- Arsenic (As) is a brittle, steel-gray metalloid with atomic number 33, positioned in Group 15 of the periodic table alongside nitrogen, phosphorus, antimony, and bismuth.
- It exhibits both metallic and nonmetallic properties and is known for its toxicity and historical use as a poison. Arsenic has five valence electrons—two in the 4s subshell and three in the 4p subshell—and commonly exhibits oxidation states of −3, +3, and +5, with +3 being the most stable in nature.
- Its atomic structure consists of thirty-three protons, typically forty-two neutrons, and thirty-three electrons arranged in four shells. Naturally occurring arsenic consists of one stable isotope, arsenic-75 (⁷⁵As).
- Arsenic is moderately abundant in Earth’s crust, at about 1.8 parts per million, and occurs naturally in over 200 mineral forms, most commonly as arsenides, sulfides, and arsenates. Major minerals include arsenopyrite (FeAsS), realgar (As₄S₄), and orpiment (As₂S₃). Arsenic is usually obtained as a by-product of refining other metals such as copper, lead, gold, and cobalt. Significant producers include China, Morocco, Russia, and Peru.
- The element has been known since ancient times, with arsenic-containing minerals used as pigments, medicines, and poisons. The name “arsenic” derives from the Greek word arsenikon, referring to yellow orpiment. Alchemists in medieval Europe recognized arsenic as a distinct substance, and by the 17th century it was classified as a chemical element. Its infamy as a poison—often referred to as “inheritance powder” in the 19th century—comes from its potency, lack of taste, and difficulty of detection before modern chemical analysis.
- Arsenic has several important industrial and technological uses, despite its toxicity. Gallium arsenide (GaAs) is a major semiconductor material used in high-speed electronics, lasers, LEDs, and solar cells, particularly in aerospace and telecommunications due to its efficiency and radiation resistance. Arsenic is also used in small amounts in alloys to improve hardness and corrosion resistance, such as in lead–acid battery plates. In the past, arsenic compounds were widely used in pesticides, herbicides, and wood preservatives (e.g., chromated copper arsenate, CCA), though these uses have declined sharply due to environmental and health concerns.
- Chemically, arsenic can exist in several allotropes: metallic gray arsenic, yellow arsenic (molecular As₄), and black arsenic, each with different stability and reactivity. It resists oxidation in dry air but tarnishes in moist air, forming arsenic oxides. Arsenic trioxide (As₂O₃) is the most commercially important arsenic compound, used historically in glassmaking, pesticides, and medicine. Arsenic forms covalent compounds with many elements, and arsenic hydride (arsine, AsH₃) is an extremely toxic, flammable gas used in semiconductor manufacturing.
- Biologically, arsenic is not essential for humans and is toxic in most of its inorganic forms. Chronic exposure, typically through contaminated groundwater, can cause skin lesions, cancer, cardiovascular disease, and neurological damage. Some organisms, including certain bacteria, can metabolize arsenic, and trace amounts of organic arsenic compounds occur naturally in seafood with relatively low toxicity. Arsenic trioxide is also used in modern medicine under strict control for the treatment of acute promyelocytic leukemia (APL).
- From an environmental perspective, arsenic contamination of groundwater is a major global health issue, particularly in regions such as Bangladesh, India, and parts of South America. Industrial emissions, mining, and improper disposal of arsenic-containing materials can also lead to soil and water contamination. Because arsenic cannot be destroyed (only transformed into different chemical forms), mitigation strategies focus on containment, stabilization, and removal from contaminated sites.
