nitrogen
It is the most abundant constituent of the atmosphere, amounting to 78.09% by volume of dry air. The molecular formula for nitrogen gas is N2; its molecular weight is 28.016. Nitrogen enters the atmosphere from volcanoes and from the decay of organic matter. It is removed from the atmosphere by certain natural nitrogen-fixing bacteria for use in plant life processes. Free nitrogen is very inactive, but can be broken down by high-energy reactions such as occur in lightning, high temperature combustion, or in the upper atmosphere. Nitrogen-containing compounds are very reactive and play integral roles in the production and destruction of ozone in the atmosphere. Atomic nitrogen, N, occurs in significant quantities only at altitudes above about 100 km.
