Nitrogen

Nitrogen was a 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. (TV: The Mutants)

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

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

Frostburn was liquid nitrogen, -200 degrees Celsius. (TV: Dragonfire)

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

The Fifth Doctor had canisters of liquid nitrogen on board the TARDIS. Peri used some to freeze the TARDIS swimming pool into a skating rink. They were later used to freeze the Great Old Ones. (AUDIO: The Roof of the World)

Earth's atmosphere was 80% nitrogen and 20% oxygen. In the 23rd century, the Eighth Doctor had altered Mars' atmosphere with the atmospheric re-ioniser on Deimos Moonbase to make it the same consistency, saving the human colonists below. (AUDIO: The Resurrection of Mars)