Hydrogen

Element
Hydrogen is the first element in 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 and is the lightest element. 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 some Hydrogen barrels in Halo 2 and 3.