Nitrogen (N)

The nitrogen molecule is held together by a triple bond that takes about 945 kilojoules per mole to break — among the strongest bonds in any common molecule, and nearly twice the strength of the double bond in oxygen. That single number explains two apparently opposite facts about the element: why the atmosphere is chemically boring, and why nitrogen compounds blow things up.

The same bond makes air safe and explosives possible

Because N≡N is so hard to break, nitrogen at room temperature reacts with almost nothing. This is convenient. Air is roughly four fifths nitrogen, and if that fraction were as reactive as the oxygen fraction, nothing organic would last a season.

Turn the argument around and the other consequence appears. If breaking the bond costs a great deal of energy, then forming it releases a great deal. An explosive is essentially a molecule that holds nitrogen atoms in a strained, high-energy arrangement and can rearrange them into N₂ — which is not only energetically far downhill but also a gas, expanding violently from a solid starting volume. Nitroglycerin, TNT and ammonium nitrate are chemically unlike one another and all work on that principle. The stability of the nitrogen molecule is the payload.

Lifeless air, and a name that lost in English only

Daniel Rutherford, a medical student in Edinburgh working under Joseph Black, described the gas in 1772 as the residue left when a flame had burned out in a closed vessel and the carbon dioxide had been absorbed. He called it noxious air, and he published first. Carl Wilhelm Scheele, Henry Cavendish and Joseph Priestley all had it independently within a couple of years, which makes the conventional credit a matter of publication date rather than of insight.

Lavoisier named it azote — without life — because a mouse would not survive in it. Jean-Antoine Chaptal proposed nitrogène in 1790 instead, meaning the generator of nitre, since the gas is a constituent of saltpetre. English adopted Chaptal's word; French, Russian and several other languages kept Lavoisier's. The symbol N works for both, which is presumably why nobody ever bothered to settle the argument.

Half the nitrogen in your body passed through a steel vessel

Nitrogen is the element that limits plant growth almost everywhere, and until the twentieth century the only large-scale sources of fixed nitrogen were manure, legume rotation, and imported Chilean saltpetre and Peruvian guano. Those supplies were visibly inadequate to a growing population; the chemist William Crookes said so publicly in 1898, and framed the problem as a race against famine.

Fritz Haber demonstrated the synthesis of ammonia from atmospheric nitrogen and hydrogen at bench scale in 1909, at a few hundred degrees and around two hundred atmospheres. Carl Bosch and BASF turned it into an industrial process, which required inventing high-pressure chemical engineering more or less from nothing, and Alwin Mittasch's group screened something on the order of twenty thousand catalyst formulations before settling on promoted iron. The first plant opened at Oppau in 1913.

The process now consumes roughly one to two per cent of the world's energy supply. Vaclav Smil's estimate, widely quoted and not seriously disputed, is that around half the nitrogen atoms in an average human body have passed through a Haber-Bosch reactor, and that several billion people are alive who could not be fed without it. Both men received Nobel prizes — Haber in 1918, in an award that was contested at the time and has been ever since, given what else he had spent the war years doing.

Bacteria do the same job in the cold

Nitrogenase, the enzyme that fixes nitrogen biologically, performs at ambient temperature and pressure the reaction that industry needs several hundred degrees and two hundred atmospheres to force. It manages this with a metal cluster at its active site — most commonly an iron-molybdenum cofactor — and it is not free: the reaction consumes something like sixteen molecules of ATP for every nitrogen molecule reduced.

The enzyme has one severe constraint. Oxygen destroys it, irreversibly, so every organism that fixes nitrogen must keep the process anaerobic while remaining alive in an oxygenated world. The solutions are ingenious and various. Some cyanobacteria build dedicated thick-walled cells called heterocysts that shut down photosynthesis internally. Legumes, in partnership with rhizobia bacteria, manufacture leghaemoglobin — an oxygen-binding protein closely related to the one in blood — to buffer oxygen down to a workable trickle inside the root nodule. It is what makes a freshly split nodule pink, and a pink nodule is the field test for whether the symbiosis is actually working.

The most beautiful experiment in biology needed a heavy isotope

Nitrogen has two stable isotopes, and nitrogen-15 makes up only about 0.36% of the natural mixture. That scarcity is what makes it useful: a molecule built from enriched nitrogen-15 is measurably heavier than its ordinary counterpart and can be followed through a system.

Matthew Meselson and Franklin Stahl exploited exactly this in 1958. They grew E. coli for many generations on nitrogen-15 so that its DNA was uniformly heavy, switched the culture to ordinary nitrogen-14, and spun samples in a cesium chloride density gradient after each round of replication. After one generation all the DNA sat at an intermediate density; after two, it split between intermediate and light bands. That pattern is what semiconservative replication predicts and what the two competing models did not. The experiment has been called the most beautiful in biology, and it hinged entirely on a mass difference of one neutron.

Nitrogen-15 ratios also serve as a trophic marker in ecology, because tissue is enriched in the heavier isotope relative to diet at each step up a food chain — which is how the diet of an extinct animal or an archaeological population can be read off its bone collagen.

Too much of a good thing, in the water and in the sky

Industrial fixation roughly doubled the amount of reactive nitrogen circulating through the biosphere, and only a fraction of what is applied to a field is taken up by the crop. The rest leaves — as nitrate into groundwater and rivers, as ammonia into the air, as nitrous oxide from soil microbes.

Downstream, that surplus fertilises algae, which bloom, die and are decomposed by bacteria that strip the oxygen out of the water. The resulting hypoxic zone in the northern Gulf of Mexico, fed by the Mississippi drainage, has covered many thousands of square kilometres in bad years.

The atmospheric leak is a distinct problem. Nitrous oxide is a greenhouse gas with a global warming potential of roughly 270 times carbon dioxide over a century, and it is long-lived enough to reach the stratosphere. With chlorofluorocarbons in decline, nitrous oxide is now the largest ongoing human emission that depletes stratospheric ozone. It is a byproduct of agriculture rather than of any industry that could simply be switched off.

Cheap because oxygen is worth more

Nearly all commercial nitrogen comes from the fractional distillation of liquefied air. The economics are back to front: nitrogen is the majority component but often the less valuable one, so at many plants it is effectively a co-product of oxygen production, and liquid nitrogen ends up the cheapest cryogenic liquid available by a wide margin.

Its industrial role follows from the inertness rather than from any reaction it takes part in. Gaseous nitrogen blankets fuel tanks and chemical reactors to keep oxygen out, purges pipelines, and fills food packaging to slow the oxidation of fats. The liquid is used to freeze rapidly enough that ice crystals stay small, which is a food-processing consideration before it is anything else.