Nitrogen

Nitrogen was chemical element present on a number of planets.

On Solos, the soil contained a nitrogen isotope unknown on Earth. This isotope reacted with ultraviolet rays to produce a mist toxic to humans. (DW: The Mutants)

Triton had a thin atmosphere composed of nitrogen. (PDA: The Devil Goblins from Neptune)

On Justicia, nitrogen was rendered as snow when the planet moved away from its sun. (NSA: The Monsters Inside)

The 456 thrived in an atmosphere composed of 20% nitrogen (in addition to other elements). (TW: Children of Earth: Day Two)