Nitrogen (N) is a fundamental element that exists in a variety of forms, driving processes from the atmosphere’s composition to the basic chemistry of life. It is the most abundant gas in Earth’s atmosphere, making up approximately 78% of the air we breathe. In this gaseous form, it is remarkably unreactive, but it transitions to highly reactive compounds indispensable to global food production, chemical manufacturing, and biological systems.
The Elemental Form and Physical State
In its most common state, nitrogen exists as dinitrogen gas (\(\text{N}_2\)), a colorless, odorless, and non-metallic substance. This diatomic molecule is characterized by a strong triple covalent bond. The high energy required to break this triple bond makes atmospheric nitrogen chemically inert under normal conditions, effectively diluting the more reactive oxygen in the air.
When cooled to extremely low temperatures, nitrogen transforms into liquid nitrogen (\(\text{LN}_2\)). This cryogenic fluid has a boiling point of approximately \(-196^\circ \text{C}\) (\(-320^\circ \text{F}\)). Liquid nitrogen is produced industrially through the fractional distillation of liquid air. Its primary function is as a powerful coolant, capable of rapidly extracting thermal energy from any material it contacts.
Nitrogen in the Biological World
Despite its atmospheric abundance, nitrogen gas cannot be directly used by most complex life forms. Organisms require nitrogen to build proteins and nucleic acids, and must obtain it in a “fixed,” or chemically combined, form. Nitrogen atoms are required for all amino acids, the building blocks of proteins, and are integral to the structure of nucleobases that form DNA and RNA.
The conversion of inert atmospheric nitrogen into biologically usable compounds is managed by the natural process of nitrogen fixation. Specialized microorganisms, known as diazotrophs, possess the nitrogenase enzyme complex necessary to break the \(\text{N}_2\) triple bond. These microbes, which include bacteria like Rhizobia that form symbiotic relationships within the root nodules of leguminous plants, convert \(\text{N}_2\) into ammonia (\(\text{NH}_3\)).
Once ammonia or its ionic form, ammonium (\(\text{NH}_4^+\)), is produced, it can be converted into nitrates (\(\text{NO}_3^-\)) and absorbed by plants through the soil in a process called assimilation. This fixed nitrogen then enters the food web when animals consume the plants. The entire series of conversions, which also includes the return of nitrogen to the atmosphere by denitrifying bacteria, is known as the nitrogen cycle, a foundational loop that sustains all terrestrial and aquatic ecosystems. The ability of microorganisms to perform this fixation is what makes atmospheric nitrogen accessible to all other living organisms.
Nitrogen as an Industrial and Chemical Agent
Human ingenuity bypassed natural nitrogen fixation through the development of the Haber-Bosch process in the early 20th century. This industrial method combines atmospheric nitrogen with hydrogen gas under pressure and high temperatures, using an iron catalyst, to synthesize ammonia (\(\text{NH}_3\)). The resulting ammonia is the chemical foundation for nearly all synthetic fertilizers, a development that dramatically increased global crop yields and supported the rise in the human population.
Beyond its role in agriculture, nitrogen gas is widely used as an inert agent due to its non-reactive nature. This application, often called “blanketing,” involves displacing oxygen to prevent unwanted reactions like oxidation or combustion. Nitrogen gas is pumped into the headspaces of fuel tanks, chemical storage vessels, and electronic component manufacturing environments to maintain an oxygen-free atmosphere.
Liquid nitrogen is employed in high-technology and commercial applications that capitalize on its extremely low temperature. In cryogenics, \(\text{LN}_2\) is used for the rapid freezing of food products to preserve texture and flavor. It is also used for the cryopreservation of biological materials, such as blood, reproductive cells, and tissue samples, allowing for their long-term storage.
Specific Inorganic Compounds and Environmental Roles
When nitrogen combines with oxygen in different ratios, it forms a range of inorganic compounds with distinct environmental and health consequences. Nitrous oxide (\(\text{N}_2\text{O}\)), known as “laughing gas,” is a potent greenhouse gas released primarily from agricultural soils. It has a warming potential nearly 300 times greater than carbon dioxide and contributes to the depletion of the stratospheric ozone layer.
Other important nitrogen-oxygen compounds are nitrates (\(\text{NO}_3^-\)) and nitrites (\(\text{NO}_2^-\)), which are highly soluble in water. While nitrates are the preferred nitrogen source for plants, runoff from heavily fertilized agricultural fields is a major environmental pollutant. Excessive nitrate concentrations in waterways can lead to eutrophication, causing dense algal blooms that deplete oxygen and create “dead zones” where aquatic life cannot survive.
Nitrates and nitrites are also commonly used in the food industry as curing and preserving agents, particularly in processed meats. They are added to inhibit the growth of bacteria, such as Clostridium botulinum, and to help maintain the characteristic pink color of cured products. However, nitrites can react with amino acids to form nitrosamines, compounds that have been linked to health concerns.