Nitrogen

Nitrogen sits at position seven on the periodic table, symbolized by N. As a nonmetal, it is the lightest element in group 15, the group known as the pnictogens. Across the Milky Way and our own Solar System, it ranks as the seventh most abundant element overall. Under standard conditions, two nitrogen atoms link up into a colorless, odorless diatomic gas called N₂. This gas makes up roughly 78 percent of Earth’s atmosphere, more than any other chemical in the air.

History

Because its compounds tend to be volatile, nitrogen is fairly scarce in the planet’s solid crust. Scottish physician Daniel Rutherford first isolated nitrogen in 1772, calling it “noxious air.” Around the same time, Carl Wilhelm Scheele and Henry Cavendish independently studied it. In 1790, French chemist Jean-Antoine-Claude Chaptal proposed the name “nitrogène,” since the element turned up in nitric acid and nitrates. Antoine Lavoisier preferred “azote,” from the Greek for “no life,” because the gas suffocates living things.

That name stuck in many languages and appears in English terms like hydrazine, azides, and azo compounds. Most commercial nitrogen gas comes from air, using pressure swing adsorption. About two-thirds of that production serves as an inert, oxygen-free gas for uses like food packaging; much of the remainder becomes liquid nitrogen for cryogenics. Nitrogen is central to many industrial compounds: ammonia, nitric acid, organic nitrates (used in propellants and explosives), and cyanides.

The N₂ molecule’s triple bond is extremely strong—second only to carbon monoxide among diatomic molecules—which makes converting it into useful compounds difficult for both organisms and industry. Yet that same bond means that burning, exploding, or breaking down nitrogen compounds releases large amounts of energy. Synthetic ammonia and nitrates are key industrial fertilizers, and fertilizer nitrates are major pollutants that cause eutrophication in waterways. Beyond fertilizers and energy storage, nitrogen appears in organic compounds ranging from aramids (used in high-strength fabrics) to cyanoacrylate (the basis of superglue).

Organic nitrogen compounds

Every living organism contains nitrogen, mainly in amino acids (and thus proteins), nucleic acids (DNA and RNA), and the energy molecule adenosine triphosphate. The human body is about 3 percent nitrogen by mass, making it the fourth most abundant element after oxygen, carbon, and hydrogen. The nitrogen cycle describes how the element moves from the air into the biosphere and organic compounds, then back to the atmosphere.

Nitrogen is a component of every major class of pharmaceutical drugs, including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signaling molecules: for instance, the organic nitrates nitroglycerin and nitroprusside regulate blood pressure by metabolizing into nitric oxide. Other notable nitrogen-containing drugs—such as natural caffeine and morphine, or synthetic amphetamines—act on animal neurotransmitter receptors. Nitrogen compounds have been known for millennia.

Ammonium chloride was familiar to Herodotus, and by the Middle Ages, alchemists were well acquainted with nitric acid (which they called aqua fortis, or “strong water”), as well as ammonium and nitrate salts. The mixture of nitric and hydrochloric acids was known as aqua regia (“royal water”), prized for its ability to dissolve gold. The element itself was discovered in 1772 by Scottish physician Daniel Rutherford, who called it “noxious air.” Although he did not recognize it as a distinct chemical element, he clearly distinguished it from Joseph Black’s “fixed air” (carbon dioxide). Rutherford understood that air contained a component that did not support combustion, even if he did not realize it was an element.

Around the same time, Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley also studied it, referring to it as “burnt air” or “phlogisticated air.” Antoine Lavoisier called it “mephitic air” or “azote,” from the Greek word for “no life,” because it suffocates. In pure nitrogen, animals died and flames went out. Though English did not adopt Lavoisier’s name—since all gases except oxygen are either asphyxiant or toxic—it persists in many languages (French, Italian, Portuguese, Polish, Russian, Albanian, Turkish, and others; German Stickstoff and Dutch stikstof similarly mean “choking substance”). It also survives in English names for nitrogen compounds like hydrazine and azides, and it inspired the term “pnictogens” for the group headed by nitrogen, from the Greek for “to choke.” The English word “nitrogen” (1794) came from the French nitrogène, coined in 1790 by Jean-Antoine Chaptal.

He formed it from nitre (potassium nitrate, also called saltpetre) and the suffix -gène (“producing”), from the Greek -genes (“begotten”). Chaptal meant that nitrogen is the essential part of nitric acid, which itself came from nitre. Earlier, nitre had been confused with Egyptian “natron” (sodium carbonate), called νίτρον (nitron) in Greek, which despite the name contained no nitrate. The earliest military, industrial, and agricultural uses of nitrogen compounds relied on saltpetre (sodium or potassium nitrate), most notably in gunpowder.