I guarantee you ... they know about hydrogen, and helium, and lithium and all the rest. The universe is filled with them. The rest of your "what ifs" are based on the same misunderstanding ... until:
What if their reality is based virtually not physically?
Whatever do you mean by that?
Whatever do you mean by that?
There are proponents of us being holograms. I don't believe in that theory but that theory is even gaining traction with respected physicists.
If we, by chance, are a hologram then nothing in our perception is physically real, only virtually real. That would include encounters with other civilizations.
I guarantee you ... they know about hydrogen, and helium, and lithium and all the rest
As for the elements being universal, yes to the extent we know them and their light emissions from other celestial measurements.
That doesn't guarantee that other life forms would perceive them as we do. What if their world is made of antimatter? What if they have no interest in atomic theory as we use it?