Bond line structures are useful and easy to read, however they are somewhat inadequate due to the mobility of electrons. Because electrons have no fixed location within a molecule or compound they can be delocalized throughout the structure; however, some spaces are more likely to have a higher electron density than others.
As a result organic chemist generally look at something called resonance. Resonance structures are used when one (Lewis) Structure cannot fully explain the bonding that occurs between the atoms. 2+ high energy structures are considered a part of the resonance hybrid which shows the overall delocalization. The more resonance structures a molecule has, the more stable it is. It is important to note that resonance structures only change the position of the electron not the structure of the molecule.
Delocalization and Resonance Rules
Structures should have the same number of electrons and lone pairs (LP)- no more, no less.
Resonance structures must follow the rules of Lewis Structures
Hybridization must remain the same
Only the electrons move - do not rearrange bonds
Resonance Hybrids
When drawing Resonance structures, use solid lines for bonds
A dotted line can be used to show a double or triple bond present
Draw only the LP found in all resonance structures
Stability Rules
Greater number covalent bonds = Greater stability due to complete octets
Most stable structure has the least number of formal charges
Stable structures have the least separation of formal charge
Negative charges on the atom with the higher electronegativity is more stable and positive charges on atom with the higher electropositivity is more stable.
Equivalent resonance structure attribute equally to stability.