About 99% of atmosphere consists of these two gases. 78% Nitrogen and 21% Oxygen.
The answer lies mostly in three facts:
1. nitrogen is volatile in most of its forms
2. it is unreactive with materials that make up the solid earth
3. it is very stable in the presence of solar radiation.
To understand the abundance of N in the atmosphere, it is useful to compare it to O (the next most abundant element in the atmosphere). Compared to O, N is 4 times as abundant in the atmosphere. However, we must also consider the relative abundances of O and N over the entire Earth (oxygen is about 10,000 times more abundant). These earthly abundances overall reflect the composition of the material from which the Earth originally formed and the process of Earth's accretion. Oxygen is a major component of the solid earth, along with Si and elements such as Mg, Ca and Na. Nitrogen is not stable as a part of a crystal lattice, so it is not incorporated into the solid Earth. This is one reason why nitrogen is so enriched in the atmosphere relative to oxygen. The other primary reason is that, unlike oxygen, nitrogen is very stable in the atmosphere and is not involved to a great extent in chemical reactions that occur there. Thus, over geological time, it has built up in the atmosphere to a much greater extent than oxygen. It is important to know that both nitrogen and oxygen are intimately involved with the cycle of life on the planet, but that chemicals cycle through this material on a short time scale relative to the geological processes that have, over time, made the earth what it is today (compositionally and physically). N and O are found in the living biosphere and fossil organic matter (and both are soluble in sea water too). But, only a small fraction (less than 1% for N and much, much less than 1% for O) of the total for these element on the planet are to be found in these places.
Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822