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)

According to Peri, "A nitrogen-rich atmosphere could be ideal for certain plant life." The atmosphere of Izdaal's tomb on Mars, while rich enough in nitrogen for the Ice Warriors to thrive in, also had enough oxygen for humans and Time Lords to breathe. (AUDIO: Red Dawn)