What if an extraterrestrial culture is familiar with chemical elements we know nothing about or they don't know about ours?
What if we find our understanding of the speed of light is arbitrary to our method of measuring not theirs? What if their laws of physics are based on their understanding of the limits of their physical world, not ours?
What if their reality is based virtually not physically?
Almost all entertainment media depicts alien cultures as super intelligent and easily capable of talking the english language. Like it or not, that is the layman's perception of a future encounter.
What if they interpret our intelligence by intercepted TV shows like...well anything on TV now.
Make no mistake about it, I believe there are lifeforms in the universe other than ours, it is statistically inevitable. I just believe that any encounter would be entirely coincidental not intentional and they would be as equally unprepared to deal with us as we would be with them except in one instance. Total domination by the superior culture.
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?
The writer, Andy Weir, just wrote a book that describes meeting a different race in deep space. It goes through the process of learning how to communicate. It starts with common items…like the atomic models of hydrogen and oxygen.
In essence…if the two parties are dedicated, they can figure it out pretty quickly.