1 Properties
Nitrogen is a group 15 element with atomic number 7 and symbol N. Under standard conditions it exists chiefly as dinitrogen, N₂, a very stable diatomic molecule. In the atmosphere it is the dominant gas by volume, and its low reactivity makes it useful wherever an oxygen-free or chemically quiet environment is required. Despite its inert appearance, nitrogen is central to both inorganic chemistry and the chemistry of life.
1.1 Atomic properties
An atom of nitrogen has seven protons and, in a neutral state, seven electrons. Its electron configuration is 1s² 2s² 2p³, giving it five valence electrons. This arrangement helps explain its tendency to form three covalent bonds in many compounds and its ability to complete an octet through sharing electrons. Nitrogen is a light nonmetal, and its small atomic size contributes to strong bonding in many of its molecules.
1.2 Physical properties
At ordinary temperature and pressure, nitrogen is a colorless, odorless gas. It condenses to a liquid at very low temperatures and freezes into a solid that consists of molecular N₂ units. The gas has low density compared with air and is only sparingly soluble in water. Because its molecules interact weakly, nitrogen has relatively low boiling and melting points for a substance of its atomic mass.
1.3 Chemical properties
Nitrogen is chemically distinctive because elemental dinitrogen is exceptionally stable. This stability arises from the strong triple bond between the two nitrogen atoms, which requires substantial energy to break. As a result, atmospheric nitrogen does not readily participate in ordinary combustion or oxidation reactions. In contrast, many nitrogen-containing compounds are highly reactive and are central to industrial chemistry, agriculture, and biology.
1.3.1 Reactivity of dinitrogen
The N≡N bond makes molecular nitrogen difficult to activate. Under normal conditions it reacts slowly with most substances, although it can be converted into more reactive forms at high temperatures, under pressure, or with catalysts. Industrial fixation processes exploit these conditions to produce ammonia and related compounds. In nature, certain microorganisms and energetic events such as lightning can also break the inertness of atmospheric nitrogen.
1.3.2 Common oxidation states
Nitrogen displays a wide range of oxidation states, from -3 in ammonia and many amines to +5 in nitrates. Intermediate states occur in compounds such as nitrites, nitric oxide, and nitrous oxide. This variability contributes to the rich chemistry of nitrogen and to its participation in redox processes across living systems and industrial reactions.
1.4 Spectral and isotopic data
Nitrogen has stable isotopes, primarily nitrogen-14 and nitrogen-15. The first is overwhelmingly abundant, while the second is used in tracer studies and isotopic analysis. Spectroscopically, nitrogen and nitrogen-bearing species are important in astronomy, atmospheric science, and plasma physics. Their emission and absorption features help identify chemical environments both on Earth and beyond.
2 Occurrence
Nitrogen is widespread in the atmosphere, in living organisms, and in numerous chemical compounds. Although elemental nitrogen dominates the air, most of the nitrogen in rocks, soils, waters, and organisms is found in combined forms rather than as free N₂. Its abundance and mobility make it one of the most important elements in geochemistry and biology.
2.1 Atmospheric abundance
The atmosphere is the principal reservoir of elemental nitrogen, where it occurs mainly as N₂. This abundance is the result of long-term planetary processes and the relative stability of the molecule. Because dinitrogen is only weakly reactive, it accumulates in air rather than being rapidly consumed. Smaller atmospheric amounts occur in compounds such as nitrogen oxides and ammonia.
2.2 Natural isotopes
Natural nitrogen consists primarily of two stable isotopes, nitrogen-14 and nitrogen-15. Nitrogen-14 is by far the more common of the two and dominates most terrestrial materials. Nitrogen-15 is less abundant but is valuable in geochemistry, biology, and environmental studies because isotopic ratios can reveal processes such as metabolism, decomposition, and nitrogen cycling.
2.3 Presence in minerals and compounds
In the lithosphere, nitrogen appears in mineral-bound ammonium, nitrates, and organic matter associated with sediments and soils. Many mineral deposits contain only trace amounts, but certain evaporite and nitrate-bearing formations can be rich in combined nitrogen. In aqueous environments, nitrogen is often present as dissolved ammonium, nitrate, or nitrite, depending on redox conditions.
2.4 Biological occurrence
Nitrogen is an essential element in all known forms of life. It is found in amino acids, proteins, nucleic acids, chlorophyll, enzymes, and many coenzymes and metabolites. Organisms acquire nitrogen in different forms, but most ultimately depend on biologically available fixed nitrogen such as ammonium or nitrate. The need for nitrogen helps shape ecosystems, agriculture, and nutrient limitations.
3 Discovery and history
Nitrogen was identified during the period when scientists began separating the components of air and recognizing gases as distinct substances. Its chemical importance became clearer as the study of acids, bases, explosives, and fertilizers expanded in the eighteenth and nineteenth centuries. The element’s historical development is closely tied to the growth of modern chemistry.
3.1 Early identification
The gas later known as nitrogen was first recognized as a component of air that did not support combustion or respiration. Early investigators observed that after oxygen was removed from air, a residual gas remained. This “phlogisticated air” or “mephitic air” was eventually understood as a distinct element in its elemental state, separate from oxygen and carbon dioxide.
3.2 Naming and etymology
The name nitrogen reflects its association with nitre, an old term for potassium nitrate, from which many nitrogen compounds were first obtained. The symbol N derives from the modern elemental name. In some historical contexts, the gas was also called azote, meaning “lifeless,” a reference to its inability to sustain breathing or burning under ordinary conditions.
3.3 Development of nitrogen chemistry
The rise of nitrogen chemistry followed the discovery of ammonia, nitric acid, nitrates, and amines. Chemists learned to prepare, analyze, and transform a wide range of nitrogen compounds, revealing the element’s remarkable versatility. The later development of coordination chemistry, structural organic chemistry, and physical chemistry deepened understanding of nitrogen bonding and reactivity.
3.4 Historical applications
Historically, nitrogen compounds were used in gunpowder, dyes, fertilizers, and medicines. Nitrate salts played major roles in explosives and agriculture, while ammonia became increasingly important in industrial processes. The ability to fix atmospheric nitrogen transformed food production and manufacturing, making nitrogen one of the foundation elements of the modern chemical industry.
4 Isolated forms and allotropes
Elemental nitrogen is most familiar as diatomic N₂, but the element can also exist in atomic, ionic, excited, and high-energy forms. These species are usually transient and occur under special conditions such as electrical discharges, plasmas, or extreme pressure. They are important in fundamental chemistry and in atmospheric and astrophysical studies.
4.1 Molecular nitrogen
Molecular nitrogen is the stable form found in air and in most laboratory and industrial contexts. Its two atoms are joined by a strong triple bond, which accounts for its low chemical reactivity. This form is the standard reference for elemental nitrogen and the main reservoir from which combined nitrogen is derived.
4.2 Atomic nitrogen
Atomic nitrogen consists of single nitrogen atoms and is far more reactive than N₂. It is commonly produced in plasmas, high-temperature systems, and certain photochemical processes. Because unpaired electrons make it highly active, atomic nitrogen readily forms new compounds when it encounters other atoms or molecules.
4.3 Excited states and ions
Nitrogen can occur in electronically excited states, especially in glowing gases, lightning, and auroral or plasma environments. Ions such as N₂⁺ and N⁺ are important in mass spectrometry, atmospheric chemistry, and ionized gases. These species often have distinctive spectra and play a major role in energetic nitrogen chemistry.
4.4 High-pressure and exotic forms
Under extreme pressure, nitrogen can adopt unusual structures with different bonding patterns from ordinary molecular N₂. Some forms are predicted or observed only under specialized laboratory conditions. These exotic phases are of interest because they reveal how bonding changes when matter is compressed and can store large amounts of energy in strained chemical structures.
5 Reactions and compounds
Nitrogen forms a vast family of compounds with hydrogen, oxygen, halogens, carbon, metals, and metalloids. Many of these compounds are indispensable in industry and biology, while some are highly reactive or hazardous. The diversity of nitrogen chemistry reflects the element’s ability to occupy several oxidation states and form multiple bond types.
5.1 Hydrides
Nitrogen hydrides are compounds of nitrogen and hydrogen. They range from simple molecules to complex organic derivatives and are widely used in synthesis, agriculture, and pharmaceuticals. Their behavior depends strongly on bonding, basicity, and oxidation state.
5.1.1 Ammonia
Ammonia, NH₃, is the most important nitrogen hydride. It is a colorless gas with a pungent odor, high water solubility, and basic character. Industrially, it is a major feedstock for fertilizers and numerous chemicals. Biologically, ammonia and its protonated form, ammonium, are central intermediates in nitrogen metabolism.
5.1.2 Hydrazine
Hydrazine, N₂H₄, is a colorless, highly reactive liquid used as a reducing agent, propellant, and chemical intermediate. It is more hazardous than ammonia and requires careful handling. Its chemistry includes powerful nucleophilic and reductive behavior, which makes it useful in synthesis and in some specialized industrial applications.
5.2 Oxides of nitrogen
Nitrogen and oxygen form several oxides with sharply different properties. Some are natural components of the atmosphere, while others are manufactured for chemical production or arise during combustion. These oxides are important in atmospheric chemistry, acid formation, and industrial synthesis.
5.2.1 Nitric oxide
Nitric oxide, NO, is a reactive gas that participates in atmospheric reactions and biological signaling. It oxidizes readily in air and can be converted into higher oxides of nitrogen. In biology, it serves as a signaling molecule with important regulatory functions in many organisms.
5.2.2 Nitrogen dioxide
Nitrogen dioxide, NO₂, is a brown, pungent gas and a significant atmospheric pollutant and chemical intermediate. It is involved in the formation of nitric acid and photochemical smog. The compound exists in equilibrium with dinitrogen tetroxide, especially at lower temperatures.
5.2.3 Dinitrogen monoxide
Dinitrogen monoxide, N₂O, is a colorless gas commonly known as nitrous oxide. It is used in medicine and some industrial processes and is also produced naturally by microbial activity. It is chemically more stable than many other nitrogen oxides but still participates in atmospheric reactions and has environmental significance.
5.3 Nitrogen halides
Nitrogen forms halides with fluorine, chlorine, bromine, and iodine, though many are unstable or highly reactive. Nitrogen trifluoride is a notable stable compound used in certain industrial settings. Other nitrogen halides can decompose explosively or require low temperatures for safe handling.
5.4 Nitrogen oxoacids
Nitrogen oxoacids include nitrous acid and nitric acid, along with their salts. These acids are crucial in redox chemistry and industrial manufacture. Nitric acid is one of the most important mineral acids, used in fertilizer production, metal processing, and synthesis of many nitrated compounds.
5.5 Organic nitrogen compounds
Organic nitrogen compounds include amines, amides, amino acids, nitriles, nitro compounds, and heterocyclic molecules. They form the basis of proteins, nucleic acids, many pharmaceuticals, and synthetic materials. Organic nitrogen chemistry is broad because nitrogen can bond in multiple ways to carbon frameworks and participate in acid-base and redox reactions.
6 Production
Industrial nitrogen is produced mainly by physical separation of air, taking advantage of the composition of the atmosphere and the low boiling point of nitrogen. Smaller quantities are also obtained in laboratories by chemical methods. The choice of method depends on purity requirements, scale, and intended use.
6.1 Fractional distillation of liquid air
The most common large-scale method is fractional distillation of liquefied air. Air is compressed, cooled, and separated into components according to their boiling points. Nitrogen, having a lower boiling point than oxygen, is collected as a purified gas or liquid. This process supplies nitrogen for food, chemical, and cryogenic uses.
6.2 Pressure swing adsorption
Pressure swing adsorption separates nitrogen from air by using materials that preferentially adsorb oxygen, water vapor, and carbon dioxide. By cycling pressure, the adsorbent can be regenerated repeatedly. This technique is widely used where moderate purity and on-site generation are desirable.
6.3 Membrane separation
Membrane systems allow nitrogen-enriched gas to pass through selective barriers at different rates from other air components. They are compact, scalable, and practical for continuous industrial supply. Membrane separation is often used where very high purity is unnecessary or where equipment simplicity is valued.
6.4 Laboratory preparation
In laboratory settings, nitrogen may be generated by decomposing suitable nitrogen compounds or by removing reactive constituents from air. Such methods are usually used for demonstrations, specialized syntheses, or small-scale work. Laboratory nitrogen is often purified further to remove oxygen, moisture, and trace impurities.
7 Industrial and laboratory uses
Nitrogen is valued for its inertness, availability, and versatility. It is used both as a bulk commodity and as a controlled atmosphere in specialized processes. Many applications rely on either elemental nitrogen or nitrogen-containing compounds produced from it.
7.1 Inert atmospheres
Because it is relatively unreactive, nitrogen is widely used to displace oxygen and moisture from containers, reactors, pipelines, and storage tanks. This helps prevent oxidation, unwanted combustion, and degradation of sensitive materials. In laboratories, nitrogen blankets protect air-sensitive reagents and samples.
7.2 Chemical synthesis
Nitrogen is an essential feedstock for ammonia production, which in turn supports fertilizer manufacture and the synthesis of numerous nitrogen compounds. It is also used indirectly in the production of amines, nitriles, polymers, and pharmaceuticals. Many industrial chemical routes depend on nitrogen fixation or nitrogen transfer reactions.
7.3 Food packaging
Nitrogen is commonly used to flush food packages and preserve freshness. Replacing oxygen with nitrogen can slow oxidation, reduce rancidity, and help maintain texture in packaged products. It is especially useful for snacks, coffee, and other foods that benefit from a low-oxygen environment.
7.4 Electronics and metallurgy
In electronics manufacturing, nitrogen provides a clean atmosphere for soldering, semiconductor processing, and storage of sensitive components. In metallurgy, it is used to limit oxidation during heat treatment or to support controlled nitriding processes. Its role is often to stabilize conditions rather than to react directly.
7.5 Cryogenic applications
Liquid nitrogen is used for cooling, freezing, and rapid preservation. Its very low boiling point makes it suitable for cryopreservation, shrink-fitting, and many laboratory procedures. It is also employed in demonstrations and in applications requiring extreme cold, such as certain analytical or materials-processing techniques.
8 Biological role
Nitrogen is indispensable for life because it is built into the molecules that store information, catalyze reactions, and form cellular structures. Although the atmosphere contains abundant nitrogen, most organisms cannot use N₂ directly. Biological systems therefore depend on processes that convert it into accessible forms.
8.1 Nitrogen in biomolecules
Amino acids, proteins, nucleic acids, and many cofactors contain nitrogen. The element is responsible for key chemical properties of these biomolecules, including bonding patterns, charge behavior, and catalytic activity. Without nitrogen, the molecular machinery of cells could not function in its known form.
8.2 Nitrogen fixation
Nitrogen fixation is the conversion of atmospheric N₂ into biologically usable compounds such as ammonia. It is carried out by certain microbes and, on a much smaller scale, by natural physical processes and industrial catalysts. This step is essential because it introduces atmospheric nitrogen into living systems.
8.3 Nitrification and denitrification
Nitrification is the microbial oxidation of ammonium to nitrite and nitrate, while denitrification is the reduction of nitrate back toward gaseous nitrogen forms. These processes connect soil, water, and atmosphere in a dynamic chemical network. They are important in agriculture, ecology, and environmental management.
8.4 Nutrient cycling in ecosystems
Nitrogen moves through ecosystems in a continuous cycle involving microbes, plants, animals, decomposers, soils, and water. Availability of fixed nitrogen often limits growth and productivity. The balance between inputs, transformations, and losses strongly influences ecosystem structure and function.
9 Nitrogen cycle
The nitrogen cycle describes the movement of nitrogen among the atmosphere, biosphere, hydrosphere, and geosphere. It includes both biological and nonbiological transformations. Because nitrogen can exist in many chemical forms, it passes through numerous interconnected pathways.
9.1 Atmospheric nitrogen
Atmospheric N₂ forms the largest reservoir in the cycle. Most organisms cannot directly assimilate it, so it must first be fixed into more reactive compounds. Atmospheric reactions, microbial activity, and industrial fixation all contribute to converting nitrogen into accessible forms.
9.2 Soil nitrogen transformations
Soils contain a mix of organic nitrogen, ammonium, nitrite, nitrate, and gaseous forms. Microorganisms mediate mineralization, nitrification, immobilization, and denitrification. These reactions determine whether nitrogen remains available to plants or is returned to the atmosphere.
9.3 Aquatic nitrogen cycling
In rivers, lakes, estuaries, and oceans, nitrogen cycles between dissolved inorganic forms and organic matter. Uptake by algae and other organisms is balanced by decomposition and microbial conversion. Water chemistry, oxygen levels, and biological productivity all influence the dominant nitrogen species present.
9.4 Human influence on the cycle
Human activity has altered the nitrogen cycle through fertilizer production, combustion, cultivation, and waste generation. These inputs increase the abundance of reactive nitrogen in soils and waters and change natural fluxes. The result is a cycle that now includes substantial industrial and agricultural contributions.
10 Safety and environmental considerations
Although elemental nitrogen is generally nonhazardous in chemical terms, it can create serious risks in enclosed spaces or under certain handling conditions. Many nitrogen compounds are more reactive or toxic than the element itself. Careful control is important in laboratories, industry, and cryogenic work.
10.1 Asphyxiation hazard
Nitrogen can displace oxygen in confined areas without warning because it is colorless and odorless. This makes leaks from tanks, lines, or liquid nitrogen systems potentially dangerous. The main hazard is not toxicity but oxygen deprivation, which can occur rapidly in poorly ventilated spaces.
10.2 High-pressure gas handling
Compressed nitrogen cylinders and pipelines must be handled with standard gas safety practices. Sudden release can cause mechanical injury, frost damage, or equipment failure. Liquid nitrogen also presents risks from extreme cold and rapid expansion during warming.
10.3 Toxic nitrogen compounds
Several nitrogen compounds are hazardous, including nitrogen oxides, hydrazine, and certain nitrated organics. Some are corrosive, poisonous, or highly reactive. Exposure limits and protective measures are required when these substances are produced, stored, or used.
10.4 Environmental effects of nitrogen compounds
Reactive nitrogen compounds can affect air quality, water quality, and ecosystems. Their presence may contribute to acid formation, nutrient imbalance, and changes in biological communities. Environmental management often focuses on reducing unwanted releases and improving nitrogen use efficiency.
11 Analytical methods
Nitrogen and nitrogen-containing substances are analyzed using a range of chemical, spectroscopic, and isotopic methods. The choice of technique depends on the sample type and the information sought. Accurate analysis is important in atmospheric monitoring, materials science, biology, and industry.
11.1 Detection and quantification
Nitrogen can be detected by measuring gas composition, chemical reactivity, or the concentration of nitrogen-containing species in a sample. Common approaches include chromatography, titration, and gas analysis. In many applications, detection focuses on specific compounds rather than elemental nitrogen itself.
11.2 Elemental analysis
Elemental analysis determines the total nitrogen content of a material, especially in soils, foods, and organic compounds. Methods may involve combustion, conversion to measurable gases, or chemical digestion followed by quantification. These techniques are widely used in agriculture, environmental science, and quality control.
11.3 Spectroscopic techniques
Spectroscopy is used to study nitrogen molecules, radicals, and compounds across gas, liquid, and solid phases. Infrared, ultraviolet-visible, Raman, and emission methods can reveal bonding and molecular structure. In some contexts, spectroscopic signatures are also used to monitor atmospheric or plasma processes.
11.4 Isotopic analysis
Isotopic measurements distinguish nitrogen-14 from nitrogen-15 and can reveal sources and pathways of nitrogen in natural systems. Mass spectrometry is a common tool for this purpose. Isotopic data are useful in ecology, geochemistry, and forensic investigations involving food, soil, or water.
12 Standards and data
Nitrogen is described in scientific references by a set of identifiers, physical constants, and thermodynamic values. These data support laboratory work, industrial design, and database indexing. Standardized information helps ensure consistency across chemical literature and technical applications.
12.1 Chemical identifiers
Nitrogen is identified by its symbol N and atomic number 7. In databases and technical references, it is associated with standardized names, registry entries, and molecular identifiers for N₂ and nitrogen-bearing compounds. These identifiers allow precise communication across scientific fields.
12.2 Thermodynamic data
Thermodynamic tables for nitrogen include values such as enthalpy, entropy, heat capacity, and phase-transition temperatures. Such data are important for process engineering, cryogenics, and reaction modeling. They also support calculations involving gas mixtures and equilibrium systems.
12.3 Physical constants
Key constants include atomic mass, boiling and melting points of elemental nitrogen, and the properties of liquid and solid nitrogen. Gas density, diffusion behavior, and solubility are also routinely tabulated. These constants are essential in laboratory design and industrial operations.
12.4 Reference materials
Reference materials for nitrogen analysis include calibration gases, isotopic standards, and certified compounds for elemental determination. They are used to validate instruments and ensure accurate measurements. In analytical chemistry, well-characterized reference samples improve comparability between laboratories.