which is more electronegative than sulphur.
Structure 2: S-C≡N with three lone pairs on S and one lone pair on N. Formal charges: S = -1, C = 0, N = 0. This is a valid resonance structure but less favoured because the negative charge is on sulphur (less electronegative than nitrogen).
The best single structure is Structure 1.
Ethene (C₂H₄): Two carbons connected by a C=C double bond, each with two H atoms. Planar molecule, about 120° bond angles around each carbon (trigonal planar local geometry).
Ethyne (C₂H₂): Two carbons connected by a C≡C triple bond, each with one H atom. Linear molecule, 180° bond angles.
Ethanol (CH₃CH₂OH): Two-carbon chain, with an -OH group on the end carbon. The O has two lone pairs. The C-O-H portion is bent.
Dimethyl ether (H₃COCH₃): Same formula as ethanol (C₂H₆O) but different connectivity: the oxygen sits between two methyl groups. This makes it a structural isomer of ethanol. The C-O-C angle is bent (about 109°), similar to water's shape around oxygen.
Acetic acid (H₃CCOOH): A methyl group bonded to a carboxyl group (C=O and C-OH on the same carbon). The carboxyl carbon has trigonal planar geometry.
Alanine (H₂NCHCH₃COOH): An amino acid with an amine group (-NH₂), a central carbon, a methyl side chain, and a carboxyl group. Multiple local geometries: tetrahedral around the central carbon, trigonal planar around the carboxyl carbon.
Cyclohexane (C₆H₁₂): Six carbons in a ring, all single bonds, each with two hydrogens. The ring is not flat; it adopts a "chair" conformation with tetrahedral angles.
Benzene (C₆H₆): Six carbons in a flat ring with alternating double bonds (resonance). All bond angles are 120°. Contrast with cyclohexane: benzene is planar and has delocalised electrons.
Carbon backbone: Organic compounds are built on chains or rings of carbon. Inorganic compounds have central atoms that can be metals, metalloids, or non-metals other than carbon.
Bonding variety: Organic compounds primarily use single, double, and triple covalent bonds between C, H, O, N, and a few other elements. Inorganic compounds can involve expanded octets, coordinate bonds, and ionic bonding.
Physical properties: Organic compounds tend to have lower melting and boiling points. Inorganic ionic compounds tend to be high-melting-point solids that dissolve in water to conduct electricity.
Examples from the lab: Organic: CH₄, C₂H₆, ethanol, benzene, alanine. Inorganic: BeCl₂, BF₃, NH₄⁺, PCl₅, SF₄, ClF₃, XeF₂.
"The molecule actually switches back and forth between resonance structures." It does not. The molecule exists as a single resonance hybrid at all times. The individual structures are a limitation of our drawing system, not a description of molecular behaviour.
"Resonance structures can differ in atom positions." Only electron positions change between resonance structures. If you have moved an atom, you have drawn an isomer, not a resonance structure.
"The best Lewis structure is always the one with a formal charge of zero on every atom." Zero formal charges are ideal, but sometimes unavoidable charges exist. In those cases, negative charges should sit on more electronegative atoms.
"Ethanol and dimethyl ether are the same molecule because they have the same formula." They are structural isomers. Same atoms, different connectivity, different properties. Ethanol is a liquid that mixes with water; dimethyl ether is a gas at room temperature.
⚠️ You will be asked to draw resonance structures for ions like NO₃⁻ and SCN⁻. Practise drawing all equivalent forms and connecting them with double-headed arrows.
⚠️ Formal charge calculation is a near-certainty on the exam. Know the formula and be able to assign charges to every atom in a Lewis structure.
⚠️ Be able to identify the "best" resonance structure using the rules: minimise charges, negative on electronegative atoms.
⚠️ Know the structural differences between ethanol and dimethyl ether as an example of structural isomers with identical molecular formulae.
⚠️ Be ready to compare organic and inorganic compounds in terms of bonding, structure, and physical properties.
True or false: Resonance structures show different arrangements of atoms in a molecule.
Fill in the blank: The formal charge formula is ___ minus ___ minus ___.
True or false: In NO₃⁻, the actual N-O bond order is 1.33, not a whole number.
Fill in the blank: Ethanol and dimethyl ether are ___ isomers because they have the same formula but different ___.
True or false: The best Lewis structure is the one where negative formal charges sit on the least electronegative atoms.
(Answers: 1. False, only electron arrangements differ. 2. Valence electrons minus bonds minus unshared electrons. 3. True. 4. Structural isomers, different connectivity. 5. False, negative charges should sit on the most electronegative atoms.)
Q: Draw the resonance structures of the nitrate ion (NO₃⁻). How many equivalent structures are there?
A: Three equivalent resonance structures. In each, nitrogen is the central atom with one N=O double bond and two N-O single bonds. The double bond rotates to a different oxygen in each structure. All three oxygens carry lone pairs, and the negative charge is distributed among them. The double-headed arrow connects each pair.
Q: In nitric acid (HNO₃), to which oxygen atom should the hydrogen be attached, and why?
A: The hydrogen should be attached to one of the singly-bonded oxygen atoms (an oxygen with a formal charge of -1 in the nitrate resonance structures). This oxygen uses one of its lone pairs to bond with H, giving it a formal charge of 0. Hydrogen bonds to oxygen, not nitrogen, because nitrogen already has three bonds and a positive formal charge in this structure.
Q: Draw two Lewis structures of SCN⁻. Assign formal charges and identify the better structure.
A: Structure 1 (S=C=N⁻): formal charges S = 0, C = 0, N = -1. Structure 2 (⁻S-C≡N): formal charges S = -1, C = 0, N = 0. Structure 1 is better because the negative formal charge is on nitrogen, which is more electronegative than sulphur.
Q: Compare inorganic and organic compounds using examples from this lab.
A: Organic compounds (CH₄, C₂H₄, ethanol, benzene, alanine) are built on carbon skeletons, use covalent bonds, tend to have lower melting points, and form molecular units. Inorganic compounds (BeCl₂, BF₃, NH₄⁺, PCl₅, SF₄, XeF₂) have varied central atoms, can have expanded octets, may be ionic, and often have higher melting points. Inorganic compounds from this lab include examples that are electron-deficient (BeCl₂, BF₃) and examples with expanded octets (PCl₅, SF₄, ClF₃, XeF₂).
Q: Why are ethanol (CH₃CH₂OH) and dimethyl ether (H₃COCH₃) different substances despite having the same molecular formula?
A: They are structural isomers. In ethanol, the oxygen is bonded to a carbon and a hydrogen (-OH group), allowing hydrogen bonding and making ethanol a liquid that mixes with water. In dimethyl ether, the oxygen is between two carbon groups (C-O-C), with no O-H bond, so it cannot hydrogen-bond as a donor and is a gas at room temperature. Same formula (C₂H₆O), very different properties.
Resonance connects to aromaticity (benzene is the starting point for understanding aromatic compounds in organic chemistry) and to molecular orbital theory, which provides a more rigorous explanation for delocalised electrons. Formal charges become critical in acid-base chemistry, where they help predict which atoms are protonated or deprotonated. The organic vs inorganic distinction threads through the rest of general chemistry and into upper-level courses: organic chemistry focuses on carbon compounds, while inorganic chemistry deals with the rest of the periodic table.
Resonance, resonance structure, resonance hybrid, double-headed arrow, formal charge, best Lewis structure, nitrate ion NO₃⁻, thiocyanate SCN⁻, benzene, structural isomers, ethanol, dimethyl ether, organic compounds, inorganic compounds, carbon skeleton, expanded octet, electron-deficient, acetic acid, alanine, amino acid, cyclohexane, general chemistry, molecular modelling lab