Hydrogen

Element
Hydrogen is the first element on the Periodic Table, with an atomic number of 1. In its natural state, it is a colorless, odorless, tasteless gas. It is a very flammable gas. Its low atomic number makes it a very popular component in many reactions and molecules. Its most popular molecule is H2O, or water.

Hydrogen In Halo
Hydrogen is a gas used to fuel some UNSC vehicles, such as the Warthog and Mongoose. There are also Hydrogen barrels in Halo 3.

Trivia

 * Hydrogen gas was used in the famous blimp, the Hindenburg. When it set fire in 1937, the Hydrogen gas caused an immediate explosion.