Rules for Writing Resonance Structures
The Nature of Resonance Structures and Resonance Hybrids.
When a single Lewis structure fails to represent a molecule or ion adequately, the molecule/ion is frequently represented as a hybrid of multiple Lewis structures. The individual Lewis structures are called resonance structures. The alternative terms resonance forms, contributing structures and contributing forms are also used.
The real molecule/ion is called a resonance hybrid. The hybrid is real, but the resonance structures are just imaginary approximations to it.
How to write Resonance Structures
Begin with a proper Lewis structure for your molecule.
Change only the positions of π electrons and unshared electron pairs to make additional resonance structures.
Obey all valence rules.
The two structures below satisfy these criteria and are contributors to the same hybrid. Resonance structures that contribute to the same hybrid are usually connected by a double-headed arrow and are frequently enclosed in curly brackets as below.
Criteria for Resonance Structures
To contribute to the same hybrid, resonance structures must satisfy the following criteria. Use them to test resonance structures.
They must have the same elemental composition. C2H5Cl and C2H3Cl cannot contribute to the same hybrid.
They must have the same number of electrons and the same overall charge. The two structures below do not satisfy this criterion.
They must correspond to the same atom connectivity. An equivalent statement is that only electrons, not atoms, can be moved to interconvert resonance structures. The two structures below do not satisfy this criterion.
They must correspond to the same 3-dimensional geometry. The two structures below do not satisfy this criterion.
The two carbons that join to complete the 4-membered ring in the right-hand structure must come closer than they are in the left-hand structure. The change in hybridization at these two carbons also implies a change in their local atom and electron geometry from trigonal planar to tetrahedral.