@cal: In Legend of Korra Lightning bending became more common, but in The Legend of Aang it was and elite skill. According to Avater wiki this is even mentioned explicitly in the Artbook.
All in all lightning bending is simply a very powerful ability.
As said with the conductivity you have a point.
Was ever something in regards to electrical machines shown in Korra?
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Aparajita: Because lightning more often than not is energy beams.
Air, Earth and water are commonly taken from the sourroundings so there hardly is any doubt about them being real.
The thing is for those four it doesn't matter, since they don't have any stat related properties (except fire colour and heat in which case it would be actually debated wether that is an magical fire or not).
For something stat related we actually have to figure out whether it is supernatural in order to reason to other natural properties. As said the other elements would be debated as well, if there was any reason to.
The fact that the rest of the elements showing element properties isn't impressive as those properties are a lot more basic. More importantly hat reasoning is like saying "a verse showed one ability can produce something with the properties that one would scientifically expect, hence every ability in the verse has those". That is not valid reasoning.
Iroh doesn't talk about electrons and protons. He talks about the energies around them, yes, but those energies can, in the given mythological believe of the verse, be the energies of nature, specifically they are yin and yang. The energies of the moon (yin) and the ocean (yang) are for example actively represented of such energies shown in the verse, which can power bending. (in this case they power water bending)
In that sense, that there are balances between yin and yang is also a common concept in series that incoporate such concepts.
