Hydrogen

Hydrogen was the most common element in the universe. A hydrogen in-rush was a crucial part of Event One. (TV: Castrovalva)

The Fourth Doctor suggested energising hydrogen as an alternative power source to the "mineral slime" oil. (TV: Terror of the Zygons)

Hydrogen could be used as a base in the process to form huon particles. (TV: The Runaway Bride)

The R101 was filled with hydrogen gas. (AUDIO: Storm Warning)

The Skelloids' life cycle relied on the absorption of hydrogen through their gill-plates. Banto Zame, posing as the Doctor, seemingly foiled an invasion of the Generios system by the Skelloids by exploiting this by using his psychic screwdriver to invert the core of their hydrogen stabilisers. In reality, the Skelloid invasion was nothing but a mere holographic illusion created by him. (AUDIO: The One Doctor)

It is unclear whether or not the Skelloids existed beyond Banto's illusion.