Definitely a classic is "Solaris" (1972 film) by Andrei Tarkovsky. It has a mix of sci-fi and drama, but I can't tell more without spoiling it. It definitely makes you think ... Also other Tarkovsky films have been praised, like "Stalker" that you mentioned. So maybe check out his filmography?
Edit: Something completely different is "The Mystery of the Third Planet" (1981) by Roman Kachanov. It is an animated sci-fi film for children, but also fun to watch as an adult because of the absurd storyline and humour.
Other films I have in mind, too, but don't remember their exact titles now. Will edit this post, if I recall ... Edit: You might want to have a look at the Wikipedia list differentiating by age and/or genre: https://en.wikipedia.org/wiki/Cinema_of_the_Soviet_Union
My ELI5 answer: The periodic table is basically "finished", there are no new additions. Meaning, the structure is is set and almost all elments found, including some (theoretical) higher-number elements that still have to be securely "discovered". This final discovery nowadays happens only in specialised labs from time to time, because the higher-number elements are extremly unstable, only created artifically and existing for a few nanoseconds or femtoseconds. Some more details you can find on Wikipedia here: https://en.wikipedia.org/wiki/Actinide
Additionally, some people discuss different models/structures for organising the elements in general. But that is a different topic.