Arsen

Arsen Description

Arsen is a chemical element with the symbol As and atomic number 33. It occurs in numerous minerals, usually in association with sulphur and metals, and it also exists as a pure elemental crystal. Arsen is classified as a metalloid. It exists in several allotropes. Only the grey allotrope with a metallic appearance holds industrial significance.

Arsen is primarily used in lead alloys. It is added to components in automotive batteries and ammunition. Arsen serves as an n-type dopant in electronic semiconductor devices. The optoelectronic compound gallium arsenide is the second most used semiconductor after doped silicon. Arsen and its compounds, particularly the trioxide, are employed in the production of pesticides, treated wood products, herbicides, and insecticides. Their use has diminished given that the toxicity of arsenic and its compounds is increasingly recognised.

Arsenic Compounds

Arsenic compounds share certain chemical properties with those of phosphorus because both elements belong to the same group in the periodic table. The predominant oxidation states of arsenic are –3 in arsenides, +3 in arsenites, and +5 in arsenates and most organoarsenic compounds. Arsenic forms bonds with itself to yield species such as the square As₃⁻₄ ions observed in the mineral skutterudite. In the +3 state, the presence of a lone electron pair produces a pyramidal molecular geometry.

Inorganic Compounds

One of the simplest arsenic compounds is the trihydride, AsH₃. This compound is highly toxic, flammable, and pyrophoric. It remains relatively stable at room temperature because it decomposes slowly. At temperatures between 250–300 °C, it decomposes rapidly into arsenic and hydrogen. Moisture, light, and certain catalysts (particularly aluminium) increase the decomposition rate. It oxidises in air to form arsenic trioxide and water, and analogous reactions occur when sulphur or selenium replace oxygen.

Arsenic forms colourless, odourless, crystalline oxides As₂O₃ (commonly known as “white arsenic”) and As₂O₅. These oxides are hygroscopic and dissolve readily in water to yield acidic solutions. Arsenic(V) acid acts as a weak acid. Its salts, called arsenates, are the most common arsenic contaminants in groundwater and affect many communities. Synthetic arsenates include Scheele’s Green (copper hydrogen arsenate, acidic copper arsenate), calcium arsenate, and lead hydrogen arsenate. These compounds have been used as agricultural insecticides and toxicants.

Protonation between arsenate and arsenic acid follows the same sequence as that observed between phosphate and phosphoric acid. Unlike phosphorous acid, arsenious acid is a true tribasic acid with the formula As(OH)₃.

A variety of sulphide compounds of arsenic have been identified. Orpiment (As₂S₃) and realgar (As₄S₄) occur relatively frequently and were historically used as pigments. In As₄S₁₀, arsenic assumes a formal oxidation state of +2 because of As–As bonds, while its overall valence remains 3. Orpiment, realgar, and As₄S₃ each have selenide analogues; the compound As₂Te₃ is found as the mineral kalgoorlieite, and the anion As₂Te⁻ is identified as a ligand in cobalt complexes.

All trihalides of arsenic(III) are well documented except the astatide, which remains unknown. Arsenic pentafluoride (AsF₅) is the only significant pentahalide, reflecting the lower stability of the +5 state. It functions as a strong fluorinating and oxidising agent. The pentachloride is stable only below –50 °C; at these temperatures, it decomposes with the release of chlorine gas into the trichloride.

Alloys

Arsenic is incorporated in group V III–V semiconductors such as gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count in GaAs is equivalent to that of a silicon atomic pair; the band structure, however, differs. A further arsenic alloy is the II–V semiconductor cadmium arsenide.

Arsenic Applications

Agriculture

The toxicity of arsenic to insects, bacteria, and fungi led to its use as a wood preservative. It was also applied in various agricultural insecticides and pesticides. In the poultry and pig farming sectors in the USA, arsenic is added to animal feed to increase weight gain, improve feed efficiency, and help prevent disease.

Medicine

During the 18th, 19th, and 20th centuries, several arsenic compounds were utilised as medicinal agents. These agents included arsphenamine, developed by Paul Ehrlich, and arsenic trioxide, developed by Thomas Fowler. Over a period of 500 years, arsenic trioxide was applied in cancer treatment and in medications such as Fowler’s solution for psoriasis. Recently, researchers have used arsenic-74, a positron emitter, to locate tumours. This isotope yields clearer PET scan images compared with radioactive iodine-124, which accumulates in the thyroid and increases background noise. Arsenic nanoparticles have been shown to kill cancer cells with lower cytotoxicity than other arsenic formulations.

Alloy

The primary application of arsenic is in lead alloys. A small quantity of arsenic is added to lead components in car batteries. Its inclusion reduces the dezincification of brass (a copper–zinc alloy). “Phosphorised arsenic-containing copper” with an arsenic content of 0.3% shows improved corrosion resistance in certain environments. Gallium arsenide is an essential semiconductor material used in integrated circuits. Circuits produced from GaAs operate at higher speeds than those produced from silicon; however, their manufacture incurs higher costs. Owing to its direct bandgap, GaAs is used in laser diodes and LEDs to convert electrical energy into light.

Military

After the First World War, the United States produced a stockpile of 20 000 tonnes of munitions-grade lewisite. This organoarsenic blister agent acts as a pulmonary irritant. The stockpile was neutralised with bleach and discharged into the Gulf of Mexico during the 1950s. During the Vietnam War, the United States deployed Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the herbicides to remove vegetative cover from North Vietnamese soldiers.