Arsenic

Few elements carry as much historical baggage as arsenic. For centuries it was the poison of choice among nobles and assassins, earning the grim nickname 'inheritance powder' for its role in expediting family fortunes. Yet this silvery-grey metalloid is far more than a toxicological curiosity. Arsenic sits at a fascinating chemical crossroads — part metal, part nonmetal — which gives it properties that modern technology has found genuinely useful. It exists in several distinct allotropic forms, behaves as both an insulator and a semiconductor depending on context, and compounds involving arsenic turn up in everything from computer chips to wood decking. Understanding arsenic means wrestling with a substance that has simultaneously shortened lives and extended the reach of human technology.

Modern industry relies on arsenic most heavily in the production of gallium arsenide (GaAs), a semiconductor prized in high-speed electronics, laser diodes, and solar cells where its electron mobility surpasses silicon. Chromated copper arsenate (CCA) was the dominant wood preservative for decades, though its use on residential lumber has been phased out in many countries due to toxicity concerns. Agriculture historically used arsenic-based pesticides and herbicides extensively, and some organoarsenic compounds are still employed as animal feed additives in certain regions. Arsenic trioxide has found an unexpected medical second life as an FDA-approved treatment for acute promyelocytic leukemia. Lead-arsenic alloys improve the hardness of lead shot and battery grids. In glass manufacturing, arsenic compounds act as refining agents, helping remove bubbles from molten glass.

Arsenic compounds have been known since antiquity — Greek and Roman physicians used orpiment (arsenic trisulfide) as a medicine and pigment. The element itself was first isolated around 1250 by the German scholar Albertus Magnus, who heated orpiment with soap and observed the metallic residue, making it one of the earliest documented isolations of an element. The name derives from the Greek arsenikon, itself borrowed from a Persian word for the yellow sulfide mineral. Through the Renaissance and into the nineteenth century, arsenic trioxide became notorious as an undetectable poison because its symptoms mimicked cholera. The 1836 invention of the Marsh test finally gave forensic scientists a reliable method for detecting arsenic in tissues, fundamentally changing criminal toxicology and helping convict several high-profile poisoners.

Arsenic is the twentieth most abundant element in Earth's crust, present at roughly 1.5 to 2 parts per million. It rarely appears as a native element; instead it occurs chiefly in sulfide minerals such as arsenopyrite (FeAsS), realgar (As4S4), and orpiment (As2S3). Arsenopyrite is by far the most common ore. Arsenic is also a common trace contaminant in sulfide ores of copper, lead, and gold, meaning it is frequently recovered as a byproduct of smelting those metals. Elevated arsenic levels in groundwater represent a serious public health problem in parts of Bangladesh, India, and other regions where natural geological sources leach into drinking water. Volcanic emissions and the weathering of arsenic-bearing rocks contribute to background levels in soils and surface water worldwide.

Arsenic trioxide (As2O3) is the most commercially important compound, serving as a starting material for most arsenic chemistry and as the active ingredient in the leukemia drug arsenic trioxide injection. Gallium arsenide (GaAs) is a III-V semiconductor foundational to high-frequency electronics and optoelectronics. Arsenic pentoxide (As2O5) is used in herbicides and wood preservatives. Organoarsenic compounds include roxarsone, once widely used in poultry farming, and melarsoprol, a treatment for late-stage African sleeping sickness. Historically, Paris green (copper acetoarsenite) served as both a pigment and an insecticide, and was even used to color Victorian-era wallpaper with sometimes fatal consequences. Arsine (AsH3), a colorless, highly toxic gas, is employed in the semiconductor industry for vapor-phase deposition of arsenic-containing layers.

• Napoleon Bonaparte may have been slowly poisoned by arsenic-laced green wallpaper in his damp St. Helena exile, a theory supported by high arsenic levels found in locks of his preserved hair.
• The Marsh test, developed in 1836, was one of the first scientific techniques used as evidence in a criminal murder trial.
• Grey arsenic, the most stable allotrope, sublimates directly from solid to vapor at 887 degrees Celsius without passing through a liquid phase at standard pressure.
• Gallium arsenide solar cells are used on most commercial satellites because they are more efficient and radiation-resistant than silicon cells in the harsh conditions of space.
• Some species of marine algae and certain bacteria can convert inorganic arsenic into organic arsenobetaine, a far less toxic form, effectively detoxifying their environment.