Resonating molecules

Question:

Can you tell me the resonating frequency of H₂O (water) and how resonance in atoms work so I could get a broader understanding of resonance in atoms.

Answer:

I think it is best dealt with in two parts.

Firstly the resonance of some diatomic molecules.
Hydrogen chloride 8.66x10¹³ Hz
Carbon monoxide 6.42x10¹³ Hz
Nitrous oxide 5.63x10¹³ Hz

You can look up theory of how these frequencies are calculated in university text books such as Physics by R.Serway. It is way above A level however.
These frequencies lie in the near infra red part of the spectrum, nowhere near the microwave area.

Now to water and microwaves.
When microwaves pass through water the water molecules twist and turn, writhing around, as they pass. However after the microwaves have gone the molecules stop moving again, remitting the energy as more microwaves. In free water molecules this does not result in a heating. It is the rubbing of one molecule against another as in liquid water that allows the energy to be retained and prevents it being reemitted as microwaves.

Microwave ovens operate at a frequency of 2.45 GHz and this is NOT the resonant frequency of a water molecule. If it were the microwaves would all be absorbed in the surface layer of a substance (liquid water or food) and so the interior of the food would not get cooked at all.

The 2.45 GHz is a kind of useful average frequency. If the frequency was much higher then the waves would penetrate less well, lower frequencies would penetrate better but are absorbed only weakly and so once again the food would not absorb enough energy to cook well.

If you want to look at a really good site with a moving applet try this:

http://www.colorado.edu/physics/2000/applets/h2o.html