​Lewis symbols and structures are used to represent the valence electrons of atoms and the way in which they are arranged in molecules. A Lewis symbol consists of the symbol for the element, surrounded by dots that represent its valence electrons.

For example, the Lewis symbol for carbon is "C" with four dots surrounding it, representing its four valence electrons.

​Lewis structures can be useful in understanding the chemical properties of molecules, including their polarity and reactivity.

They are also used in predicting molecular shapes and identifying isomers, which are molecules that have the same chemical formula but different arrangements of atoms.

​Formal Charge and Resonance

Formal charge is a measure of the electron distribution in a molecule or ion, calculated by subtracting the number of valence electrons in an isolated atom from the number of electrons associated with that atom in a molecule or ion. The formula for calculating formal charge is:

Formal Charge = Valence Electrons - (Number of Unshared Electrons + 0.5 x Number of Shared Electrons)

​Resonance

​Resonance is a phenomenon in which two or more Lewis structures can be drawn for a single molecule or ion, each structure differing only in the arrangement of electrons.

The actual electronic structure of the molecule or ion is a combination of these resonance structures, and is known as the resonance hybrid.

Resonance is observed when a molecule or ion has a delocalized system of electrons, meaning that the electrons are not confined to a single bond or atom, but are shared over several atoms in the molecule.

This can result in more stable molecules, as the delocalization of electrons can decrease the energy required for bond formation and increase the stability of the molecule.

The concept of resonance is often used to explain the behavior and properties of molecules, particularly in organic chemistry.

It is important to note that resonance structures are not real structures in the traditional sense, but are simply representations of the actual electronic structure of the molecule or ion.