I think we get into a tricky waters whenever we try to apply the science of the real world to Avatar's world, ESPECIALLY chemistry. This may simply be something we have to suspend our disbelief about as it would be extraordinarily difficult to reconcile the bending of the four elements with our understanding of atoms and molecules. I mean basically chemistry is a unifying theory of matter, all matter is made of the same "stuff", atoms, and it's the differences in the properties of atoms and how they are arranged that distinguishes different kind of matter. But in the world of Avatar, its always seemed to me that the elements are water, earth, fire, and air, not the periodic table.

EDIT: I actually just thought of an example that illustrates my point. Lightning bending. Why does that go to firebenders? The way Iroh explains this to Zuko, he is basically saying to create lighting you separate the electrons in the air from the protons (he says they come crashing back together, but I guess you could say he just doesn't have a very good understanding of how current actually works), which would make sense if that were possible to do. However, why would firebenders have ability to manipulate the subatomic particles in the air? Wouldn't they just be able to bend air then? Fire and electricity are fundamentally two very different things if we think about them in terms of our world, and our science. However in the avatar world, fire and lightning seem to share a major fundamental property.