Nitrogen (N) is the fifth most abundant element in the solar system with the symbol “N.” On Earth, the abundance of N in the Earth’s crust is not noticeable, ranking about 30th. But N is the most abundant element in the Earth’s atmosphere, where dinitrogen (N₂) which is a diatomic molecule of N accounts for 78% of the atmospheric constituents.

N atom itself is highly reactive and can combine with almost all elements. In the environment, N mainly combines with hydrogen, oxygen, carbon, etc. to form various compounds. Due to the high reactivity, N₂, which is a combination of two N atoms, is very stable so that N compounds other than N₂ are collectively called reactive N. N₂ is only decomposed at high temperature and pressure, but some microorganisms can break the bond of N₂ and create ammonia at room temperature and pressure. This process is called biological N fixation.

N is an essential element to form amino acids and nucleobases. Proteins and DNAs are formed by assembling these molecules in meaningful ways. In other words, N is deeply involved in the basics of life: body formation, metabolisms, and expressions and inheritance of genetic information. Therefore, living things need N.