Finding the more stable resonance structure?

[Chem_Mod]

Question:
a.) When drawing resonance structures, how do you determine what the more stable resonance structure?

b.) In the structures, which atoms would get the negative charge?

c.) What if there are more than one electronegative atoms with the same electronegativity? (say 2 oxygens? or 2 chlorines?) Is it possible to draw and select a resonance structure that shows both areas of delocalization?

[Chem_Mod]

Answer:
a.) Use formal charge. Formal charge closer to zero is the more stable structure.

b.) If there is a choice between say C or O having a negative charge, put the negative charge on O.

c.) Yes. For example CH3COO- has two equivalent resonance structures. A more stable structure is one in which the electrons delocalize towards the more electronegative atom, positively charged atom (ie carbocation), or to an sp2 hybrid to create stability.