Kryptonite!

[Great Rao]

Kryptonite is already Subcategory of Elements.

just see here: https://www.supermanthroughtheages.com/wiki/index.php/Category:Elements

I know that Kryptonite is any surviving fragment of the doomed planet Krypton - but a planet is composed of many elements, each with their own atomic number.  Supermanica states

When the planet Krypton exploded into fragments as the result of a cataclysmic chain reaction originating at the planet’s core, all of Krypton’s atomic elements “fused to become one deadly compound,” a compound later to become known as Kryptonite (S No. 61/3, Nov/Dec 1949: “Superman Returns to Krypton!”).

But becoming one compound isn't the same as becoming one element.  http://www.nyu.edu/pages/mathmol/textbook/compounds.html

[Super Monkey]

Yes I know, but it was the best name I could come up with at the time.

[MatterEaterLad]

It seems like elements, compounds, minerals, and even serums are going to have to be loosely defined, I suppose the category could be "elements" in quotes, or substances...