Nitrogen

• 1. The Air We Cannot Breathe

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  Two technicians walked into a confined space inside the Space Shuttle's mobile launcher platform on the 19th of March 1981, shortly before the program's first launch. The space had been filled with pure nitrogen as a guard against fire. Both men died from asphyxiation, with few warning symptoms. The gas that killed them is the same gas that fills more than three-quarters of every breath you take, yet your body cannot warn you when oxygen disappears around it.

  Nitrogen carries the chemical symbol N and the atomic number 7. It makes up about 78% of Earth's atmosphere, the most abundant chemical species in the air. It is colourless, odourless, and tasteless. So how did a gas this ordinary earn names that mean "no life" and "to choke"? Why does half of the world's food now depend on pulling it out of thin air? And how can something so unreactive sit at the heart of explosives, fertiliser, superglue, and your own DNA? This is the story of an element defined by a bond so strong that nature itself struggles to break it.

• 2. No Life

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  Daniel Rutherford, a Scottish physician, isolated nitrogen in 1772 and called it noxious air. He noticed that this part of the air would not support combustion, and he clearly separated it from Joseph Black's "fixed air", which we now call carbon dioxide. Rutherford did not realise he had found an element. At about the same time, Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley studied the same gas, with Priestley naming it burnt air or phlogisticated air.

  Antoine Lavoisier called the gas "mephitic air" or azote, borrowing the Greek word for "no life". In an atmosphere of pure nitrogen, animals died and flames went out. English speakers rejected Lavoisier's name, pointing out that nearly every gas but oxygen is either asphyxiant or toxic. The name azote survived elsewhere, living on in French, Italian, Portuguese, Polish, Russian, and other languages. The German name Stickstoff carries the same idea, drawing on ersticken, meaning to choke or suffocate.

  The English word nitrogen entered the language in 1794, from the French nitrogène coined four years earlier by Jean-Antoine Chaptal. He built it from nitre, the old name for potassium nitrate or saltpetre, and a suffix meaning "producing". His point was that the element forms the essential part of nitric acid, which was made from nitre. This same theme of choking later named the whole chemical family. The Greek word for "to choke" gave us pnictogens, the group that nitrogen heads.

• 3. An Ancient Chemistry

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  Ammonium chloride was known to Herodotus, giving nitrogen compounds a history that stretches back to antiquity. By the Middle Ages, alchemists worked with nitric acid under the name aqua fortis, or strong water. They prized a mixture of nitric and hydrochloric acids called aqua regia, royal water, for its power to dissolve gold, which they regarded as the king of metals.

  Saltpetre supplied the earliest military, industrial, and agricultural uses of nitrogen, most famously in gunpowder and later as fertiliser. In 1910, Lord Rayleigh sent an electrical discharge through nitrogen gas and produced what he called "active nitrogen", a single-atom form of the element. His apparatus threw off a whirling cloud of brilliant yellow light, and that glowing gas reacted with mercury to form explosive mercury nitride.

  Natural sources of nitrogen compounds were once scarce, drawn only from biology or from nitrate deposits made by reactions in the air. The Frank-Caro process between 1895 and 1899 and the Haber-Bosch process between 1908 and 1913 broke that scarcity open. Today half of global food production rests on synthetic nitrogen fertilisers. The Ostwald process of 1902 turned fixed nitrogen into nitrates on an industrial scale, feeding the explosives factories of the World Wars.

• 4. The Unbreakable Bond

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  An N2 molecule is held together by a triple bond, written N is identical to N, that dominates the entire chemistry of the element. This bond has a length of just 109.76 picometres and a dissociation energy of 945.41 kilojoules per mole. It is the second strongest bond in any diatomic molecule, surpassed only by carbon monoxide. Because of it, dinitrogen sits mostly unreactive at room temperature, reacting only with lithium metal and a handful of transition metal complexes.

  Nitrogen melts at minus 210 degrees Celsius and boils at minus 196 degrees Celsius, far lower than the rest of its group. Its diatomic molecules cling together only through weak van der Waals forces, with too few electrons to build strong instantaneous dipoles. A nitrogen atom carries seven electrons and five valence electrons, three of them unpaired. Its electronegativity of 3.04 on the Pauling scale is among the highest of any element, beaten only by chlorine, oxygen, and fluorine.

  That unbreakable bond cuts two ways. It makes converting N2 into useful compounds hard for both living organisms and industry. Yet the same strength means that burning, exploding, or decomposing nitrogen compounds back into nitrogen gas releases large amounts of energy. The difficulty of fixing nitrogen and the energy stored in releasing it are two faces of a single chemical fact.

• 5. Inside Every Organism

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  The human body is about 3% nitrogen by mass, making it the fourth most abundant element in the body after oxygen, carbon, and hydrogen. Nitrogen lives in amino acids and therefore in proteins, in the nucleic acids DNA and RNA, and in adenosine triphosphate, the molecule that carries energy. The nitrogen cycle traces the element from the air into living things and their organic compounds, then back into the atmosphere.

  Nitrogen must be "fixed" into a plant-usable form, usually ammonia, before life can use it. Lightning strikes fix some nitrogen by making nitrogen oxides, but most of the work falls to diazotrophic bacteria using enzymes called nitrogenases. Plants take up the ammonia to build proteins, animals eat the plants and build their own proteins, and decomposition returns free dinitrogen to the air. The decay of animal flesh gives off its characteristic odour through long-chain amines such as putrescine and cadaverine, breakdown products of the amino acids ornithine and lysine.

  Nitrogen sits in every major class of pharmacological drug, including antibiotics. Many drugs mimic natural nitrogen-bearing signal molecules. The organic nitrates nitroglycerin and nitroprusside lower blood pressure by turning into nitric oxide in the body. Caffeine, morphine, and synthetic amphetamines all act on the receptors of animal neurotransmitters. Inside mammals, nitric oxide itself works as a regulatory molecule for circulation, a stable molecule remarkable for holding an odd number of electrons.

• 6. A Family Of Hazards

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  Pierre Louis Dulong first synthesised nitrogen trichloride in 1811, and the explosive liquid cost him three fingers and an eye. The compound is dense and volatile, yet as a dilute gas it is tamed enough to bleach and sterilise flour. Its cousins grow more dangerous down the line. Nitrogen tribromide, first prepared in 1975, explodes even at minus 100 degrees Celsius. Nitrogen triiodide, made only in 1990, is less stable still.

  Nitrogen triiodide's ammonia adduct is so shock-sensitive that the touch of a feather, a shift in air currents, or even an alpha particle can set it off. For that reason small amounts are sometimes made as a demonstration for high school chemistry students, an act of so-called chemical magic. Hydrogen azide, first produced in 1890 by oxidising hydrazine with nitrous acid, is violently explosive even in dilute solution, with a disagreeable smell and the power to poison.

  Nitrogen forms nine molecular oxides, all thermally unstable toward breaking down into their elements. Nitrous oxide, known as laughing gas, comes from heating molten ammonium nitrate to 250 degrees Celsius and once served as an anaesthetic. Dinitrogen tetroxide became the storable oxidiser of choice for many rockets in the United States and the USSR by the late 1950s, since it pairs as a hypergolic propellant with hydrazine-based fuel. Hydrazine itself, a fuming colourless liquid that smells like ammonia, is widely used as rocket fuel.

• 7. Stored Cold And Inert

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  Two-thirds of the nitrogen made by industry is sold as gas and the remaining third as liquid. The gas is produced from air by fractional distillation of liquid air or by pressure swing adsorption, sometimes supplied compressed in cylinders as oxygen-free nitrogen. Commercial-grade nitrogen holds at most 20 parts per million of oxygen, and purified grades cut that to 2 parts per million.

  Nitrogen gas works mainly as a low-reactivity safe atmosphere wherever oxygen would bring a fire or explosion risk. It preserves the freshness of packaged foods, carries the European food additive number E941, fills incandescent light bulbs as a cheaper alternative to argon, and protects information technology equipment in fire suppression systems. Its smaller bubbles give stouts and British ales a smoother, headier pour, packaged in cans through a pressure-sensitive capsule called a widget. Nitrogen tanks are also replacing carbon dioxide as the power source for paintball guns.

  Liquid nitrogen, a colourless fluid that resembles water, boils at 77 kelvin and carries 80.8 percent of water's density. Its main use is cooling to low temperatures. It preserves blood and reproductive cells, freezes off cysts and warts in cryotherapy, chills heat-sensitive infrared and X-ray detectors, and consolidates unstable soil during excavation. Solid nitrogen reaches far beyond Earth, coating Pluto and the moon Triton, where geysers of nitrogen gas erupt from the polar ice cap.

• 8. The Silent Danger

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  A young woman in England had to have her stomach removed in 2012 after drinking a cocktail made with liquid nitrogen. The liquid causes cold burns on contact, though the Leidenfrost effect shields against very brief exposure of about a second. Its quiet hazards come not from toxicity but from physics and displacement.

  Nitrogen expands from liquid to gas at a ratio of 1 to 694 at 20 degrees Celsius, which can generate tremendous force in a sealed space. On the 12th of January 2006, at Texas A&M University, the pressure-relief devices on a tank of liquid nitrogen failed and were then sealed shut. The tank ruptured catastrophically. The blast drove the tank through the ceiling, shattered a reinforced concrete beam beneath it, and pushed the laboratory walls off their foundations.

  The gas itself kills by displacing oxygen, and the human carotid body is a poor, slow sensor of low oxygen. Under high pressure below about 30 metres in scuba diving, nitrogen causes a state of mental impairment called nitrogen narcosis. Rapid decompression lets nitrogen bubble out of the blood, causing the condition once called the bends. That same asphyxiating power has been turned to deliberate use. In January 2024, Kenneth Eugene Smith became the first person executed by nitrogen asphyxiation.