Gallium is unambiguously a metal. It sits in Group 13 of the periodic table alongside aluminum and indium, and every major chemistry authority classifies it as a post-transition metal. The confusion is understandable, though, because gallium behaves in ways that seem distinctly un-metallic: it melts just above room temperature, it can shatter certain aluminum alloys on contact, and its crystal structure involves covalent bonding more reminiscent of a nonmetal. These quirks make gallium one of the most fascinating elements on the periodic table, but they do not push it into metalloid territory.
Why People Think Gallium Might Be a Metalloid
The metalloid label gets applied to elements that straddle the line between metals and nonmetals, typically showing a mix of properties from each category. Silicon and germanium are classic metalloids: they look shiny like metals but conduct electricity poorly unless you tweak their chemistry, which is exactly why they are useful in semiconductors. Gallium does not fit this description. It conducts electricity well, it conducts heat, it has a metallic luster, and it forms positive ions in chemical reactions. All of these are hallmarks of a true metal.
The source of the confusion usually comes down to two things. First, gallium has a melting point of about 29.8 °C, meaning it will literally liquefy in your hand on a warm day. Most people associate metals with solid, hard materials, so a metal that melts at body temperature feels wrong. Second, gallium’s crystal structure is genuinely unusual for a metal. Researchers have described it as a “molecular metal” because its atoms form covalently bonded pairs, with metallic bonding existing primarily in the plane perpendicular to those pairs.1Chemical Physics Letters. On the bonding of Ga2, structures of Ga*n* clusters and the relation to the bulk structure of gallium That covalent character is atypical for metals, but it does not change gallium’s classification. Plenty of metals have idiosyncratic bonding without being reclassified.
Where Gallium Sits on the Periodic Table
Gallium occupies a spot in Group 13, directly below aluminum. Its existence was predicted in 1871 by Dmitri Mendeleev, who noticed a gap in his periodic table and called the missing element “eka-aluminum” because he expected it to sit one position away from aluminum and share similar chemistry. Four years later, the French chemist Paul-Emile Lecoq de Boisbaudran discovered gallium in a zinc-sulfide mineral called sphalerite using spectroscopy, confirming Mendeleev’s prediction. He named it “gallia” after the Latin name for France.2U.S. Geological Survey Fact Sheet. Gallium–A smart metal The discovery was a landmark moment for the periodic table itself, because it validated the idea that the table could predict the properties of elements not yet found.
Mendeleev was right that gallium would resemble aluminum chemically, and in many ways it does. Both form +3 ions, both are soft and silvery, and both are reactive enough to oxidize in air. But gallium also shows a phenomenon called the scandide contraction, where the filling of the 3d electron shell between calcium and gallium causes the outer electrons to be held more tightly by the nucleus than you would expect based on simple periodic trends. Research on Group 13 compounds has shown that gallium’s valence electrons are noticeably more stabilized compared to aluminum’s, a direct electronic consequence of that contraction.3Inorganic Chemistry. Electronic Manifestations of Scandide Contraction: Theoretical Photoelectron Spectroscopy of Monovalent Group 13 Compounds This affects how gallium bonds and helps explain why its chemistry sometimes diverges from what you would predict by simple extrapolation from aluminum.
Unusual Physical Properties for a Metal
Beyond its low melting point, gallium has some striking physical characteristics. Its boiling point is around 2,400 °C, giving it one of the largest liquid ranges of any element. That huge gap between melting and boiling temperatures makes liquid gallium useful in high-temperature applications where mercury or water would long since have evaporated.
Gallium is also remarkably anisotropic in its solid form, meaning its physical properties differ dramatically depending on which direction you measure them through a crystal. Single crystals of gallium show electrical conductivity ratios of roughly 1:3.2:7 along the three crystal axes, and thermal expansion coefficients in the ratio 1:0.7:1.9. Those ratios hold with relatively little change down to extremely low temperatures.4Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences. The electrical resistivity of gallium and some other anisotropic properties of this metal In practical terms, this means a gallium crystal conducts electricity seven times better in one direction than another. That level of anisotropy is greater than in most other metals and is another consequence of gallium’s unusual dimeric crystal structure, where the covalent pairs create a strongly directional bonding pattern.
One more oddity: gallium expands when it solidifies, just like water does when it freezes into ice. This is rare among metals and means you should never store liquid gallium in a sealed glass container and then refrigerate it, because the expanding solid can crack the vessel.
Gallium as a Liquid Metal
Gallium’s low melting point and low toxicity compared to mercury have made it the basis for a new generation of liquid-metal materials. The two most widely used are eutectic gallium-indium (EGaIn), which is about 75% gallium and 25% indium and melts at 15.7 °C, and Galinstan, a eutectic of roughly 68% gallium, 22% indium, and 10% tin with a melting point near 11 °C.5PubMed Central. Attributes, Fabrication, and Applications of Gallium‐Based Liquid Metal Particles Both remain liquid well below room temperature, making them practical for applications ranging from flexible electronics to thermal management in computers.
What makes gallium-based liquid metals particularly interesting is their surface behavior. When exposed to air or water, they rapidly form a thin oxide layer, predominantly composed of gallium oxides, that is only nanometers thick. This “oxide nano-skin” passivates the surface and allows the liquid metal to be shaped into stable microstructures and films despite having high surface tension.6PubMed. Interface of gallium-based liquid metals: oxide skin, wetting, and applications Researchers have measured the surface tension of this oxide skin at roughly 350 to 365 millinewtons per meter, and the value is similar whether the underlying alloy is EGaIn or Galinstan, because the oxide skin’s composition is dominated by gallium oxides regardless of which alloy sits beneath it.7PubMed. Surface Tension of the Oxide Skin of Gallium-Based Liquid Metals
This oxide skin is both a feature and a nuisance. It allows engineers to pattern liquid metal into circuits that flex and stretch, which is valuable for wearable electronics and soft robotics. But it also makes the liquid metal sticky and hard to remove from surfaces, and it changes the apparent surface tension that researchers measure depending on whether they account for the skin or not. Much of the current research on gallium-based liquid metals is focused on controlling or removing this oxide layer to get predictable behavior.
Gallium’s Trick with Iron in Biology
Gallium has no known biological function in any living organism, but it has a useful chemical coincidence: it mimics iron. Gallium ions are similar enough in size and charge to iron ions that biological systems can be fooled into taking up gallium where iron is expected. The difference is that gallium cannot participate in the same redox chemistry that iron does, so once it replaces iron in an enzyme, that enzyme stops working.8PubMed. Gallium and its competing roles with iron in biological systems
This iron-mimicry gives gallium compounds potential as both anticancer and antimicrobial agents. Tumor cells and infectious bacteria both depend heavily on iron for growth. By disrupting iron-dependent processes, gallium-based compounds have shown activity against certain cancers and bacterial infections in research settings. As an antimicrobial strategy, gallium is especially appealing because it targets multiple iron-dependent biological pathways simultaneously, which makes it harder for bacteria to develop resistance compared to conventional antibiotics that hit a single target.9PubMed Central. Antimicrobial Activity of Gallium(III) Compounds: Pathogen-Dependent Targeting of Multiple Iron/Heme-Dependent Biological Processes Gallium nitrate, for instance, has been used clinically to treat cancer-related hypercalcemia and is being investigated for use against drug-resistant infections.
When Gallium Destroys Other Metals
If you have spent any time watching science videos online, you have probably seen gallium turn a solid aluminum can or beam into a crumbly mess. This is liquid metal embrittlement, and it is not a party trick. It is a real engineering concern.
Liquid gallium infiltrates the grain boundaries of aluminum alloys, which are the tiny seams between individual crystal grains in the metal. Once there, it lowers the binding energy between grains and causes cracks to form and spread along those boundaries. The fracture mode shifts entirely: what was a ductile material that could bend and deform becomes a brittle one that snaps along its grain boundaries.10PubMed Central. Experimental Study on Liquid Metal Embrittlement of Al-Zn-Mg Aluminum Alloy (7075): From Macromechanical Property Experiment to Microscopic Characterization The infiltrating gallium also creates discontinuities that block the normal movement of dislocations, which are the microscopic sliding motions that give metals their ability to bend without breaking. With those motions blocked, stress concentrates until the metal cracks.
This effect is not limited to aluminum. Testing on both 7075 aluminum alloy and 4340 steel compact tension specimens showed extremely rapid crack propagation to failure after gallium exposure, with the fracture surfaces showing clear intergranular patterns in both materials.11Journal of Testing and Evaluation. Liquid-Metal Embrittlement of 7075 Aluminum and 4340 Steel Compact Tension Specimens by Gallium The practical implication is that gallium must be kept away from structural metals in any setting where mechanical integrity matters. Airports, for instance, restrict liquid metals in carry-on luggage partly because of the risk to aircraft aluminum.
Environmental Toxicity of Gallium
Given gallium’s growing use in electronics, solar panels, and experimental medical applications, the question of what happens when it gets into the environment is worth asking. The short answer is that gallium appears to be relatively benign compared to many other metals, though “relatively” is doing some work in that sentence.
In one comparative toxicity study on freshwater shrimp, gallium ranked as much less toxic than copper and cadmium, and less toxic than antimony, but more toxic than indium. The lethal concentration required to kill half the test shrimp over 96 hours was about 2.8 milligrams per liter for gallium, compared to 0.03 mg/L for copper and 0.05 mg/L for cadmium.12PubMed Central. Comparative acute toxicity of gallium(III), antimony(III), indium(III), cadmium(II), and copper(II) on freshwater swamp shrimp (Macrobrachium nipponense) In other words, gallium was roughly a hundred times less acutely toxic to those organisms than copper. A separate ecotoxicity assessment that tested gallium ions against multiple aquatic species found that gallium was “not or only mildly toxic” in those experiments, and its ecotoxicity was much lower than that of arsenic species tested alongside it.13PubMed. Ecotoxicity assessment of ionic As(III), As(V), In(III) and Ga(III) species potentially released from novel III-V semiconductor materials
These results do not mean gallium is harmless. Any metal concentrated in waste streams can cause problems, and gallium’s increasing use in gallium arsenide and gallium nitride semiconductors means more of it will eventually end up in electronic waste. But the available evidence suggests gallium itself is far less of an environmental worry than metals like cadmium, lead, or mercury. The arsenic in gallium arsenide is the more pressing toxicological concern in semiconductor waste.
What Actually Counts as a Metalloid
Part of the reason gallium gets confused for a metalloid is that the metalloid category itself is fuzzy. There is no single universally agreed-upon list. Most chemists include boron, silicon, germanium, arsenic, antimony, and tellurium. Some lists add polonium or astatine. A few older textbooks included aluminum or even selenium. The boundary is more of a gradient than a bright line, and different sources draw it in slightly different places depending on which properties they weight most heavily.
The key distinction is about semiconduction. True metalloids are intrinsic semiconductors or close to it: their electrical conductivity falls between that of metals and insulators, and it increases with temperature rather than decreasing. Gallium’s conductivity behaves like a metal’s, decreasing as temperature rises. Its anisotropic conductivity may be unusual for a metal, but the overall magnitude and temperature dependence are firmly metallic. Gallium also lacks the brittle, glassy appearance that many metalloids have in their pure form; instead, it is soft and silvery, with a distinctive way of wetting glass surfaces when liquid.
If you encounter a source calling gallium a metalloid, it is almost certainly either outdated, colloquial, or confusing gallium with germanium, its neighbor one column to the right. Germanium is a genuine metalloid and a semiconductor. Gallium is the metal next door that just happens to melt in your coffee.
Gallium in Semiconductors Without Being a Semiconductor
Here is where things get slightly ironic: gallium is central to the semiconductor industry despite being a metal. Gallium arsenide (GaAs) and gallium nitride (GaN) are among the most important compound semiconductors in the world. GaAs is the backbone of many high-frequency electronics and solar cells, while GaN is increasingly used in power electronics, LED lighting, and fast chargers for consumer devices. In both cases, gallium provides half of a compound whose combined electronic structure has semiconductor properties. Neither gallium by itself nor arsenic or nitrogen by itself would do the job. The semiconductor behavior arises from the crystal structure of the compound, not from either element alone.
This distinction matters because it is another source of the metalloid confusion. People hear “gallium” and “semiconductor” in the same sentence and assume gallium must be a semiconductor. It is not. It is a metallic ingredient in semiconductor compounds. Oxygen is essential for making silicon dioxide, but nobody calls oxygen a metalloid. The same logic applies.
The demand for gallium in these compounds is significant and growing, particularly as GaN technology displaces older silicon-based power electronics in everything from electric vehicle chargers to data centers. Almost all of the world’s primary gallium production comes as a byproduct of refining aluminum from bauxite ore or zinc from sphalerite, the same mineral where gallium was first discovered in 1875.2U.S. Geological Survey Fact Sheet. Gallium–A smart metal This means gallium supply is tied to aluminum and zinc markets rather than being independently mined, which creates supply-chain vulnerabilities that have drawn attention from governments classifying gallium as a critical material.