Gallium

Gallium is the chemical element with the atomic number 31 and the symbol Ga. It was first identified in 1875 by the French chemist Paul-Émile Lecoq de Boisbaudran, working in Paris. At standard temperature and pressure, this metal is soft and silvery, with a complex orthorhombic crystal structure; when it melts, it takes on a silvery-white appearance. If struck with enough force, solid gallium can break in a conchoidal fracture, similar to glass. In nature, gallium never appears as a free element but exists only in trace amounts within zinc ores like sphalerite and in bauxite, where it forms gallium(III) compounds.

The metal’s melting point is 29.7646 °C (85.5763 °F; 302.9146 K), a value used as a standard temperature reference. Gallium is one of just four metal elements that are liquid at or near normal room temperature; it will liquefy simply from the heat of a human hand, which is about 37 °C (99 °F). Low-temperature gallium alloys serve as a non-toxic, environmentally friendly alternative to mercury in thermometers and can handle higher temperatures than mercury can. One such alloy, galinstan (composed of 62–95% gallium, 5–22% indium, and 0–16% tin by weight), is claimed to melt at −19 °C (−2 °F)—well below water’s freezing point—though this may actually be its freezing point due to supercooling.

Electronics are the main application for gallium. Gallium arsenide, its primary electronic compound, appears in microwave circuits, high-speed switching circuits, and infrared circuits. Semiconducting gallium nitride and indium gallium nitride are used to produce blue and violet light-emitting diodes and diode lasers. Gallium also goes into artificial gadolinium gallium garnet for jewelry. It has no known natural biological role, but gallium(III) behaves similarly to ferric salts in biological systems and has found some medical uses, including in pharmaceuticals and radiopharmaceuticals.

Pure gallium is not found naturally but is easily obtained through smelting. In its highly pure form, it is a silvery-blue metal that fractures conchoidally like glass. It forms alloys with most metals and readily seeps into cracks or grain boundaries of materials such as aluminum, aluminum–zinc alloys, and steel, causing a severe loss of strength and ductility known as liquid metal embrittlement.

Under normal conditions, gallium does not crystallize into any simple structure. Its stable phase (Ga-I) is a complex orthorhombic arrangement with eight atoms per conventional unit cell. In this cell, each atom has only one nearest neighbor, at a distance of 244 pm; the other six neighbors are spaced 27, 30, and 39 pm farther away, grouped in pairs at each distance. The bonding between the two nearest neighbors is covalent, forming Ga₂ dimers that act as the crystal’s basic building blocks. This explains gallium’s low melting point compared to its neighbors aluminum and indium. The structure closely resembles that of iodine and likely arises from interactions between the single 4p electrons of gallium atoms, which are farther from the nucleus than the 4s electrons and the [Ar]3d¹⁰ core. A similar phenomenon occurs with mercury, which has a “pseudo-noble-gas” electron configuration and is also liquid at room temperature. Because the 3d¹⁰ electrons do not shield the outer electrons well from the nucleus, gallium’s first ionization energy is higher than that of aluminum.

Gallium’s physical properties are highly anisotropic—they differ along the three major crystallographic axes a, b, and c—leading to a significant difference between its linear and volume thermal expansion coefficients. These properties are strongly temperature-dependent, especially near the melting point; for instance, the coefficient of thermal expansion increases by several hundred percent upon melting.

The melting point of gallium, 302.9146 K (29.7646 °C; 85.5763 °F), is just above room temperature and roughly matches average summer daytime temperatures in Earth’s mid-latitudes. This melting point is one of the formal temperature reference points in the International Temperature Scale of 1990 (ITS-90), set by the International Bureau of Weights and Measures (BIPM). The US National Institute of Standards and Technology (NIST) uses the triple point of gallium, 302.9166 K (29.7666 °C; 85.5799 °F), instead of the melting point.

Among the four non-radioactive metals that are liquid at or near room temperature (the others being caesium, rubidium, and mercury), gallium is the only one that is neither highly reactive like rubidium and caesium nor highly toxic like mercury. This makes it suitable for use in metal-in-glass high-temperature thermometers. It also has one of the largest liquid ranges for a metal and, unlike mercury, a low vapor pressure at high temperatures. Gallium’s boiling point is 2,676 K (2,403 °C; 4,357 °F), nearly nine times its melting point on the absolute scale—the greatest ratio of any element. However, liquid gallium wets glass, skin, and most other materials (exceptions include quartz, graphite, gallium(III) oxide, and PTFE), making it mechanically trickier to handle than mercury, even though it is far less toxic and requires fewer precautions. Gallium painted onto glass forms a brilliant mirror. Because of this wetting behavior, along with contamination and freezing-expansion issues, gallium samples are usually supplied in polyethylene packets inside other containers.

When gallium solidifies from a liquid, its volume expands by 3.10%, so care is needed to avoid storing it in containers that might rupture during the phase change.