Nitrogen
Nitrogen is a chemical element with symbol N and atomic number 7. It is the lightest pnictogen and at room temperature, it is a transparent, odorless diatomic gas. Nitrogen is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System.
Identity
- Atomic mass
- [14.00643, 14.00728] u
- Category
- Reactive non-metals
- Group
- 15
- Period
- 2
- Block
- p
- Oxidation States
- −3, +1, +2, +3, +4, +5
- Phase (25 °C)
- Gas
- Appearance
- colorless gas, liquid or solid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p3
- Short configuration
- [He] 2s2 2p3
- Electrons per shell
- 2, 5
- Electronegativity (Pauling)
- 3.04
- Electron affinity
- -6.80 kJ/mol
- 1st ionization energy
- 1.40e+3 kJ/mol
- Ionization energies
- 1402 · 2856 · 4578 · 7475 · 9445… kJ/mol
Physical properties
- Melting point
- -210.0 °C (63 K)
- Boiling point
- -195.8 °C (77 K)
- Density
- 0.0012506 g/cm³
- Molar heat capacity
- —
Chemical behavior of nitrogen
Nitrogen has five valence electrons ([He] 2s2 2p3) and high electronegativity (3.04), so it acts almost always as -3 in covalent bonds with nonmetals and hydrogen. Its defining trait is the N triple N bond of the N2 molecule, at 941 kJ/mol: so stable that dinitrogen is inert at room temperature. With oxygen it shows all positive states, from +1 to +5, in a family of oxides and oxoacids; with electropositive metals it forms nitrides (-3).
Reactivity
N₂ + 3 H₂ → 2 NH₃
It only reacts with hydrogen at 400-500 C and 150-300 atm with an iron catalyst (Haber-Bosch). Without those extreme conditions the triple bond stays intact: the industrial fixation of nitrogen.
N₂ + O₂ → 2 NO
With oxygen it only reacts at lightning or engine temperatures (thousands of degrees), forming nitrogen monoxide. It is the natural and human origin of atmospheric NOx.
3 Mg + N₂ → Mg₃N₂
With metals such as magnesium or lithium it forms nitrides on heating, with nitrogen in the -3 state. These nitrides release ammonia on contact with water.
NH₃ + HNO₃ → NH₄NO₃
Ammonia, a weak base, combines with nitric acid giving ammonium nitrate, the most used nitrogen fertilizer and, at once, an industrial explosive.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Ammonia | NH₃ | Nitrogen fertilizers; refrigerant and nitric acid precursor |
| Nitric acid | HNO₃ | Fertilizers, explosives, metal and circuit etching |
| Ammonium nitrate | NH₄NO₃ | Leading fertilizer and mining explosive |
| Nitrous oxide | N₂O | Anaesthetic (laughing gas); propellant and greenhouse gas |
| Hydrazine | N₂H₄ | Rocket and satellite propellant; boiler reducer |
Electron shell distribution
View in the atom builderIsotopes
Natural nitrogen combines nitrogen-14 (14N), with seven neutrons at 99.6%, and nitrogen-15 (15N), with eight neutrons at 0.4%. Stable 15N is used as a tracer in agriculture, ecology and medicine to follow nitrogen through soils, plants and proteins.
Occurrence and production
It forms 78% of air as N2, but that reservoir is inert and life depends on its fixation. Fixing bacteria and lightning convert it into usable forms; large deposits are Chilean saltpetre and cave nitrates. Industrially it is obtained by fractional distillation of liquid air and fixed by Haber-Bosch (about 180 million tonnes of ammonia yearly).
History and origin of the name
Daniel Rutherford isolated it in 1772 as the noxious air that does not sustain combustion, and Lavoisier called it azote (lifeless). Chaptal renamed it nitrogen, generator of saltpetre. Cavendish and Priestley studied its oxides, and in 1909 Fritz Haber and Carl Bosch synthesized ammonia from air, with Nobel prizes in 1918 and 1931: the process feeding half the planet.
Applications and relevance of nitrogen
Uses of the element
As N2 it is an inert atmosphere: it preserves packaged foods, shields welding and electronics, pressurizes cables and serves as liquid refrigerant (-196 C) in cryogenics, laboratories and dermatology. Ammonia synthesis is its largest chemical use.
Uses of its compounds
Nitric acid and nitrates feed fertilizers and explosives; nitrous oxide anaesthetizes; hydrazine propels rockets and satellites.
Health, safety and environment
Biological role
Nitrogen is essential: proteins, DNA and chlorophyll all contain it. Animals obtain it from proteins; plants need fixed forms such as nitrate or ammonium.
Safety
N2 is harmless in air but lethal in confined spaces when it displaces oxygen without warning. Liquid nitrogen burns by cold; nitrates are oxidizers and ammonium nitrate can detonate.
Environment
Fertilizer runoff eutrophies rivers and seas and emits N2O, a greenhouse gas 265 times stronger than CO2. Traffic NOx causes acid rain and ground-level ozone; matching farm doses is key.
Frequently asked questions about Nitrogen
If air is 78% nitrogen, why do we not use it when breathing?
Because N2 is almost inert: its triple bond, at 941 kJ/mol, is among the strongest in chemistry and our lungs cannot break it. We inhale and exhale it unchanged; it only dilutes oxygen. Only some bacteria with nitrogenase, and industry with Haber-Bosch at 500 C and 200 atmospheres, manage to fix it into useful forms.
Is the doctors' nitrous oxide the same as in tuned cars?
The same molecule, N2O, with opposite uses: in medicine it is an anaesthetic (laughing gas) since 1844; in engines it supplies extra oxygen to burn more fuel and gain power. As a recreational drug it destroys vitamin B12 and damages nerves. And as a greenhouse gas it is 265 times stronger than CO2.
What is eutrophication?
The drowning of rivers and coasts by excess nutrients: fertilizer nitrates trigger algal blooms, which die and decompose consuming all oxygen. Fish die and dead zones are born, like the Gulf of Mexico's. It is the price of nitrogen escaping fields: matching fertilizer doses and dates avoids it.
What was the saltpetre battle about?
The War of the Pacific (1879-1884), in which Chile seized the saltpetre provinces of Atacama from Peru and Bolivia. Saltpetre was then the only nitrogen fertilizer and the raw material of gunpowder. Haber-Bosch ammonia synthesis (1909) ruined the business: air nitrogen replaced mine nitrogen.
How can the same element be fertilizer and explosive?
Because nitrogen compounds store much energy in their N-O and N-N bonds. Ammonium nitrate feeds plants, but heated or contaminated it decomposes at once releasing hot gases: from fertilizer to bomb. Dynamite and TNT are the same story. The difference between nourishing and detonating is the speed of that release.
Why do legumes need no nitrogen fertilizer?
Because they host Rhizobium bacteria in root nodules that fix air N2 with nitrogenase, gifting them ammonium for sugars. This symbiosis supplies free what other crops demand as industrial fertilizer. Rotating cereals with legumes is the oldest, most ecological fertilization: it enriches soil with nitrogen for free.
Is liquid nitrogen dangerous?
Yes, for three reasons: cold burns on contact (-196 C), embrittlement of materials and, above all, evaporation displacing oxygen and asphyxiating without warning in labs and basements. With ventilation, cryogenic gloves and goggles it is safe; it is the same nitrogen as in air, only very cold and concentrated.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.