I'd be surprised if this hasn't been discussed before, but I thought I'd shared some of my thoughts on the subject. By chemistry, I mean what chemicals/atoms/molecules/ions can benders bend? I'm not trying to explain how they do it, just what they use. I think I've got pretty decent answers for (almost) every type of bending.
Waterbending: This one is the easiest, because it's right in the name. Waterbenders can control good old H2O. Most waterbenders can also create ice, which would mean they are forcing the water to vibrate less, so they cool it down, and they could assist this process by expanding the water (water is one of the few substances that expands when it freezes). Plantbending and bloodbending are pretty obvious, as plants, animals and humans are most made of water. The moon requirement for bloodbending could be nothing more than a psychological requirement, which would explain why the Blues Brothers (Amon and Tarrlok) could do it without a moon. Healing could easily be a form of bloodbending where the user controls the water in someones body to help the healing process.
Airbending: This one is pretty easy too as there are really one of two molecules that airbenders could use: N2 or O2. N2 is molecular nitrogen and makes up 70% of the air we breathe, while O2 is molecular oxygen, and it makes up 21%. I would say that nitrogen is the more likely of the two as it is the more plentiful. It would still allow the user to choke someone like the Earth Queen by creating a pocket of low pressure around her head, causing her lungs to collapse. As for becoming lighter than air, that's a tough one, and could be nothing more than some pretty strong control over the air directly below your body. Just like bloodbending, there seems to be a psychological block keeping everyone from doing it.
Earthbending: This one was simple until last season. Silicon (Si) makes up most of the earth's crust (after oxygen of all things), and is a major component of steel. In fact the silicon content can greatly change steel's properties. So this explains earth, metal, sand and lavabending, with lavabending being to earthbending what icebending is to waterbending, nothing more than a change in temperature. BUT WAIT, what was the Red Lotus Poison? It wasn't any real life liquid metal, most heavy metal poisoning both takes much longer and has different effects. Also, if the poison itself was a metal, it would have to have some silicon in it for Korra to be able to bend it, but that doesn't really help because silicon is in many metals, even platinum to an extent. I don't know of any naturally liquid metal that would both cause the effects AND have silicon in it. I think that this one comes done to Deus Ex Jinora once again just somehow knowing what to do to save Korra. So, platinum and the poison aside, I'm pretty sure that Silicon is what earthbenders bend.
Firebenders: This is where stuff gets cool. There isn't a specific element or molecule for them to bend that would explain their powers. Which means that what they are doing is actually bending plasma, or atoms that have become so energized that they lose their electrons. Once they electrons are stripped, the plasma can take on a number of properties, which include EXTREMELY hot flames and lightning. So the average firebender can cause the air around his fists to become so hot that they iodize and become plasma, while lightning would be the firebender taking the electrons from the air around them and shooting it at their opponents. Like bloodbending and flight(bending?), this would require a very specific state of mind. Now combustionbending is somewhat harder to define, but if they can use the individual atoms of anything around them to create fire and lightning, I think it would stand to reason that they can also have enough control over an atom to be able to fuse it with other atoms. Yes, I think that combustion is actually a miniaturized nuclear fusion reactor. Fusion releases a LOT of energy, but doesn't have all that much radiation, in fact after the initial blast, there isn't any.
So what do you think? Am I over thinking it? Did I miss an element? Am I completely right?