arsenic arsenic arsenic is a chemical element with the symbol as and atomic number 33. arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. arsenic is a metalloid. it has various allotropes, but only the gray form, which has a metallic appearance, is important to industry. this article is about the chemical element. for the poison commonly called "arsenic", see arsenic trioxide. for other uses, see arsenic (disambiguation). arsenic, 33asarsenicpronunciation/ˈɑːrsənɪk/(ar-sən-ik)as an adjective: /ɑːrˈsɛnɪk/(ar-sen-ik)allotropesgrey (most common), yellow, black (see allotropes of arsenic)appearancemetallic greystandard atomic weight ar°(as)74.921595±0.00000674.922±0.001 (abridged) arsenic in the periodic table hydrogen helium lithium beryllium boron carbon nitrogen oxygen fluorine neon sodium magnesium aluminium silicon phosphorus sulfur chlorine argon potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon caesium barium lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury (element) thallium lead bismuth polonium astatine radon francium radium actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson p↑as↓sb germanium ← arsenic → selenium atomic number (z)33groupgroup 15 (pnictogens)periodperiod 4block p-blockelectron configuration 3d10 4s2 4p3electrons per shell2, 8, 18, 5physical propertiesphase at stpsolidsublimation point887 k ​(615 °c, ​1137 °f) density (near r.t.)5.727 g/cm3when liquid (at m.p.)5.22 g/cm3 triple point1090 k, ​3628 kpa critical point1673 k, ? mpa heat of fusiongrey: 24.44 kj/mol heat of vaporization34.76 kj/mol (?)molar heat capacity24.64 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 propertiesoxidation states−3, −2, −1, 0, +1, +2, +3, +4, +5 (a mildly acidic oxide)electronegativitypauling scale: 2.18 ionization energies1st: 947.0 kj/mol 2nd: 1798 kj/mol 3rd: 2735 kj/mol (more) atomic radiusempirical: 119 pm covalent radius119±4 pm van der waals radius185 pm spectral lines of arsenicother propertiesnatural occurrenceprimordialcrystal structure ​rhombohedralthermal expansion5.6 µm/(m⋅k) (at r.t.)thermal conductivity50.2 w/(m⋅k) electrical resistivity333 nω⋅m (at 20 °c) magnetic orderingdiamagnetic molar magnetic susceptibility−5.5×10−6 cm3/molyoung's modulus8 gpa bulk modulus22 gpa mohs hardness3.5 brinell hardness1440 mpa cas number7440-38-2 historydiscoveryarabic alchemists (before ad 815)isotopes of arsenic main isotopes decay abun­dance half-life (t1/2) mode pro­duct 73as synth 80.3 d ε 73ge γ – 74as synth 17.8 d ε 74ge β+ 74ge γ – β− 74se 75as 100% stable the primary use of arsenic is in alloys of lead (for example, in car batteries and ammunition). arsenic is a common n-type dopant in semiconductor electronic devices. it is also 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 toxicity of arsenic and its compounds. a few species of bacteria are able to use arsenic compounds as respiratory metabolites. trace quantities of arsenic are an essential dietary element in rats, hamsters, goats, chickens, and presumably other species. a role in human metabolism is not known. 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 as number 1 in its 2001 priority list of hazardous substances at superfund sites. arsenic is classified as a group-a carcinogen. characteristics physical characteristics crystal structure common to sb, assb and gray as the three most common arsenic allotropes are gray, yellow, and black arsenic, with gray being the most common. gray arsenic (α-as, space group r3m no. 166) adopts a double-layered structure consisting of many interlocked, ruffled, six-membered rings. because of weak bonding between the layers, gray 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. gray arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 ev if amorphized. gray arsenic is also the most stable form. yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus (p4). 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, as4. it is rapidly transformed into gray 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 100–220 °c 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. as arsenic's triple point is at 3.628 mpa (35.81 atm), it does not have a melting point at standard pressure but instead sublimes from solid to vapor at 887 k (615 °c or 1137 °f). isotopes arsenic occurs in nature as one stable isotope, 75as, a monoisotopic element. as of 2003, at least 33 radioisotopes have also been synthesized, ranging in atomic mass from 60 to 92. the most stable of these is 73as with a half-life of 80.30 days. all other isotopes have half-lives of under one day, with the exception of 71as (t1/2=65.30 hours), 72as (t1/2=26.0 hours), 74as (t1/2=17.77 days), 76as (t1/2=1.0942 days), and 77as (t1/2=38.83 hours). isotopes that are lighter than the stable 75as tend to decay by β+ decay, and those that are heavier tend to decay by β− decay, with some exceptions. at least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. the most stable of arsenic's isomers is 68mas with a half-life of 111 seconds. chemistry arsenic has a similar electronegativity and ionization energies to its lighter congener phosphorus and accordingly 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 (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 k (614 °c). the triple point is 3.63 mpa and 1,090 k (820 °c). 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 group oxidation state of +5 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 (column) 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 as3−4 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 (ash3). this compound is generally regarded as stable, since at room temperature it decomposes only slowly. at temperatures of 250–300 °c decomposition to arsenic and hydrogen is rapid. several factors, such as humidity, presence of light and certain catalysts (namely aluminium) 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 ("white arsenic") and as2o5 which are hygroscopic and readily soluble in water to form acidic solutions. arsenic(v) acid is a weak acid and the salts are called arsenates, the most common arsenic contamination of groundwater, and a problem that affects many people. synthetic arsenates include scheele's green (cupric hydrogen arsenate, acidic copper arsenate), 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 as(oh)3. a broad variety of sulfur compounds of arsenic are known. orpiment (as2s3) and realgar (as4s4) 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(iii) are well known except the astatide, which is unknown. arsenic pentafluoride (asf5) 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. (the pentachloride is stable only below −50 °c, at which temperature it decomposes to the trichloride, releasing chlorine gas.) 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 trimethylarsine 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 (even though arsenic is not a true metal) and was named from the greek κακωδία "stink" for its offensive odor; it is very poisonous. occurrence and production a large sample of native arsenic arsenic comprises about 1.5 ppm (0.00015%) of the earth's crust, and is the 53rd most abundant element. 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. minerals with the formula mass and mas2 (m = fe, ni, co) are the dominant commercial sources of arsenic, together with realgar (an arsenic sulfide mineral) and native (elemental) arsenic. an illustrative mineral is arsenopyrite (feass), which is structurally related to iron pyrite. many minor as-containing minerals are known. arsenic also occurs in various organic forms in the environment. arsenic output in 2006 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,