|There is more information available on this subject at Nitrogen on the English Wikipedia.|
Nitrogen (pronounced /nī'trə-jən/) is a chemical element that has the symbol "N," atomic number 7 and atomic weight 14.007. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78.1% by volume of Earth's atmosphere.
Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The very strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in converting the N₂ into useful compounds, and releasing large amounts of energy when these compounds burn or decay back into nitrogen gas.
Nitrogen occurs in all living organisms — it is a constituent element of amino acids and thus of proteins, and of nucleic acids (DNA and RNA); resides in the chemical structure of almost all neurotransmitters; and is a defining component of alkaloids, biological molecules produced by many organisms.
Almost all known planets in the Halo Universe possess nitrogen-rich atmospheres, ranging from Earth to Sanghelios, presumably necessary for the formation of DNA and therefore sentient and non-sentient life.