Arsenic

Arsenic is a chemical element; it has the symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is notoriously toxic. It occurs naturally in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.
The primary use of arsenic is in alloys of lead. Arsenic is also a common n-type dopant in semiconductor electronic devices, and a component of the III–V compound semiconductor gallium arsenide. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. These applications are declining with the increasing recognition of the persistent toxicity of arsenic and its compounds.
Arsenic has been known since ancient times to be poisonous to humans. However, a few species of bacteria are able to use arsenic compounds as respiratory metabolites. Trace quantities of arsenic have been proposed to be an essential dietary element in rats, hamsters, goats, and chickens. Research has not been conducted to determine whether small amounts of arsenic may play a role in human metabolism. However, arsenic poisoning occurs in multicellular life if quantities are larger than needed. Arsenic contamination of groundwater is a problem that affects millions of people across the world.
The United States' Environmental Protection Agency states that all forms of arsenic are a serious risk to human health. The United States Agency for Toxic Substances and Disease Registry ranked arsenic number 1 in its 2001 prioritized list of hazardous substances at Superfund sites. Arsenic is classified as a group-A carcinogen.

Characteristics

Physical characteristics

The three most common arsenic allotropes are grey, yellow, and black arsenic, with grey being the most common. Grey arsenic adopts a double-layered structure consisting of many interlocked, ruffled, six-membered rings. Because of weak bonding between the layers, grey arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next. This relatively close packing leads to a high density of 5.73 g/cm³. Grey arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized. Grey arsenic is also the most stable form. Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus. Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecular, is the most volatile, least dense, and most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapor,. It is rapidly transformed into grey arsenic by light. The yellow form has a density of 1.97 g/cm³. Black arsenic is similar in structure to black phosphorus.
Black arsenic can also be formed by cooling vapor at around and by crystallization of amorphous arsenic in the presence of mercury vapors. It is glassy and brittle. Black arsenic is also a poor electrical conductor.
Arsenic sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at. However, at and 2.84 MPa, arsenic melts. The triple point is at 3.63 MPa and.

Isotopes

Arsenic occurs naturally as a single stable isotope, ⁷⁵As. Synthetic radioisotopes are known from ⁶⁴As to ⁹⁵As, as well as at least 11 isomers.
The most stable of these are ⁷³As with a half-life of 80.30 days and ⁷⁴As with a half-life of 17.77 days, followed by ⁷¹As, ⁷⁷As, ⁷⁶As, and ⁷²As. All others have half-lives under 100 minutes and most under one minute. Isotopes lighter than the stable one generally decay by positron emission or electron capture to germanium isotopes, while those heavier beta decay to selenium isotopes. A notable exception is that ⁷⁴As decays both ways.

Chemistry

Arsenic has a similar electronegativity and ionization energies to its lighter pnictogen congener phosphorus and therefore readily forms covalent molecules with most of the nonmetals. Though stable in dry air, arsenic forms a golden-bronze tarnish upon exposure to humidity which eventually becomes a black surface layer. When heated in air, arsenic oxidizes to arsenic trioxide; the fumes from this reaction have an odor resembling garlic. This odor can be detected on striking arsenide minerals such as arsenopyrite with a hammer. It burns in oxygen to form arsenic trioxide and arsenic pentoxide, which have the same structure as the more well-known phosphorus compounds, and in fluorine to give arsenic pentafluoride. Arsenic makes arsenic acid with concentrated nitric acid, arsenous acid with dilute nitric acid, and arsenic trioxide with concentrated sulfuric acid; however, it does not react with water, alkalis, or non-oxidising acids. Arsenic reacts with metals to form arsenides, though these are not ionic compounds containing the As³⁻ ion as the formation of such an anion would be highly endothermic and even the group 1 arsenides have properties of intermetallic compounds. Like germanium, selenium, and bromine, which like arsenic succeed the 3d transition series, arsenic is much less stable in the +5 oxidation state than its vertical neighbors phosphorus and antimony, and hence arsenic pentoxide and arsenic acid are potent oxidizers.

Compounds

Compounds of arsenic resemble, in some respects, those of phosphorus, which occupies the same group of the periodic table. The most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compounds, +3 in the arsenites, and +5 in the arsenates and most organoarsenic compounds. Arsenic also bonds readily to itself as seen in the square ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal owing to the influence of the lone pair of electrons.

Inorganic compounds

One of the simplest arsenic compounds is the trihydride, the highly toxic, flammable, pyrophoric arsine. This compound is generally regarded as stable, since at room temperature it decomposes only slowly. At temperatures of 523–573 K decomposition to arsenic and hydrogen is rapid. Several factors, such as humidity, presence of light and certain catalysts facilitate the rate of decomposition. It oxidises readily in air to form arsenic trioxide and water, and analogous reactions take place with sulfur and selenium instead of oxygen.
Arsenic forms colorless, odorless, crystalline oxides As₂O₃ and As₂O₅ which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic acid is a weak acid and its salts, known as arsenates, are a major source of arsenic contamination of groundwater in regions with high levels of naturally occurring arsenic minerals. Synthetic arsenates include Scheele's Green, calcium arsenate, and lead hydrogen arsenate. These three have been used as agricultural insecticides and poisons.
The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. Unlike phosphorous acid, arsenous acid is genuinely tribasic, with the formula As₃.
A broad variety of sulfur compounds of arsenic are known. Orpiment and realgar are somewhat abundant and were formerly used as painting pigments. In As₄S₁₀, arsenic has a formal oxidation state of +2 in As₄S₄ which features As-As bonds so that the total covalency of As is still 3. Both orpiment and realgar, as well as As₄S₃, have selenium analogs; the analogous As₂Te₃ is known as the mineral kalgoorlieite, and the anion As₂Te⁻ is known as a ligand in cobalt complexes.
All trihalides of arsenic are well known except the astatide, which is unknown. Arsenic pentafluoride is the only important pentahalide, reflecting the lower stability of the +5 oxidation state; even so, it is a very strong fluorinating and oxidizing agent.

Alloys

Arsenic is used as the group 5 element in the III-V semiconductors gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count of GaAs is the same as a pair of Si atoms, but the band structure is completely different which results in distinct bulk properties. Other arsenic alloys include the II-V semiconductor cadmium arsenide.

Organoarsenic compounds

A large variety of organoarsenic compounds are known. Several were developed as chemical warfare agents during World War I, including vesicants such as lewisite and vomiting agents such as adamsite. Cacodylic acid, which is of historic and practical interest, arises from the methylation of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry. Cacodyl was the first organometallic compound known and was named from the Greek κακωδία "stink" for its offensive, garlic-like odor; it is very toxic.

Occurrence and production

Arsenic is the 53rd most abundant element in the Earth's crust, comprising about 1.5 parts per million . Typical background concentrations of arsenic do not exceed 3 ng/m³ in the atmosphere; 100 mg/kg in soil; 400 μg/kg in vegetation; 10 μg/L in freshwater and 1.5 μg/L in seawater. Arsenic is the 22nd most abundant element in seawater and ranks 41st in abundance in the universe.
Minerals with the formula MAsS and MAs₂ are the dominant commercial sources of arsenic, together with realgar and native arsenic. An illustrative mineral is arsenopyrite, which is structurally related to iron pyrite. Many minor As-containing minerals are known. Arsenic also occurs in various organic forms in the environment.
In 2014, China was the top producer of white arsenic with almost 70% world share, followed by Morocco, Russia, and Belgium, according to the British Geological Survey and the United States Geological Survey. Most arsenic refinement operations in the US and Europe have closed over environmental concerns. Arsenic is found in the smelter dust from copper, gold, and lead smelters, and is recovered primarily from copper refinement dust. Arsenic is the main impurity found in copper concentrates to enter copper smelting facilities. There has been an increase in arsenic in copper concentrates over the years since copper mining has moved into deep high-impurity ores as shallow, low-arsenic copper deposits have been progressively depleted.
On roasting arsenopyrite in air, arsenic sublimes as arsenic oxide leaving iron oxides, while roasting without air results in the production of gray arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum, in a hydrogen atmosphere, or by distillation from molten lead-arsenic mixture.

Rank	Country	2014 As₂O₃ Production
1		25,000 T
2		8,800 T
3		1,500 T
4		1,000 T
5		52 T
6		45 T
—	World Total	36,400 T