Fluorine

Fluorine is a highly reactant element, existing normally as a diatomic molecule - never as a singular element, at least on Earth.

Borg drones are apparently immune to oxidizing, as fluorine is one of the compounds found in the preferred Borg atmosphere, along with carbon monoxide and methane. 

In an alternate timeline where the Borg were able to assimilate entire star systems in the Alpha Quadrant, increasing levels of fluorine were noted in the atmospheres of Alpha Centauri and Andor while the Borg adapted their worlds.