na'vi and human air

[Raiden]

Originally Posted by Ja'k Dawsiin

why is there 80% nitrogen in earth's atmosphere? jw

No reason.

It was just a random event that happened billions of years ago when the Earth formed; of the gaseous elements that coalesced into Earth after our star (Sol, or "The Sun") shed some of its outer layers (this is a normal part of the "life cycle" of stars), Nitrogen was the most abundant. It is likely that Earth has always had a high concentration of Nitrogen, but there are other features that affect its abundance.

Nitrogen gas (N2, since Nitrogen gas only exists as a pair of bonded Nitrogen atoms) is relatively unreactive. Oxygen gas reacts with many different metals, including Iron and Magnesium, and so some of it is trapped as a part of other compounds, such as iron oxide (also known as rust, or Fe2O3), or covalently bonded to two hydrogen atoms, forming water. Carbon dioxide, CO2, is a very important part of plant and animal metabolism, and it is transformed directly into plant biomass via photosynthesis; the amount of CO2 in our atmosphere has changed dramatically between different geological eras, and until recently was tied to the amount of functioning plant biomass on the planet versus decaying plant biomass and animal metabolism.

Basically, of the three major gases in our atmosphere (oxygen, carbon dioxide, and diatomic nitrogen), nitrogen is the least reactive. Some plants harbor symbiotic bacteria that can transform nitrogen gas into nitrate (NO3-) which is an important plant nutrient, and the surging energy of lightning bolts is enough to force a reaction between N2 and O2 to form NO3- in the soil at the site of a lightning strike. But these two things happen in limited amounts; gaseous nitrogen doesn't react much, and when it does, it is only in small quantities, and there are a myriad of aquatic bacteria in marine and freshwater biomes that produce more N2 gas from NO3-. These processes keep the balance between gaseous nitrogen and nitrogen in living organisms, but the vast majority of Earth's nitrogen will always be in the atmosphere.