covalent model - resonance structures + benzene
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When do resonance structures occur?
Resonance structures occur when there is more than one possible position for a double bond in a molecule or ion.
What condition must be met for resonance to be possible?
There must be more than one valid Lewis structure that differs only in the position of electrons, not the arrangement of atoms.
What does resonance indicate about electron behaviour in a molecule?
It indicates that some bonding electrons are delocalised and are not confined to a single bond position.
Define delocalisation in the context of resonance.
Delocalisation is the spreading of bonding electrons over more than one bonding position within a molecule or ion.
Why does delocalisation increase stability?
Because delocalised electrons are spread over a larger volume, reducing electron–electron repulsion and lowering the overall energy of the system.
Why are resonance structures not real, separate molecules?
Because the actual molecule does not switch between structures; the resonance structures are models used to represent electron delocalisation.
What is meant by a resonance hybrid?
A resonance hybrid is the true structure of a molecule, representing the average of all valid resonance structures.
How does the resonance hybrid differ from individual resonance structures?
The hybrid shows delocalised electrons spread across all possible positions, rather than localised double bonds.
How are resonance structures usually represented in diagrams?
By Lewis structures connected by a double-headed arrow (↔), indicating delocalisation rather than equilibrium.
What does the double-headed arrow between resonance structures mean?
It shows that the real structure is a hybrid of the forms, not that the molecule alternates between them.
What experimental evidence supports resonance in molecules?
Measured bond lengths and bond strengths are often equal and intermediate between single and double bonds.
How does ozone (O₃) demonstrate resonance?
Ozone has two valid Lewis structures with the double bond in different positions, but experimentally both O–O bonds are equal.
What would Lewis structures incorrectly predict for ozone?
One O–O single bond and one O=O double bond with different lengths and strengths.
What is the actual bonding situation in ozone?
Both oxygen–oxygen bonds are equal and intermediate in length and strength between a single and double bond.
How is this (O3) explained using resonance?
The π electrons from the double bond are delocalised and shared equally between both O–O bonds.
Why are dashed bonds sometimes used in resonance hybrids?
To indicate partial double-bond character and delocalised electrons.
Why are lone pairs not drawn in resonance hybrid structures?
Because the electrons involved in delocalisation do not have fixed positions.
How many resonance structures exist for carbonate, CO₃²⁻?
Three resonance structures, corresponding to three possible positions of the C=O double bond.
What does resonance explain about the C–O bonds in CO₃²⁻?
All three C–O bonds are equal in length and strength, intermediate between single and double bonds.
How can you predict the number of resonance structures for a molecule or ion?
By counting the number of possible positions for double bonds in valid Lewis structures.
Why does resonance not violate conservation of electrons?
Because resonance structures redistribute the same electrons; no electrons are added or removed.
How does resonance affect bond enthalpy?
Resonance lowers the overall energy of the molecule, so bonds are stronger than expected for single bonds but weaker than pure double bonds.
Why is resonance important for explaining reactivity?
Because delocalisation stabilises molecules and ions, often making them less reactive than predicted by Lewis structures.
Give a textbook example where resonance explains acidity/basicity differences.
Differences in the strengths of organic acids and bases can be explained by the stability of resonance-stabilised conjugate ions.