Resonance and Formal Charge

Resonance in Molecular Structures

Resonance refers to a condition that occurs when more than one valid Lewis structure can be written for a particular molecule or ion.
These multiple structures, known as resonance structures, do not represent different structures changing over time but illustrate the same single structure with electrons delocalised over the entire molecule.
Resonance structures contribute to the resonance hybrid, which is the actual structure of the molecule. The stability of a resonance hybrid is greater than the stability of any individual resonance structure.
Differences between resonance structures lie in electron placement, not in the arrangement of atoms.
In order to draw valid resonance structures, one must follow the octet rule, an atom cannot have more than eight electrons in its valence shell, with exception for central atoms in the third period and beyond which can accommodate more than eight valence electrons due to d-orbital participation.

The formal charge of an atom in a molecule or ion is used to determine the most stable Lewis structure when more than one is possible.
Formal charge is calculated as the number of valence electrons in an atom, minus the number of electrons it ‘owns’, where ‘ownership’ is defined as the number of unshared electrons plus half of the shared electrons.
A formal charge of zero is preferred for all atoms in a molecule. When this is not possible, atoms closer to electronegativity should have more negative charges.
When calculating formal charge, it should always be remembered that the sum of the formal charges on all atoms in a molecule or ion must equal the total net charge of that species.
Comparing different resonance structures, those with minimal formal charges and with any negative charges residing on the more electronegative atoms are more stable and thus, more likely to represent the actual placement of electrons in the molecule or ion.
Formal charge does not necessarily indicate real charge; it is a tool for deciding among potential structures. It can help understand or predict molecular reactivity and interactions based on charge distribution.

Resonance and formal charge are intertwined concepts.
Molecules exhibiting resonance may have different formal charges for different resonance structures.
The most stable resonance structure will have a formal charge distribution closest to zero while maintaining the total net charge.