Boron (B)

When a nuclear reactor has to be stopped, the substance that stops it is almost always boron. Boron-10 absorbs slow neutrons with a cross-section of about 3,840 barns, a figure exceeded by only a handful of nuclides, and it does so in a particularly convenient way: it captures the neutron and immediately breaks into lithium and an alpha particle, both of which stop within micrometres. Nothing long-lived is created and no further neutrons are released. It is as clean a neutron sink as nuclear engineering has.

How a reactor is switched off

The applications follow directly. Control rods are typically boron carbide. Pressurised water reactors dissolve boric acid in the primary coolant and adjust its concentration slowly over the fuel cycle — chemical shim — so the control rods can stay mostly withdrawn. Spent fuel storage racks are made of borated steel or aluminum to keep the assemblies from reaching criticality while sitting in a pool.

It is also the emergency measure of last resort. Helicopter crews dropped several thousand tonnes of boron carbide and other materials into the burning reactor at Chernobyl. At Fukushima Daiichi, boric acid was mixed into the seawater pumped into the damaged cores. In both cases the reasoning was the same: whatever else is going wrong, adding boron-10 makes it less likely that fission will restart.

The same property has been turned into a cancer treatment. Boron neutron capture therapy delivers a boron-containing compound that concentrates preferentially in tumour cells, then irradiates the region with low-energy neutrons that pass through tissue almost harmlessly on their own. Only where boron has accumulated does anything happen, and the alpha particle released travels roughly the diameter of one cell. Japan approved the technique for recurrent head and neck cancers in 2020, after decades in which it was a permanently promising idea.

A shape that will not tile space

Elemental boron is built from icosahedra — twelve atoms at the vertices of a twenty-faced solid. That choice creates a problem, because an icosahedron has five-fold symmetry and five-fold symmetry cannot fill space periodically. Boron is geometrically frustrated with itself.

The result is a set of allotropes of extraordinary complexity, with unit cells containing dozens or hundreds of atoms, partial site occupancies, and structures that took decades to determine. Which one is genuinely the ground state at ordinary conditions was disputed into the twenty-first century. A high-pressure phase designated γ-B28, characterised by Artem Oganov's group in 2009, turned out to consist of B₁₂ icosahedra and B₂ pairs with measurable charge transfer between them — which makes it, oddly, a partially ionic compound of an element with itself.

Boron carbide inherits the icosahedral architecture and is one of the hardest materials known, behind only diamond and cubic boron nitride. It is the ceramic in most rifle-rated body armour, chosen because it is both extremely hard and very light.

Not enough electrons to go round

Boron has three valence electrons and four orbitals available to bond with, which means it cannot complete an octet by forming three ordinary bonds. Everything unusual about boron chemistry descends from that shortfall.

The simple consequence is that boron compounds are avid electron acceptors. Boron trifluoride is the textbook Lewis acid, and boric acid works as an acid not by donating a proton but by grabbing a hydroxide ion out of water.

The interesting consequence is what boron does when it has no electron donor available. Diborane, B₂H₆, has too few electrons to form the eight two-electron bonds its formula appears to demand, and for two decades chemists proposed structures analogous to ethane that simply did not fit the evidence. The resolution was that two hydrogen atoms bridge the boron atoms in three-centre, two-electron bonds — a single pair of electrons holding three atoms together. William Lipscomb worked out the structures and the bonding rules of the boranes and received the 1976 chemistry Nobel for it, and the concept has since turned up across organometallic chemistry.

Twenty mules, and then Turkey

Borax was hauled out of Death Valley in the 1880s by wagon trains pulled by teams of eighteen mules and two horses, over a hundred and sixty-five miles of desert to the railhead at Mojave. The Harmony Borax Works operated for only about five years. The 20 Mule Team brand attached to the product outlasted the mining by more than a century and is still on supermarket shelves.

Modern supply is concentrated to a degree few commodities match. Turkey holds the majority of world boron reserves — figures around seventy per cent are commonly cited — in deposits at Kırka, Bigadiç and Emet, all worked by the state enterprise Eti Maden. The Rio Tinto operation at Boron, California, is the other significant producer. Boron is a marine evaporite mineral concentrated by volcanic and hydrothermal activity, and the geological coincidences required have occurred in very few places.

The glass that survives a temperature shock

Ordinary soda-lime glass cracks when heated unevenly because it expands, and the hot part pushes against the cold part until something gives. Replacing some of the silica with boron oxide drops the coefficient of thermal expansion to roughly a third of that value, and the stresses never build high enough to matter.

Otto Schott developed borosilicate glass in Jena in 1887 for thermometers and lamp chimneys. Corning commercialised it as Pyrex in 1915, initially to stop railway lantern globes shattering when rain hit them, and then — reportedly after an engineer's wife baked in a cut-down battery jar — as ovenware.

There is a genuine and widely misunderstood divergence here. Pyrex-branded kitchenware sold in the United States has been made from tempered soda-lime glass since the 1990s, while the European licensee continued with borosilicate. The two products carry the same name and behave differently under thermal shock, which is the source of a great deal of confused consumer experience. Laboratory glassware remains borosilicate everywhere.

Two claims in one year, both to impure material

Joseph Louis Gay-Lussac and Louis Jacques Thénard announced boron in Paris on 30 June 1808, having reduced boric acid with potassium. Humphry Davy in London reached the same result independently by the same general approach in the same year.

Neither had the element in any pure form. Modern assessments put both products at somewhere around half to sixty per cent boron, contaminated with borides and unreacted material. Berzelius established in 1824 that the substance was an element rather than a compound, and reasonably pure boron was not produced until Ezekiel Weintraub managed it in 1909, a full century after the discovery it is dated from.

Essential to plants, and easy to overdo

Every vascular plant requires boron, and it has exactly one established structural role: it cross-links two chains of a pectin called rhamnogalacturonan-II in the cell wall, forming a borate diester bridge. Without it, cell walls do not assemble properly and growing tips die back.

Boron is unusual among plant nutrients in how narrow the workable range is. The gap between deficiency and toxicity is smaller than for any other micronutrient, sometimes only a factor of two or three in soil concentration, which makes it awkward to fertilise and a real problem in irrigated arid soils where boron accumulates. Deficiency is nonetheless among the most widespread micronutrient limitations in world agriculture. No comparable requirement has been established in animals.

A pH meter for ancient oceans

Boron's two stable isotopes are distributed about four to one in favour of boron-11, and the ratio between them has become the leading proxy for the acidity of past oceans.

In seawater, boron exists as both boric acid and the borate ion, and the balance between them depends on pH. The two species have measurably different isotopic compositions, and — critically — only borate is incorporated into growing calcium carbonate. The boron isotope signature locked into a fossil shell therefore records the pH of the water it grew in.

Because ocean pH is set largely by dissolved carbon dioxide, δ¹¹B measurements on foraminifera and corals are one of the few ways to estimate atmospheric carbon dioxide concentrations from before the ice-core record begins, and they underpin much of what is claimed about the Pliocene and Miocene atmosphere.