God, most disruption but without affecting the universe? I would say probably Tungsten? It's not particularly important in any major "universe" stuff, but it is very important for humans with it's super high melting point. I feel it's an element that would very much disrupt humans but not make the universe implode like if you chose Hydrogen.

i don't know about chemistry to answer this :3

Hydrogen obv, all the stars go out at once and the universe instantly becomes dark and devoid of any life that may have existed...

i'm not smart enough for this type of question,, just at a guess maybe titanium? that seems important for humanity but maybe not so much for the overall universe

Lead. All unstable elements now no longer decay into a stable element.

i assume either one of the nobles or the heaviest ones that dont like to exist at all... but its been a while since chemistry

no what i should do is make an element that decays into lead and gold for the memes

This would have been a good question for Our younger self.. We have forgotten much of what We were taught in Our chemistry lessons.