Antimony

The name “antimony” comes from the Greek words anti (meaning “against”) and monos (meaning “alone”). It is “a metal not found alone,” so named because it rarely occurs in its pure elemental form. Although it bonds readily with heavy metals such as copper, lead, and silver, antimony is a chalcophile at heart, meaning its preferred companion is sulfur. Thus, the most common form of antimony is the mineral antimony trisulfide (Sb₂S₃), also called stibnite. It is unknown who first isolated antimony in its pure, elemental form—or when—but as a mineral it has been part of human history for thousands of years.

Like many elements, antimony’s symbol (and our word for its sulfide form, stibnite) comes from its Latin name, stibium (Sb), which in turn comes from the ancient Greek word for “mark.” The name is a reference to the practice of using crushed stibnite as an eyeliner and mascara, both to accentuate the eyes and to protect them from diseases and the sun. This practice dates to 3000 BCE, when it was popular in the Middle East and much of Africa.

In the first century CE, Roman naturalist Pliny the Elder—who is often credited with first using the term “stibnite”—described its medicinal uses and offered a curious warning that heating stibnite for too long would produce the element lead. The product he was referring to was most likely pure antimony. In the Middle Ages and beyond (as late as the 1800s), antimony remained in vogue as a remedy for various ailments. In tablet form, it was commonly used as a laxative capable of being ingested, recovered, and re-ingested. Despite a long tradition of medicinal, cosmetic, and material use (in pigments, glazes, and pewter), we now know that antimony and many of its compounds are toxic, with health effects comparable to those of arsenic poisoning.

Pure antimony is brittle, flaky, and easily powdered. It is one of five elements that, like water, expands as it cools, and it lends this property to some of its alloys. Antimony is a metalloid, meaning its characteristics fall somewhere between those of metals (to which it is chemically most similar) and solid nonmetals (with greatest similarity to sulfur). It is a poorer conductor of heat and electricity than most metals.

Antimony is highly valued for its ability to increase the hardness and strength of metals. It is often paired with lead, an almost buttery-soft metal in its pure form. Lead-antimony alloys (or mixtures) are durable and corrosion-resistant, especially with the addition of tin, antimony’s neighbor to the left on the periodic table. The major uses of this alloy are for solder, bullet castings, cable sheathing, and, most commonly, the grid plates and terminals of lead-acid storage batteries.

Antimony in its pure form is a critical “doping agent” in the manufacture of semiconductor devices. Semiconductors are elements like silicon that fall near the middle of the electrical conductivity spectrum—a range of ability to conduct electricity. Adding antimony to a semiconductor, called doping, shifts the material’s electron configuration and can increase its conductivity by several orders of magnitude. Antimony-doped semiconductor devices include lasers, solar cells, and the microchips used in modern computing.

Antimony trioxide (Sb₂O₃) is a component of the flame-retardant compounds applied to everyday items like mattresses, toys, electronic devices, and car seat covers. These compounds account for more than half the annual consumption of antimony in the United States. Sb₂O₃ is also used to facilitate chemical reactions in the production of plastics and rubber and is added to some enamels, ceramics, and glass as an “opacifier” to reduce their transparency, making them harder to see through.

Besides the stable, grey, metallic form, there are three allotropes (or forms) of antimony that are only stable under certain conditions: black, yellow, and explosive white.

Black antimony forms when antimony vapor cools rapidly. It has no definite shape and is susceptible to spontaneous combustion. It transforms into the stable, metallic form of antimony above 100° C.

Yellow antimony is the most unstable allotrope and forms when the compound stibine (SbH₃) oxidizes below -90° C. Above this temperature, it will transform to black antimony.

White antimony is a solid solution of antimony trichloride (SbCl₃). It can be made to explode easily by scratching it with a sharp object; when it vaporizes, heat is released and a toxic cloud of SbCl₃ is emitted as the elemental antimony returns to its stable, metallic form.

Antimony is almost always encountered as a component of mineral ores, mainly stibnite, from which it is extracted and purified. The first recorded discovery of naturally occurring pure antimony was not made until 1783 in Sweden. Today, more than 50% of the world’s antimony is mined in China. As an alternative to mining, it can be extracted from lead, copper, and silver ores during the smelting process.