Arsenic

Arsenic is a metalloid in group 15, below phosphorus, with five valence electrons ([Ar]3d10 4s2 4p3). It forms compounds in the −3, +3 and +5 oxidation states. Its most stable form, gray arsenic, is a brittle, metallic-looking solid that turns straight to vapor at 615 °C without melting at ordinary pressure. Arsenic is toxic because it imitates and interferes: arsenate (AsO4^3−) resembles phosphate and disrupts the production of ATP, while arsenite binds tightly to sulfur atoms in key enzymes.

White arsenic, arsenic trioxide, is nearly tasteless and was once so widely used in murders that it was nicknamed inheritance powder. The same compound is now an approved drug for acute promyelocytic leukemia, and an arsenic compound, Salvarsan, introduced in 1910 against syphilis, was the first modern synthetic chemotherapy drug. Today arsenic is used in gallium arsenide semiconductors and in hardening lead alloys. Naturally arsenic-rich groundwater poisons tens of millions of people drinking from wells, especially in Bangladesh and India.

Discovery

Arsenic minerals such as yellow orpiment and red realgar were used as pigments and poisons in the ancient world. The thirteenth-century German scholar Albertus Magnus is often credited with first isolating the element, around 1250. The name comes through Greek arsenikon, used for orpiment, which was borrowed from a Persian word for gold-colored, zarnikh.