Gallium (Ga)

Gallium is a metal that cannot cope with a warm room. Its melting point sits just under 30 °C, which is below body temperature and below a hot afternoon in most of the world, so a lump of it held in a closed palm slumps into a silver puddle. That is the party trick. The consequence worth knowing is where the other end of the liquid range sits: 2477 K, which is more than eight times the melting point read off the same scale. The multiple is 8.18, and it is the highest of the ninety-three elements whose melting and boiling points are both firmly established. Second place belongs to tin at 5.69 and third to indium at 5.46; no fourth element manages five at all.

State that range as a plain subtraction, though, and gallium is unremarkable — 2174 kelvin ranks it fourteenth, a long way behind neptunium, thorium and the rest of the early actinides, which simply start hot and finish hotter. Gallium's distinction is the proportion, and the proportion is achieved at the cold end. Nothing about its boiling point is extreme. Its melting point is.

Two further oddities follow from the same structure. Gallium expands as it solidifies, by roughly three per cent, so it splits glass and steel containers from the inside as it sets — the crystal it forms is a peculiarly open one, built from covalently bonded pairs of atoms rather than a close-packed lattice, which is why melting has so little to undo while vaporising has to dismantle the full metallic bonding the liquid adopts instead. And it supercools readily, sitting as a liquid for hours tens of degrees below the temperature at which it should have frozen, because those pairs have to reassemble into an awkward lattice before freezing can start.

Mendeleev's eka-aluminium, and the letter about density

In 1871 Dmitri Mendeleev used the gap below aluminum in his table to predict an unknown metal he called eka-aluminium. He specified an atomic weight near 68, a low melting point, and a density around 5.9 grams per cubic centimetre.

Four years later Paul-Émile Lecoq de Boisbaudran, examining a zinc blende sample from the Pyrenees, saw two unfamiliar violet lines in a spectroscope and traced them to a new element. He isolated the metal and measured its density at 4.7. Mendeleev, reading the paper in St Petersburg, wrote to tell a man who had the substance in front of him that his measurement was wrong. Boisbaudran repurified his sample — the first batch was contaminated with sodium from the reduction — and got 5.9.

That exchange did more for the periodic law than any argument Mendeleev published. A classification scheme had described an object nobody had ever seen, and then corrected the person holding it.

A country, or possibly a rooster

Boisbaudran named the element for Gallia, the Latin name for France. Almost immediately his colleagues pointed out that Lecoq, his own surname, translates into Latin as gallus — a rooster. He denied the pun in print, and there is no way now to know whether the denial was honest. It remains the only element whose etymology carries a credible accusation of vanity.

Nitride, arsenide, and almost nothing else

Very little gallium is used as gallium. Nearly all of it goes into two compound semiconductors, and between them they account for the metal's entire economic existence.

• Gallium nitride has a wide band gap that emits in the blue and ultraviolet. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura solved the problem of making it emit efficiently, work that took the 2014 Nobel Prize in Physics and gave the world the blue LED — and therefore the white LED, since white light is made by putting a yellow phosphor in front of a blue chip. The same wide band gap lets gallium nitride switch high voltages with low losses, which is why phone chargers shrank and why the material now competes with silicon in electric-vehicle inverters and radar transmitters.
• Gallium arsenide moves electrons faster than silicon and handles high frequencies cleanly. It dominates the radio front-end of mobile phones and the amplifiers in base stations, and multi-junction gallium arsenide cells are the standard photovoltaic on spacecraft, where efficiency per kilogram outweighs cost per watt by a wide margin.

Beyond semiconductors, gallium alloyed with indium and tin gives galinstan, a liquid at room temperature that has replaced mercury in some thermometers and in liquid-metal cooling loops.

Sixty tonnes of it, buried, waiting for neutrinos

Gallium-71 absorbs a neutrino and becomes germanium-71, and the energy threshold for that capture is low enough to catch neutrinos from the proton–proton reaction at the very core of the Sun. Two experiments were built around it: SAGE, under a mountain in the Caucasus, using about sixty tonnes of liquid metallic gallium, and GALLEX and its successor GNO at Gran Sasso in Italy, using a gallium chloride solution.

They found fewer neutrinos than the Sun should emit, contributing to the solar neutrino problem that was eventually resolved by neutrino oscillation. Their calibration runs left a residue of their own: exposed to intense artificial neutrino sources, the gallium detectors consistently counted about 20% too few events. The gallium anomaly has survived every attempt to explain it away and is one of the standing hints that a fourth, sterile neutrino might exist.

There is no gallium mine

Nobody digs for gallium. It is too evenly diffused through the crust to concentrate into an ore, and every gram of it arrives as a passenger on something else. Most comes from the caustic liquor of the Bayer process, where bauxite is dissolved on the way to becoming aluminum; the rest is stripped from zinc refining residues.

This makes gallium output a function of aluminum demand rather than gallium demand, and it makes the supply extraordinarily concentrated, because most of the world's alumina refining capacity with gallium extraction bolted on is in China. When China placed gallium and germanium under export licensing in July 2023 and then banned shipments to the United States in December 2024, the constraint bit immediately, because there was no idle Western capacity to switch on. Recovery projects have since restarted in several countries, but the underlying economics have not changed: gallium is worth extracting only where somebody is already refining something else.

What it does to aluminum

Liquid gallium wets aluminum and then diffuses along the grain boundaries, and the aluminum loses essentially all of its strength — a structural metal reduced to something that tears like foil. The effect is dramatic enough that gallium and gallium-bearing alloys are restricted as air cargo, since an aircraft is largely made of the metal gallium destroys.