Atomic number 31

Gallium

Ga
+ phase
− phase

Shells Orbitals 3d 4s 4p

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31 electrons in 4 shells: 2, 8, 18, 3.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Post-transition metal
Period: 4
Group: 13
Block: p
Standard atomic weight: 69.723(1) Da
Electron configuration: [Ar] 3d¹⁰ 4s² 4p¹
Phase at 25 °C: solid
Melting point: 29.76 °C
Boiling point: 2400 °C
Density: 5.91 g/cm³
Electronegativity (Pauling): 1.81
Discovered: 1875, Paul-Émile Lecoq de Boisbaudran, spectroscopically

Neighbours in the table

Zn Ge

Dmitri Mendeleev described gallium four years before anyone found it. In 1871 he proposed it from a gap in his newly created periodic table, calling the missing element eka-aluminium. In 1875 Paul-Émile Lecoq de Boisbaudran, a French chemist, detected it spectroscopically, by the light it emits, in zinc blende, says CIAAW, IUPAC’s atomic-weights commission. Los Alamos and Jefferson Lab say he had the free metal that same year, by electrolysing gallium hydroxide dissolved in potassium hydroxide. CIAAW dates the first isolation to 1878 instead, and shares it with Émile-Clément Jungflesch.

By shell, gallium’s thirty-one electrons go 2, 8, 18 and 3: [Ar] 3d¹⁰ 4s² 4p¹. OpenStax counts the full 3d subshell as core, so gallium has three valence electrons in the ns² np¹ pattern of group 13. It commonly gives up all three to form Ga³⁺. It also shows a +1 state, two below the group’s +3; OpenStax calls this the inert pair effect, the pair being the valence s pair. Los Alamos lists a longer range, down to −5.

The metal melts at 29.76 °C. Liquid gallium tends to supercool, staying liquid below its freezing point, so a seed crystal may be needed before it sets. On freezing it swells by 3.1 per cent, which is why Los Alamos warns that glass or metal containers holding it can break.

Where it is found

Gallium is spread thinly. Jefferson Lab puts it at 19 milligrams per kilogram of the Earth’s crust and 3 × 10⁻⁵ milligrams per litre of seawater. Los Alamos finds it as a trace in diaspore, sphalerite, germanite, bauxite and coal, and some flue dusts from burning coal hold as much as 1.5 per cent. Bauxite, the rock mined for alumina, averages 50 parts per million, and some zinc ores in the United States reach the same level.

Those two hosts supply almost all of it. The US Geological Survey (USGS) reports that most primary gallium is a byproduct of processing bauxite, with the rest drawn from zinc-processing residues. Of the gallium held in those bauxite and zinc resources, the USGS judges under 10 per cent potentially recoverable. China produced 99 per cent of the world’s primary low-purity gallium in 2024, about 750,000 of 760,000 kilograms.

The United States has recovered no primary gallium since 1987. A single company in New York refines imported low-purity metal to high purity, together with the scrap left over from making gallium arsenide devices.

Why it matters

The USGS counts integrated circuits as 79 per cent of American gallium consumption and optoelectronic devices as 20 per cent. About 83 per cent went in as wafers of gallium arsenide, GaAs, gallium nitride, GaN, and gallium phosphide. Gallium arsenide makes laser diodes, light-emitting diodes, photodetectors and solar cells, and Los Alamos notes that it turns electricity straight into coherent light. In many defence applications, the USGS finds, circuits built on GaAs and GaN have no effective substitute.

Gallium also alters silicon. Los Alamos names it with boron, phosphorus and arsenic among the elements silicon can be doped with, that is, given a deliberate trace of another element. An atom bringing three valence electrons into a silicon site, as gallium does, is an acceptor, and the doped crystal is p-type, carrying current mainly as positive holes.

Its long liquid range earns it a place in thermometers. Jefferson Lab ranks that range among the largest of any metal, starting near room temperature, and credits it for gallium’s use in high-temperature thermometers. Galinstan, gallium alloyed with indium and tin, now stands in for mercury in traditional thermometers, the USGS reports. Brushed onto glass, gallium wets the surface and leaves a brilliant mirror, Los Alamos adds.

Properties

Properties of gallium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	31
Standard atomic weight	69.723(1) Da	Da	IUPAC/CIAAW 2024
Melting point	29.76 °C	°C	Los Alamos: 29.76 °C
Boiling point	2400 °C	°C	Los Alamos: 2400 °C
Density	5.91 g/cm³	g/cm³
Electronegativity (Pauling)	1.81
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹
Phase at 25 °C	solid

Safety

Gallium metal carries the signal word Danger in company reports to the European Chemicals Agency, and 57.2% of them say it causes severe skin burns and eye damage. Smaller shares add harm if swallowed, corrosion of metals and harm to aquatic life, while 9 of 388 reports find it short of the Globally Harmonized System’s criteria. Los Alamos says its toxicity seems low, pending more data, and Haz-Map calls it probably a skin and eye irritant. The PubChem Laboratory Chemical Safety Summary gives the classification in full.

Questions

Does gallium really melt in your hand?

It does. Its melting point is 29.76 °C, low enough that OpenStax says a piece will melt in your hand, yet at 25 °C it is still a solid. Los Alamos's gallium page counts it among the metals, with mercury, caesium and rubidium, that can be liquid near room temperature.

Why is gallium used in medical scans?

Two radioactive isotopes do the work, according to IUPAC's periodic table project. Gallium-68, with a half-life of 68 minutes, emits positrons for positron emission tomography (PET), and a generator holding its parent, germanium-68, supplies it cheaply. It attaches readily to peptides aimed at receptors that tumour cells carry in excess. Gallium-67, with a half-life of 78 hours, emits gamma rays and is used in scintigraphy, imaging that maps the gamma rays a radioactive tracer gives off inside the body.

Why do China's gallium export controls matter?

Nearly all primary gallium is Chinese, and the United States buys in all it uses: the USGS puts American net import reliance at 100 per cent every year from 2020 to 2024. After China introduced export controls, its domestic price rose through the first half of 2024 and reached $420 a kilogram by October. In December 2024 China banned every gallium export to the United States.

Where does the name gallium come from?

From Gallia, the Latin name for France, the country of its discoverer, the chemist Paul-Émile Lecoq de Boisbaudran. Los Alamos adds a second Latin root, gallus, a cock, which translates his surname Lecoq. Before anyone had found the element, Mendeleev had given it the placeholder name eka-aluminium.