Arsenic

Arsenic is a metalloid with one of the most sinister reputations in history. It was the poison of choice for murderers for centuries, colourless, tasteless, almost undetectable, earning the nickname "inheritance powder" in Renaissance Europe. Yet arsenic compounds are also used in medicine, wood preservatives and semiconductor electronics.

  • Atomic Number3333 protons, 33 electrons
  • Atomic Mass74.92159 uAbout 75× heavier than hydrogen
  • State at Room TempSolidgrey metalloid solid
  • Density5.776 g/cm³About 5.7× denser than water
  • Melting / Boiling816.9°C / 613.9°CSublimes at 616°C, no liquid phase at normal pressure
  • DiscoveredAncientAlbertus Magnus, c.1250

Arsenic sits between germanium and selenium in Period 4.

It is a metalloid sitting at the boundary of the p-block.

Atomic Mass Comparison
Germanium72.6 u
Arsenic74.9 u
Selenium79.0 u
Bromine79.9 u
Iron55.8 u

Arsenic at 74.9 u is a metalloid, sitting between germanium (a semiconductor) and selenium (a non-metal with semiconductor properties). It has five outer electrons and usually forms three bonds, similar to its lighter neighbour nitrogen and phosphorus above it.

What is arsenic?

Arsenic is a metalloid in Group 15 of the periodic table, sitting below phosphorus. It has 33 protons and exists in several allotropes: the most stable is the grey or metallic form, which has a silvery appearance and conducts electricity poorly. Arsenic does not melt at normal atmospheric pressure; instead it sublimes at 616°C (turning directly from solid to vapour). It has five electrons in its outer shell and usually forms three bonds in compounds.

Arsenic gets its name from the Persian word zarnikh (meaning yellow orpiment, an arsenic sulfide mineral), which passed through Greek arsenikon and Latin arsenicum. The element was first isolated around 1250 CE by the German scholar Albertus Magnus, who reduced arsenic sulfide with soap. The symbol As comes from the Latin name.

Fact Arsenic was called "inheritance powder" in Renaissance Europe because it was used so frequently to poison wealthy relatives and inherit their estates. Arsenic trioxide is colourless, tasteless and nearly odourless, almost undetectable in food or drink. Until James Marsh developed a chemical test for arsenic in 1836, it was virtually impossible to prove arsenic poisoning, making it the perfect historical murder weapon.

Where you find arsenic

On Earth

Arsenic is present in small amounts throughout the Earth's crust and is often released by volcanic activity, erosion of rocks and mining activities.

  • Arsenopyrite (FeAsS). The most common arsenic mineral, found worldwide associated with gold and silver deposits. Most commercial arsenic is a by-product of smelting metal ores.
  • Orpiment and realgar. These golden-yellow and red arsenic sulfide minerals were used as pigments in ancient and medieval art, despite their toxicity.
  • Groundwater contamination. Natural arsenic leaches from rocks into groundwater in parts of Bangladesh, India, China and South America. Millions of people drink arsenic-contaminated water, causing a major public health crisis.

How we use arsenic

  • Gallium arsenide semiconductors. Gallium arsenide (GaAs) is an important compound semiconductor used in high-speed electronics, LED lights, solar cells for satellites and microwave devices.
  • Wood preservative (historically). Chromated copper arsenate (CCA) was widely used to treat outdoor timber until the early 2000s. It has now been largely phased out due to toxicity concerns.
  • Medicine. Arsenic trioxide is used to treat a form of leukaemia (acute promyelocytic leukaemia). Like many poisons, arsenic is a medicine in small controlled doses.
  • Agriculture (historically). Arsenic-based pesticides were common until the mid-20th century. Most are now banned.
Did you know? Napoleon Bonaparte may have been slowly poisoned by arsenic-containing wallpaper in his damp home on the island of Saint Helena. Samples of his hair preserved after his death contain levels of arsenic many times higher than normal. The green pigment in his wallpaper, Scheele's green, copper arsenite, was known to produce toxic arsenic gas (arsine) when damp moulds broke down the pigment. The debate about whether Napoleon was deliberately poisoned or accidentally exposed by his wallpaper continues among historians.

How it was discovered

Arsenic compounds were known in antiquity, orpiment and realgar were used as pigments and medicines for thousands of years. The element itself was first isolated around 1250 CE by Albertus Magnus, who heated arsenic sulfide with soap and obtained the metallic form. However, the understanding of arsenic as a true chemical element came much later, Lavoisier listed it as an element in his 1789 treatise on chemistry, establishing arsenic's place in the chemical canon.

Deeper dive: arsenic toxicity and the Marsh test

Arsenic trioxide (As₂O₃) is the most toxic common form of arsenic. In the body, arsenic interferes with enzymes that produce ATP: the cell's energy currency, and disrupts enzyme systems involved in DNA repair, producing serious cellular damage. Chronic low-level arsenic exposure causes skin thickening, peripheral nerve damage, liver disease and several types of cancer. The World Health Organisation recommends that drinking water should contain no more than 10 micrograms of arsenic per litre; tens of millions of people in South and Southeast Asia drink water far above this level from naturally contaminated wells.

The Marsh test, developed in 1836 by British chemist James Marsh, was the first reliable chemical test for arsenic in a body or food. Marsh was asked to provide evidence in a murder trial; he wanted to demonstrate arsenic poisoning but had no reliable method. He developed a procedure in which suspected samples were treated with acid and zinc, producing arsine gas (AsH₃) that burned to leave a characteristic black arsenic mirror on a glass surface. The test was sensitive enough to detect fractions of a milligram. It changed forensic toxicology, making arsenic murders far easier to detect, and is considered the beginning of forensic chemistry as a scientific discipline.

Arsenic is a double-edged element, toxic in large amounts, yet medicinally useful and important in modern electronics. Moving to 34 protons brings us to selenium, an element essential to life in tiny amounts but poisonous in large ones.