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 also 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)