Nitrogen makes up almost four fifths of the air we breathe, but being unreactive is not used in respiration at all - we simply breathe the nitrogen back out again, unchanged. However, nitrogen is essential for the growth of most living things, and is found as a vital ingredient of proteins.
Under normal conditions nitrogen is a gas, composed of a simple diatomic molecule of formula N2. It is colourless and odourless, but unlike its partner in the atmosphere, oxygen, forms triple bonds 
. This makes it one of the strongest chemical bonds known (DHdissociation = 944.7 kJ mol-1), and this is primarily responsible for the inertness of N2. Indeed, if it were not for nitrogen's reluctance to participate in chemical reactions, life could probably not have evolved on Earth, as the concentration of oxygen would have been so high that all vegetation would have ignited at the merest spark, e.g. a lightning flash.
Once the strong triple bond is broken, however, nitrogen atoms are very reactive indeed and nitrogen forms many compounds with different bonding types. For example, nitrogen can form nitride ions, N3-, normal (s-bonds), (either single bonds as in ammonia (NH3), or double bonds as in the diazo-dyes (R-N=N-R) or triple bonds as in N2 or cyanides), or the ammonium ion, NH4+.
In order for plants and animals to utilise nitrogen, two things must happen: (a) the nitrogen gas in the atmosphere must find its way into the soil, and (b) the inert N2 must be broken down into more reactive compounds that are easier to metabolise. One mechanism for this occurs during thunderstorms, when lightning flashes provide the high energies necessary to split the N2 bond and allow it to react with oxygen to form nitrogen oxides, such as NO and NO2. These can then dissolve in the water vapour in clouds to form nitric acid and nitrous acid (HNO3 and HNO2, respectively), which then rain down onto the waiting plants, which absorb it through their root systems.
Special types of plants, however, especially legumes (like clover, peas, beans, alfalfa and acacia), do not need to rely on this complicated procedure to obtain their nitrogen. The roots of these plants are colonised by Rhizobium bacteria, which have the ability to directly convert N2 into useful nitrogen compounds via the use of a special enzyme called nitrogenase. This process is called nitrogen fixation.
 | A runner bean plant can fix nitrogen directly from N2 in the air. |
Nitrogen is such a vital ingredient to plant life, that most industrial fertilizers contain high concentrations of nitrogen compounds, such as ammonium nitrate. Animals then eat the plants and turn the nitrogen into proteins. When the animals die, bacteria break down the proteins releasing N2 back into the atmosphere to begin the cycle again.
Nitrogen is obtained by cooling air to temperatures below the boiling point of N2, 77.3K (-196°C), and separating it (fractionation) from the rest of the components of air (mainly oxygen). Most of the nitrogen produced each year is converted to ammonia, and one third of that goes to make nitric acid, for use in the fertilizer and explosives industries. N2 gas is used as an inert atmosphere in the manufacture of semiconductors and glass. Liquid nitrogen is used as a coolant.