Arsenic

33 germanium ← arsenic → selenium P ↑ As ↓ Sb periodic table
General
Name, Symbol, Number	arsenic, As, 33
Chemical series	metalloids
Group, Period, Block	15, 4, p
Appearance	metallic gray
Atomic mass	74.92160(2) g/mol
Electron configuration	[Ar] 3d10 4s2 4p3
Electrons per shell	2, 8, 18, 5
Physical properties
Phase	solid
Density (near r.t.)	5.727 g/cm³
Liquid density at m.p.	5.22 g/cm³
Melting point	1090 K (817 °C, 1503 °F)
Boiling point	subl. 887 K (614 °C, 1137 °F)
Critical temperature	1673 K
Heat of fusion	(gray) 24.44 kJ/mol
Heat of vaporization	? 34.76 kJ/mol
Heat capacity	(25 °C) 24.64 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 553 596 646 706 781 874
Atomic properties
Crystal structure	rhombohedral
Oxidation states	±3, 5 (mildly acidic oxide)
Electronegativity	2.18 (Pauling scale)
Ionization energies (more)	1st: 947.0 kJ/mol
	2nd: 1798 kJ/mol
	3rd: 2735 kJ/mol
Atomic radius	115 pm
Atomic radius (calc.)	114 pm
Covalent radius	119 pm
Van der Waals radius	185 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 333 nΩ·m
Thermal conductivity	(300 K) 50.2 W/(m·K)
Speed of sound (thin rod)	(r.t.) 8 m/s
Bulk modulus	22 GPa
Mohs hardness	3.5
Brinell hardness	1440 MPa
CAS registry number	7440-38-2
Notable isotopes
Main article: Isotopes of arsenic iso NA half-life DM DE (MeV) DP 73As syn 80.3 d ε - 73Ge γ 0.05D, 0.01D, e - 74As syn 17.78 d ε - 74Ge β+ 0.941 74Ge γ 0.595, 0.634 - β- 1.35, 0.717 74Se 75As 100% As is stable with 42 neutrons

Arsenic (IPA: (chemical symbol As, atomic number 33) is a notoriously poisonous metalloid. It comes in many allotropic forms, of which the yellow, black, and gray forms are regularly seen. Arsenic and its compounds are used as pesticides, herbicides, insecticides, and various alloys.

Some compounds—particularly Paris Green, calcium arsenate, and lead arsenate—have been used as agricultural insecticides and poisons. Other compounds, namely, orpiment and realgar, were formerly used as painting pigments, though they have somewhat fallen out of use due to their toxicity and reactivity.

Given its incredible potency as a poison and the ease with which it could be used surreptitiously, arsenic was used by murderers, including members of the ruling class, to gain wealth, position, and power. It was therefore called the "Poison of Kings and the King of Poisons."

Occurrence

Massive native arsenic.

The most common mineral of arsenic is arsenopyrite, also called mispickel (FeSAs). When this mineral is heated, the arsenic sublimes (goes directly to the vapor state), leaving ferrous sulfide. Other arsenic minerals include realgar, mimetite, cobaltite, and erythrite.

<>It is sometimes found native, but usually combined with silver, cobalt, nickel, iron, antimony, or sulfur.

<>The most important inorganic compounds of arsenic are white arsenic, orpiment, realgar, Paris Green, calcium arsenate, and lead hydrogen arsenate. In addition, arsenic occurs in various organic forms in the environment. Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized to a less toxic form of arsenic through a process of methylation.

History

Arsenic has been known and used in Persia and elsewhere since ancient times. The word arsenic is borrowed from the Persian word زرنيخ Zarnikh, meaning "yellow orpiment." Zarnikh was borrowed by the Greek language as arsenikon. During the Bronze Age, arsenic was often included in the bronze (mostly as an impurity), which made the alloy harder.

Albertus Magnus is believed to have been the first to isolate the element in 1250. In 1649 Johann Schroeder published two ways of preparing it.

The alchemical symbol for arsenic is shown below.

As the symptoms of arsenic poisoning were somewhat ill-defined, and it could be used discreetly, arsenic was frequently used as an agent for murder. Its use for this purpose declined after the English chemist James Marsh perfected a sensitive chemical test (the "Marsh test") to detect its presence. A less sensitive but more general test is the Reinsch test.

Notable characteristics

In the periodic table, arsenic is located in group 15 (formerly group 5A), between phosphorus and antimony. It is thus a member of the nitrogen family of elements, sometimes called the pnictogens (or pnicogens). It lies in period 4, between germanium and selenium.

Elemental arsenic is found in two solid forms: yellow and gray/metallic, with specific gravities of 1.97 and 5.73, respectively. When heated rapidly, it oxidizes to arsenic trioxide; the fumes from this reaction have an odor resembling garlic. The element and some of its compounds can also sublimate upon heating, converting directly to a gaseous form.

As it is chemically very similar to phosphorus, arsenic will partly substitute for phosphorus in biochemical reactions, thereby poisoning the living organism.

Isotopes

Multiple isotopes of arsenic are known, but nearly all of them are radioactive, with extremely short half-lives. Only one isotope, ⁷⁵As, is stable.

Arsenic has been proposed as a "salting" material for nuclear weapons—that is, as an agent that could increase the radioactivity of the weapon's fallout for several hours. Such a weapon is not known to have ever been built, tested, or used.

Applications

Lead hydrogen arsenate has been used, well into the twentieth century, as an insecticide on fruit trees, (resulting in brain damage to those working the sprayers), and Scheele's Green has even been recorded in the nineteenth century as a coloring agent in sweets. In the last half century, monosodium methyl arsenate (MSMA), a less toxic organic form of arsenic, has replaced lead arsenate's role in agriculture.

The application of most concern to the general public is probably that of wood which has been treated with chromated copper arsenate ("CCA", or "Tanalith", and the vast majority of older, "pressure-treated" wood). CCA timber is still in widespread use in many countries, and was heavily used during the latter half of the twentieth century as a structural, and outdoor building material, where there was a risk of rot, or insect infestation in untreated timber. Although widespread bans followed the publication of studies which showed low-level leaching from in-situ timbers (such as children's playground equipment) into surrounding soil, the most serious risk is presented by the burning of CCA timber. Recent years have seen fatal animal poisonings, and serious human poisonings resulting from the ingestion - directly or indirectly - of wood ash from CCA timber (the lethal human dose is approximately 20 grams of ash). Scrap CCA construction timber continues to be widely burnt through ignorance, in both commercial, and domestic fires. Safe disposal of CCA timber remains patchy, and little practiced, there is concern in some quarters about the widespread landfill disposal of such timber.

During the eighteenth, nineteenth, and twentieth centuries, a number of arsenic compounds have been used as medicines, including arsphenamine (by Paul Ehrlich) and arsenic trioxide (by Thomas Fowler). Arsphenamine as well as Neosalvarsan was indicated for syphilis and trypanosomiasis, but has been superseded by modern antibiotics. Arsenic trioxide has been used in a variety of ways over the past 200 years, but most commonly in the treatment of cancer. The Food and Drug Administration in 2000 approved this compound for the treatment of patients with acute promyelocytic leukemia that is resistant to ATRA. It was also used as Fowler's solution in psoriasis.

Copper acetoarsenite was used as a green pigment known under many different names, including Paris Green and Emerald Green. It caused numerous arsenic poisonings.

Other uses;

• Various agricultural insecticides and poisons.
• Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made using the compound are much faster (but also much more expensive) than those made in silicon. Unlike silicon it is direct bandgap, and so can be used in laser diodes and LEDs to directly convert electricity into light.
• Also used in bronzing and pyrotechny.

Arsenic in drinking water

Arsenic contamination of groundwater has been reported to occur in various parts of the world. The most notable case, in the late twentieth century, was that of a massive epidemic of arsenic poisoning in Bangladesh and neighboring countries. This arsenic is of natural origin, being released from sediment into the groundwater due to anoxic (oxygen-deficient) conditions of the subsurface. The groundwater began to be used after Western non-governmental organizations began a massive, tube-well drinking-water program. The program was intended to prevent people from drinking bacterially contaminated surface waters, but it unfortunately failed to test for arsenic in the groundwater. Many other countries in South and Southeast Asia—such as Vietnam, Cambodia, and Tibet—are thought to have geological environments similarly conducive to the generation of high-arsenic groundwaters.

The northern United States, including parts of Michigan, Wisconsin, Minnesota and the Dakotas, are known to have significant concentrations of arsenic in groundwater.

Arsenic can be removed from drinking water through co-precipitation of iron minerals by oxidation and filtering. When this treatment fails to produce acceptable results, more of the arsenic can be removed by using specialized media to which arsenic will bind. Several such "adsorptive media systems" have been approved for point-of-service use, based on a study funded by the U.S. Environmental Protection Agency (EPA) and the National Science Foundation (NSF).

Precautions

Arsenic and many of its compounds are especially potent poisons. They inhibit the normal action of enzymes in the body and disrupt various metabolic pathways. These metabolic interferences lead to death from multi-system organ failure. The presence of arsenic in drinking water, especially in the form of arsenate and arsenite ions, is known to cause arsenicosis.

The European Union has classified elemental arsenic and arsenic compounds as "toxic" and "dangerous for the environment." It lists arsenic trioxide, arsenic pentoxide, and arsenate salts as category 1 carcinogens.

Growing the Brake (fern) Pteris vittata will remove arsenic from the soil.

Compounds

• Arsenic acid (H₃AsO₄)
• Arsenous acid (H₃AsO₃)
• Arsenic trioxide (As₂O₃)
• Arsine (Arsenic Trihydride AsH₃)
• Cadmium arsenide (Cd₃As₂)
• Gallium arsenide (GaAs)
• Lead hydrogen arsenate (PbHAsO₄)

See also Arsenic compounds.

See also

• Chemical element
• Periodic table
• Aqua Tofana
• Arsenic poisoning
• Arsenicosis

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Arsenic

External links

• A Small Dose of Toxicology
• Case Studies in Environmental Medicine: Arsenic Toxicity
• National Pollutant Inventory - Arsenic
• WebElements.com – Arsenic
• Dartmouth Toxic Metals Research Program
• origen.net – CCA wood and arsenic: toxicological effects of arsenic
• Contaminant Focus: Arsenic by the EPA.
• Environmental Health Criteria for Arsenic and Arsenic Compounds, 2001 by the World Health Organization.
• A summary of the above report by GreenFacts.
• Evaluation of the carcinogenicity of arsenic and arsenic compounds by the IARC.

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