ARSENIC

Name: Arsenic (adopted from the original Persian: "zarnikh", "golden")

Symbol: As

Group: 15

Period: 4

Block: p

Category: Metalloids

Atomic Number: 33

Atomic Mass: 74.921 u

Electrons per Shell: 2, 8, 18, 5

Electronegativity: 2.18

Density: 5.727 g/cc

Melting Point: Does not melt, sublimes directly

Boiling Point: 817°C

Thermal Conductivity: 50 W (m K)

Electrical Conductivity: 3.3 × 10^6 S/m

Magnetic Order: Diamagnetic

Ordinary State: Solid

Oxidation States: 5, 4, 3, 2, 1, -1, -2, -3

Mohs Hardness: 3.5

Vickers Hardness: 1510 MPa

Brinell Hardness: 1440 MPa

Most stable isotopes: As-75 (100%)

Discoverer: Known since ancient times

Arsenic (As), with atomic number 33, is a metalloid from group 15, with a density of 5.73 g/cm³ and an abundance of approximately 1.8 ppm in the Earth's crust. Notable for occurring in elemental (native) form in nature, though more commonly in minerals such as arsenopyrite (FeAsS) or realgar (As₄S₄), arsenic has been known since Antiquity for its extreme toxicity and ease of reduction. Its name derives from the Greek arsenikon, related to yellow pigments like orpiment (As₂S₃), and has been documented since at least the 13th century in European texts, although its use dates back to ancient civilizations such as Egypt and China.

Although the German monk Albertus Magnus (13th century) is frequently cited for obtaining pure arsenic from mineral ores by heating, he is not formally credited with the discovery due to a lack of conclusive evidence and the element's prior use in various cultures. For example, in Antiquity, arsenic was used in pigments, medicines, and poisons, exploiting its lethal toxicity. Known as the "king of poisons," its odorless and tasteless nature made it infamous in historical poisonings, from ancient Rome to medieval Europe. In the 19th century, its use as a pesticide and rodenticide became widespread, a practice that persists in some less developed countries in 2025, despite strict environmental regulations in other regions due to its carcinogenicity (classified as a Group 1 carcinogen by IARC).

In alchemy, arsenic held a prominent place, represented by a specific symbol (resembling a swan or an inverted cross). Alchemists, working with a limited number of known elements, valued it both in its pure state and in compounds, such as orpiment and realgar, for transmutation experiments and pigments. Although they lacked the modern notion of the atom, they associated it with transformative properties, often linking it to mercury (Hg) and sulfur (S) in their theories. The global production of arsenic (~35,000 tons annually in 2025) reflects its restricted use in modern applications, such as semiconductors (gallium arsenide, GaAs) and pesticides, while its historical toxicity and role in alchemy make it an element of cultural and scientific relevance.

Arsenic, with atomic number 33, is a metalloid from group 15, with a density of 5.73 g/cm³ and an abundance of approximately 1.8 ppm in the Earth's crust. Although it shares characteristics with other metalloids like silicon (Si) and germanium (Ge), arsenic is distinguished by its more metallic appearance, presenting as a silver-gray solid with a metallic luster that, to untrained eyes, could be mistaken for a p-block metal, such as lead (Pb) or bismuth (Bi). However, its brittle and soft nature, with a hardness of 3.5 on the Mohs scale, makes it easily pulverizable, which limits its mechanical resistance. Its rigidity is low, with a compression strength of ~4 GPa, and it is a poor thermal conductor (50 W/(m·K)) and electrical conductor (~3.3 × 10⁴ S/m), contrasting with true metals like copper (Cu, ~5.9 × 10⁷ S/m).

Chemically, arsenic is remarkably resistant to corrosion, a characteristic that explains its occurrence in elemental (native) form in nature, although it is more common in combined minerals such as arsenopyrite (FeAsS) or orpiment (As₂S₃). This relative stability, comparable to that of noble metals like gold (Au) or silver (Ag), allows arsenic to resist reaction with oxygen (O₂) and sulfur (S) under ambient conditions, although it forms compounds easily in the presence of stronger reagents. It resists oxidizing acids like nitric acid (HNO₃) at room temperature, but is attacked by reducing acids like hydrofluoric acid (HF) and by hot alkalis, such as sodium hydroxide (NaOH). Its native state, although rare, makes it more dangerous than its compounds, as elemental arsenic releases fine particles when pulverized, increasing its toxicity by inhalation or ingestion. In contrast, compounds like gallium arsenide (GaAs) or arsenic trioxide (As₂O₃) are less volatile, although they remain dangerous due to their carcinogenicity (classified as a Group 1 carcinogen by IARC).

The duality of arsenic, with a metallic appearance but metalloid properties, distinguishes it from its group 15 neighbors. Its fragility and low conductivity make it unsuitable for structural applications, but its chemical stability and intrinsic toxicity have defined its historical use as a poison and pesticide. In 2025, global arsenic production (~35,000 tons annually) focuses on limited applications, such as semiconductors and chemical treatments, where its toxicity requires strict handling. Similar to fluorine (F), whose elemental form is extremely reactive and dangerous while its compounds are more manageable, pure arsenic represents a significant risk, reinforcing its reputation as the "king of poisons" in history and culture.

Arsenic, with atomic number 33, is a metalloid from group 15, with a density of 5.73 g/cm³ and an abundance of approximately 1.8 ppm in the Earth's crust. Known since Antiquity for its extreme toxicity, arsenic has been historically used as a poison, pesticide, and in medicinal applications, although its modern use is strictly regulated due to its carcinogenicity (classified as a Group 1 carcinogen by IARC). With a global production of about 35,000 tons annually in 2025, its chemical versatility and physical properties have made it relevant in metallurgy, electronics, and agriculture, despite the associated risks.

Historically, the most infamous use of arsenic was as a lethal poison. Finely pulverized, either in elemental form or as compounds like arsenic trioxide (As₂O₃), its odorless and tasteless nature allowed its use in poisonings, from ancient Rome to medieval Europe, acting as a primitive biological weapon. Its ability to cause death without evident traces made it a feared tool, although its toxicity, explained by interference with cellular enzymatic processes, lacks any intrinsic moral burden. Beyond its use as a poison, during the Victorian era in England, arsenic was applied in cosmetics or ingested in small doses to whiten the skin, a symbol of social status associated with paleness, in contrast to the tanned complexion of working classes exposed to the sun. This use, which produced a pale tone with rosy undertones on the cheeks, continued in some cultures until the 20th century, despite known risks.

In metallurgy, arsenic was used until the 20th century to harden lead-based alloys, such as those used in bullets and batteries, and in certain bronzes (Cu-Sn-As), improving mechanical resistance. However, it has been replaced by antimony (Sb), which offers similar properties with less toxicity. The chemical similarity between both elements, both from group 15, led to historical confusions, but antimony is now preferred in applications such as pewter or solder alloys. In medicine, arsenic played a significant role until the early 20th century, particularly in the treatment of syphilis through compounds like Salvarsan (an organoarsenic derivative), in an era where the lack of alternatives justified its use despite its known toxicity.

Currently, arsenic is mainly employed in the manufacture of pesticides, such as lead arsenate, in countries with less strict regulations, although its use is prohibited or restricted in many nations due to its environmental and health impact. In electronics, gallium arsenide (GaAs) is a key semiconductor, with a band gap of 1.43 eV that makes it ideal for light-emitting diodes (LEDs), solar cells, and high-frequency circuits, surpassing silicon (Si) in high-speed applications. Arsenic's resistance to corrosion (~6 Mohs, resistant to acids like HNO₃, but vulnerable to HF) and its ability to form stable compounds, such as As₂S₃ in historical pigments, reinforce its relevance, although its handling requires strict precautions due to its toxicity. Limited production and environmental regulations have reduced its use, but its historical legacy and specialized applications keep it as an element of scientific and cultural importance.