[poobear]

I'm just reading about why ATP is good as energy currency, and came across this sentence.

ADP and, particularly, Pi, have greater resonance stabilization than does ATP. Orthophosphate has a number of resonance forms of similar energy (Figure 14.4), whereas the γ-phosphoryl group of ATP has a smaller number. - http://www.ncbi.nlm.nih.gov/books/NBK22439/

I'm a molecular biology student so my knowledge about organic chemistry etc is very limited. But why does more resonance forms give a lower energy? Should I think in terms of entropy and electrons that can be in more places have a higher entropy? Or should I think in terms that localized charged particles are somehow more destabilizing that non-localized particles? Or how should I think? 

Thanks!

[Grundalizer]

Maintaining a negative charge on an atom is unstable, maintaining two negative charges like the ortho phosphate is even harder.  You are basically forcing extra electrons into the system, whose negative (like) charges repel each other, adding instability into the system.  It's sort of like trying to hold a bunch of magnets with the same poles near each-other, they always push and pull in strange directions, but if you can give them enough space where they can each spend time AWAY from each-other, then that adds stability to the system because they aren't constantly interacting with each other, and even if they are, it's from a greater distance so less force is involved.

Resonance structures ALL contribute to the overall structure of the molecule, but none of them by themselves are how the molecule REALLY looks.  Lewis structures are fine for a basic idea, but if you want more insight into the bonding of particular species molecular orbital theory is probably the best, although they all have their  (VSPR, lewis dot, MO theory,,,)

http://www.ncbi.nlm.nih.gov/books/NBK22439/figure/A1948/?report=objectonly

[poobear]

Excellent answer!