Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where arsenic sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 119 pm — that is 0.119 nm, so about 4202 million of them side by side would span a millimetre.
Thermal range
At ordinary pressure arsenic has no liquid phase at all. It sublimes at 887 K, passing straight from solid to vapour. The 1090 K melting point is real, but only above about 3.63 MPa.
The story
Arsenic is probably the most infamous metalloid in the periodic table - known throughout history as a deadly poison, yet absolutely essential for modern technology. This steel-gray element has a darker reputation than any other element, but it's also crucial for semiconductors, pesticides, and even medicine. The name comes from the Greek word "arsenikos" meaning yellow orpiment, a beautiful but deadly arsenic mineral. Known since ancient times, arsenic compounds have been used for both murder and medicine for over 2,000 years. Ancient Egyptian, Greek, and Roman texts describe arsenic poisoning, and it became known as the "king of poisons" during the Renaissance because it was tasteless, odorless, and nearly impossible to detect. Yet paradoxically, arsenic compounds were also used as medicines, including the famous "Fowler's solution" used to treat everything from fevers to skin conditions. What makes arsenic scientifically fascinating is its unusual properties. It exists in several allotropes: the stable gray metallic form, a yellow non-metallic form, and a black amorphous form. Gray arsenic is actually a semiconductor, not a true metal, and it sublimates (goes directly from solid to gas) when heated, creating toxic arsenic vapor. Modern technology relies heavily on arsenic despite its toxicity. Gallium arsenide (GaAs) semiconductors are faster and more efficient than silicon for high-frequency applications. Your cell phone, GPS system, and LED lights all contain arsenic compounds. Solar panels, laser diodes, and satellite communications wouldn't exist without arsenic-based semiconductors. Here's something surprising: arsenic is actually essential for life in tiny amounts. Some organisms use arsenic in their biochemistry, and humans need trace amounts for proper growth and development. However, the difference between essential and toxic doses is extremely small.
Arsenic has been known to humanity since ancient times, but its recognition as a distinct chemical element developed gradually over centuries through the work of numerous alchemists and chemists. Ancient civilizations, including the Greeks, Romans, and Chinese, were familiar with arsenic compounds, particularly the sulfide minerals orpiment and realgar, which were used as pigments and in traditional medicine despite their toxicity. The element's name derives from the Greek word "arsenikos," meaning "masculine" or "potent," reflecting the ancient understanding of its powerful and dangerous properties. Arab alchemists in the 8th century, including Jabir ibn Hayyan (known in Latin as Geber), described methods for preparing various arsenic compounds and recognized their distinct properties. The first clear isolation of metallic arsenic is credited to German alchemist Albertus Magnus around 1250 AD, who obtained the element by heating orpiment with organic matter, though he may not have fully understood what he had produced. Dominican friar and scholar Albert the Great documented procedures for obtaining "a metallic substance" from arsenic minerals, representing one of the earliest recorded isolations of elemental arsenic. The systematic study of arsenic began in earnest during the 18th century when Swedish chemist Georg Brandt provided detailed descriptions of arsenic's properties and chemical behavior. German chemist Johann Schroeder published comprehensive studies of arsenic compounds in the 17th century, contributing significantly to understanding the element's chemistry. The development of analytical chemistry in the 18th and 19th centuries led to more precise characterization of arsenic's properties and its recognition as a distinct element in the emerging periodic system. The infamous historical association of arsenic with poisoning led to extensive research into its detection and analysis, spurring advances in analytical chemistry and forensic science.
Applications
Arsenic, despite its notorious reputation as a poison, has several important industrial and technological applications when properly handled and controlled. The semiconductor industry utilizes ultra-pure arsenic in the production of gallium arsenide (GaAs) semiconductors, which are essential for high-frequency electronic devices, LED lighting, laser diodes, and solar cells. These arsenic-containing semiconductors offer superior performance characteristics compared to silicon in applications requiring high speed or high power efficiency. The electronics industry relies on arsenic compounds for manufacturing integrated circuits, particularly in specialized applications such as microwave devices and optoelectronic components. Wood preservation historically represented a major use of arsenic compounds, particularly chromated copper arsenate (CCA), though this application has been largely phased out in many countries due to health and environmental concerns. The glass industry uses arsenic trioxide as a clarifying agent to remove bubbles and improve optical clarity, particularly in the production of high-quality optical glass and television screens. Arsenic compounds serve as dopants in the semiconductor manufacturing process, where precise amounts are introduced to modify electrical properties of silicon and other semiconductor materials. The medical field has found specialized applications for arsenic compounds in cancer treatment, particularly arsenic trioxide for certain types of leukemia, where it has shown remarkable effectiveness despite its toxicity. Agricultural applications include certain pesticides and herbicides, though these uses are heavily regulated and declining due to environmental and health concerns. Research continues into arsenic's potential applications in advanced technologies, including quantum computing and specialized alloys for aerospace applications. The metallurgical industry uses arsenic as an alloying agent in lead-based materials to improve hardness and corrosion resistance.
The most significant modern application of arsenic is in semiconductor manufacturing, where it forms gallium arsenide compounds used in virtually all smartphones, LED lights, and wireless communication devices. Solar panels, particularly those designed for space applications, rely on gallium arsenide cells that contain arsenic for maximum efficiency in converting sunlight to electricity. High-speed internet infrastructure depends on arsenic-containing laser diodes and photodetectors used in fiber optic communication systems. Automotive LED headlights and taillights incorporate arsenic-based semiconductors for their brightness and energy efficiency. Military and aerospace applications utilize arsenic compounds in radar systems, satellite communications, and electronic warfare equipment. The glass industry continues to use small amounts of arsenic trioxide in specialty glass production, including optical lenses and high-end television screens. Medical oncology departments use arsenic trioxide as a chemotherapy agent for specific types of blood cancers, where it has proven highly effective. Research laboratories worldwide employ ultra-pure arsenic in experimental semiconductor devices and quantum research applications. Some specialized metal alloys still incorporate small amounts of arsenic to improve material properties for specific industrial applications. Environmental monitoring equipment uses arsenic-based sensors and detectors to measure air and water quality. Even consumer electronics like computer processors and graphics cards contain trace amounts of arsenic in their semiconductor components. The renewable energy sector employs arsenic compounds in advanced photovoltaic systems designed for maximum power generation efficiency.
Where it comes from
1.8 mg/kg of Earth's crust · more abundant than 56% of elements
Arsenic is widely distributed throughout the Earth's crust with an average concentration of approximately 1.8 parts per million, making it more abundant than many other trace elements but rarely found in concentrated deposits. The element occurs naturally in over 200 mineral species, though only a few are economically significant for extraction. Arsenopyrite (FeAsS) is the most common arsenic-bearing mineral and serves as the primary source for commercial arsenic production. This mineral is often found in association with gold, silver, and copper ores, making arsenic recovery a byproduct of precious metal mining operations. Realgar (As4S4) and orpiment (As2S3) are distinctive orange and yellow arsenic sulfide minerals that were historically important sources, though they are less significant commercially today. Arsenic commonly occurs as a trace element in sulfide ores of copper, lead, zinc, silver, and gold, often complicating the processing of these metals. Coal deposits frequently contain arsenic concentrations ranging from 1 to 100 parts per million, with some high-arsenic coals containing much higher levels, leading to environmental concerns when these coals are burned. Groundwater in certain geological formations naturally contains elevated arsenic levels due to the weathering of arsenic-bearing rocks and minerals, creating significant public health challenges in affected regions. Volcanic activity can release arsenic into the environment through geothermal processes, leading to naturally elevated concentrations in some hot springs and geothermal areas. Marine sediments often contain arsenic that has been concentrated through biological and chemical processes, particularly in areas with high organic matter content. Some plants can accumulate arsenic from contaminated soils, leading to elevated concentrations in certain agricultural products grown in affected areas. The geographic distribution of arsenic is closely tied to specific geological formations, with notable concentrations in areas of past volcanic activity and certain sedimentary basins.
Handling
Arsenic is extremely toxic and requires the highest level of safety precautions during handling, storage, and disposal, as it is classified as a Group 1 carcinogen by the International Agency for Research on Cancer. All forms of arsenic, including the metallic element and its compounds, should be treated as highly hazardous materials requiring specialized training and safety equipment. Direct contact with arsenic must be prevented through the use of appropriate personal protective equipment, including chemical-resistant gloves, protective clothing, and respiratory protection. Inhalation of arsenic dust or vapors can cause severe acute poisoning and long-term health effects, including cancer, cardiovascular disease, and neurological damage. Ingestion of even small amounts of arsenic compounds can be fatal, making secure storage and proper labeling absolutely critical. Workers in industries using arsenic must follow strict occupational exposure limits and undergo regular health monitoring to detect early signs of arsenic exposure. Chronic exposure to low levels of arsenic can cause skin lesions, peripheral neuropathy, and increased risk of various cancers, particularly skin, lung, and bladder cancer. Pregnant women must avoid any exposure to arsenic compounds, as the element can cause developmental abnormalities and adverse pregnancy outcomes. Emergency procedures for arsenic exposure include immediate decontamination, removal from the exposure source, and urgent medical attention, as prompt treatment is critical for preventing severe poisoning. Disposal of arsenic-containing materials requires compliance with hazardous waste regulations and specialized disposal facilities equipped to handle toxic materials safely. Laboratory work with arsenic compounds must be conducted in properly ventilated fume hoods with emergency procedures clearly posted and readily accessible. The general public should be aware that naturally occurring arsenic in drinking water poses health risks, and water testing is recommended in areas with known arsenic contamination.
Quick answers
Arsenic (symbol As) is element 33 on the periodic table, a metalloid in period 4, group 15. The classic poison of history — tasteless, undetectable for centuries, and still a groundwater crisis for millions. At room temperature it is a solid.
[Ar] 3d¹⁰ 4s² 4p³, giving 4 occupied shells holding 2, 8, 18, 5 electrons respectively. Its outer shell holds 5 electrons, which is what sets its bonding behaviour.At ordinary pressure arsenic does not melt at all — it sublimes, passing straight from solid to vapour at about 887 K (613.9 °C). A liquid phase only exists above roughly 3.63 MPa, where it melts near 1090 K. That is why a single "melting point" for arsenic is misleading without a stated pressure.
The standard atomic weight of Arsenic is 74.922 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
Arsenic has a density of 5.727 g/cm³. Water is 1.0 g/cm³, so a block of arsenic is about 5.7× heavier.
Arsenic has a Pauling electronegativity of 2.18. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Arsenic has been known since antiquity — it occurs in a form usable without smelting, so no single person can be credited with its discovery. The name comes from greek arsenikon, yellow orpiment.
Arsenic makes up about 1.8 mg/kg of the Earth's crust — uncommon, but not rare.