Hydrogen

Element
Hydrogen has an atomic number of 1; it is the first element in the Periodic Table and is the lightest with a molar mass of 1.00794 g/mol. It has an atomic radius of 78 pm. In its natural diatomic 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 common molecule is H2O, or water.

Hydrogen is the most common element in the entire universe because it makes up almost all stars (the rest of stars' mass is mostly Helium). When purely combusted with Oxygen, the (chemical equation 2H2 + O2 ---> 2H2O) it forms water and heat, a very useful reaction with a clean result.

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. As fossil fuels have probably been abandoned, hydrogen is most likely humanity's current top power source: A UNSC Reactor use hydrogen instead of nuclear power to power buildings and heavy vehicles.