TLDR; Earth benders are the only true element benders
Of the four element benders, the only ones who are actually bending an element are earthbenders.
As we know very well, there aren't four elements irl, but rather 118 (including a handful of which are effectively completely synthetic). My theory came after realising that of the four elements we see in the avatar universe, only 3 are actually matter. Fire, isn't even matter, so that doesn't count here.
That leaves us with the remaining three. Of which we have, air benders bend a mixture of any gasses, and water benders bend a specific chemical (H2O in this case, not an individual element)
Leaving us with earth benders. The only true element benders. Which in this case, is Silicon.
Earth benders bend rocks- silicon
Earth benders bending sand- silicon
Earth benders bending glass- silicon
Earth benders bending lava- again, silicon.
Now, one might argue that, "some earth benders can bend metals, that's objectively not silicon", to which I say, "watch the show again."
Metal benders are canonically bending impurities in the metal, i.e. RESIDUAL ORE, ROCKS, SILICON.
The reason why they can't bend platinum, is because it is supposedly extremely pure. i.e. no residual ore for them to bend.
Therefore, my fellow nerds, earth benders are the only true element benders and should be more accurately renamed to be silicon benders.
Thank you for reading my ted talk.
Edit: spelling