What single element could destroy the world?
Some of the answers seem to be - while amusing - either 100% wrong (the proposed method wouldn't destroy ALL life), or outside the question's clearly delineated parameters (e.g. near future means, no magi-tech, no hand-wavium)
Hydrogen atom. 20 upvotes. Requires magi-tech to accelerate the atom to needed energy, clearly NOT feasuble with near future means.
Neutron star. 10 upvotes. Leaving aside requiring magi-tech to acquire one, the question explicitly mentioned periodic table of elements, so no amount of "looks like an atom" hand-waving can make it be an on-topic answer.
2 answers have serious flaws but at least are fixable. My question really is about 2 answers above, not the following.
Astatine. 3 votes. While nowhere near as bad as other answers, it doesn't even remotely show that it can be used to wipe out all life - as opposed to a couple of beings. Mind you, this answer can probably be improved to show "how", so it's the 2-d least objectionable one.
Hydrogen explosion - this one has only one problem. It wouldn't affect life on deep sea floor. (though, with enough hydrogen, maybe? That requires some calculations to be sure; as well as knowing how much Oxygen you have available).