What is resonance

Resonance effect describes the case when there are more than one possible molecular structures possible for the given molecular formula ( canonical structures) but none of them actually describe the molecule totally correct. This is due to delocalisation of π electrons. The physical properties of the actual compound show intermediate characteristics of these canonical structures. Eg bond length of all C-C bonds of benzene in its canonical structure is different , single bond longer than double bond. But actual experimentally determined data through spectroscopy shows that all C-C bond lengths are equal . Thus a single canonical structure is incorrect and a mixture that js a hybrid of those canonical structures is used to describe the molecule know as resonance hybrid.