Gallium Ga

Discovery

Gallium was predicted before it was found. In 1871 Dmitri Mendeleev left a gap below aluminium in his periodic table and described the missing "eka-aluminium" in surprising detail, including its approximate atomic weight, density and the behaviour of its oxide.

In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran examined zinc blende from the Pyrenees with a spectroscope and spotted two violet lines that matched no known element. Later that year he obtained a small amount of the metal by electrolysing a solution of its hydroxide.

Lecoq first reported a density of about 4.7 g/cm³. Mendeleev wrote to him suggesting the sample was impure, because his prediction was close to 5.9. After further purification, the measured value came out at about 5.9. The episode convinced many chemists that the periodic table was a genuine predictive theory rather than a convenient list.

Origin of the name

The name comes from Gallia, the Latin name for France. Because the Latin word gallus means rooster, and "le coq" is French for the same bird, some suspected Lecoq had hidden his own name in the element; he denied it. The symbol Ga shares the same root.

Main uses

• Gallium nitride (GaN) made efficient blue LEDs possible, earning Isamu Akasaki, Hiroshi Amano and Shuji Nakamura the 2014 Nobel Prize in Physics. GaN also powers white LED lighting, small fast chargers and 5G base-station amplifiers.
• Gallium arsenide (GaAs) carries electrons faster than silicon and is used in radio-frequency chips in phones, high-efficiency solar cells on satellites and infrared LEDs.
• Low-melting alloys such as gallium–indium–tin are liquid at room temperature and replace mercury in some thermometers.
• Medicine uses radioactive gallium-67 and gallium-68 for diagnostic imaging.

Isotopes

Natural gallium consists of two stable isotopes: gallium-69 (about 60%) and gallium-71 (about 40%). Among the radioactive ones, gallium-67 has a half-life of roughly 3.3 days and has long been used to image lymphomas and infections. Gallium-68, with a half-life of about 68 minutes, is drawn on site from a germanium-68 generator and attached to targeting molecules for PET scans, especially of prostate cancer and neuroendocrine tumours.

In everyday life

Gallium is not mined on its own. It is recovered as a by-product when alumina is refined from bauxite or when zinc ores are processed. Small amounts end up in phone RF chips, LED bulbs and laptop chargers.

Gallium itself has low toxicity, but it is aggressive towards aluminium: it creeps along grain boundaries and leaves the metal brittle. For this reason it is restricted on aircraft. It also wets glass and skin and leaves grey stains, so it is best handled with gloves.