Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an atomic weight of 1.00794 u, hydrogen is the lightest and most abundant chemical element, constituting roughly 75 % of the Universe's elemental mass. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.
The Race had made use of hydrogen as a fuel for their vehicles for millennia before they invaded Earth in 1942. The Race was initially perplexed by Tosevites' decision to use petroleum for transportation, as they saw it as an utter waste of the resource.