This is something that got me thinking for a while. Especially after learning the basic of physics and chemistry. Basically, pretty much every movies, novel, etc that feature an advanced race of aliens(or any alien in general), they gives them some sort of technology or their organic composition that is made of materials unknown to us. Since the Periodic Table is pretty much "complete" (no missing(space) between elements) It would means that they build pretty much everything from Very heavy/dense elements right? Wouldn't that mean that they are basically very radioactive and unstable? If it's right, it would mean that any actual aliens would have pretty much the exact same materials available as us. I'm alright giving them the benefit of the doubt of a superior technology so they can create different isotopes which could result by a better and more efficient material. I always feel like the authors of such scenarios went the lazy path and decided to use the excuse that it's sci-fi so it doesn't matter. What do you guys think?