Gallium (pronounced /ˈɡцliəm/) is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the Ga (III) salt, in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquifies slightly above room temperature, it will melt in the hand. Its melting point is used as a temperature reference point, and from its discovery in 1875 to the semiconductor era, its primary uses were in high-temperature thermometric applications and in preparation of metal alloys with unusual properties of stability, or ease of melting; some being liquid at room temperature (Ga-In eutectic, 75% Ga, 25% In, mp = 15.5АC).

In semiconductors, an important application is in the compounds gallium nitride and gallium arsenide, used most notably in light-emitting diodes (LEDs). Semiconductor use is now the primary industrial market for gallium, but new uses in alloys and fuel cells continue to be discovered.

Gallium is not known to be essential in biology, but because of the biological handling of gallium’s primary ionic salt Ga(III) as though it were iron(III), gallium ion localizes to and interacts with many processes in the body in which iron(III) is manipulated. As these processes include inflammation, which is present as a marker for many disease states, several gallium salts are used, or are in development, as both pharmaceuticals and radiopharmaceuticals in medicine.