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Nitrogen (N) is the element with atomic number 7, symbol N. Its most common isotope has mass number 14, with 15 being another stable isotope. It is the seventh most common element in the universe and is formed by fusion of carbon and hydrogen in supernovae. It is volatile in its atomic form but very stable in its molecular form, N2, which has a triple bond; typically it is easer to convert another compound to N2 than the reverse (e.g., chemical explosive compounds often include nitrogen, the formation of N2 powering their explosions). N2 constitutes about 80% of Earth's atmosphere. The mechanisms of Earth life depend upon nitrogen, but only a few life-forms can break down N2 to use it. Some N2 is disassembled abiotically (e.g., "fixed", converted to ammonium, NH4+) by ultraviolet light, lightening, and energetic particles. Plant life extracts such fixed nitrogen from the soil, and a substantial amount is fixed artificially in the production of fertilizer, a primary function of which is to increase soil's fixed nitrogen.
Nitrogen is transparent to infrared and visible light radiation, but absorbs some ultraviolet. The spectral lines of atomic nitrogen are grouped into NI (neutral nitrogen), NII (singly ionized), NIII (doubly ionized), etc.