Resonance

Theory of resonance: many molecules and ions are best described by writing two or more Lewis structures and considering the real molecule or ion to be a composite of these structures.
Contributing structures: individual Lewis structures; resonance structures
Resonance hybrid: a molecule, an ion, or a radical described as a composite of a number of contributing structures.
Resonance structures are not in equilibrium with each other.

All contributing structures must have the same number of valence electrons.
All contributing structures must obey the rules of covalent bonding
* No contributing structure may have more than two electrons in the valence shell of hydrogen or more than eight electrons in the valence shell of a second-period element.
* Third period elements, such as phosphorus and sulfur, may be drawn indicating up to 12 electrons in their valence shells
The positions of all nuclei must be the same in all contributing structures; that is, contributing structures differ only in the distribution of valence electrons.
All contributing structures must have the same number of paired and unpaired electrons.

Not all structures contribute equally to a resonance hybrid.
there is only one actual way that the electrons are distributed in the molecule, but the limitations of how we draw molecules with dot pairs and lines require us to rely on examining two or more depictions
Resonance structures that follow the most preferences contributes the most to the hybrid
  * Structures in which all atoms have filled valence shells (completed octets) contribute more than those in which one or more valence shells are unfilled.
  * Structures with a greater number of covalent bonds contribute more than those with fewer covalent bonds.
  * Structures that involve separation of unlike charges contribute less than those that do not involve charge separation because separation of charges costs energy.
  * Structures that carry a negative charge on a more electronegative atom contribute more than those with the negative charge on a less electronegative atom.
    * Conversely, structures that carry a positive charge on a less electronegative atom contribute more than those that carry the positive charge on a more electronegative atom
It is important to realize that if resonance structures contribute unequally, the actual structure of the hybrid most resembles the structure that contributes most
In MO theory, when the starting atomic orbitals are not equal in energy, the resulting molecular orbitals most resemble (have a greater contribution from) the atomic orbital that is closest in energy.

Delocalization: the spreading of charge and/or an electron density over a larger volume of space
* Always a stabilizing effect
Conjugation: lack of atoms between pi bonds or between pi bonds and lone-pair electrons.
We consider delocalization only for molecules containing p bonds adjacent to pi bonds and/or pi bonds adjacent to lone pairs
For an atom to be involved in p bonding, it must have a 2p orbital
In general, whenever atoms are involved in resonance that has one or more contributing structures involving a pi bond to that atom, the atoms must be sp2 hybridized (or on rare occasions, sp hybridized, but never sp3 hybridized).