Arsenic is a chemical element of the nitrogen group (group 15 of the periodic table). It is a brittle substance (α-arsenic) gray with a metallic sheen with a rhombohedral crystal lattice. When heated to 600°C, As sublimates. When the vapor is cooled, a new modification appears - yellow arsenic. Above 270°C, all As forms turn into black arsenic.
Discovery history
Arsenic was known long before it was recognized as a chemical element. In the IV century. BC e. Aristotle mentioned a substance called sandarac, which is now believed to be realgar, or arsenic sulfide. And in the 1st century A. D. e. the writers Pliny the Elder and Pedanius Dioscorides described orpiment - the dye As2S3. In the XI century. n. e. three varieties of "arsenic" were distinguished: white (As4O6), yellow (As2S 3) and red (As4S4). The element itself was probably first isolated in the 13th century by Albert the Great, who noted the appearance of a metal-like substance when arsenicum, another name As2S3, was heated with soap. But there is no certainty that this natural scientist received pure arsenic. The first authentic evidence of the isolation of a pure chemical elementdated 1649. German pharmacist Johann Schroeder prepared arsenic by heating its oxide in the presence of coal. Later, Nicolas Lemery, a French physician and chemist, observed the formation of this chemical element by heating a mixture of its oxide, soap and potash. By the beginning of the 18th century, arsenic was already known as a unique semimetal.
Prevalence
In the earth's crust, the concentration of arsenic is low and amounts to 1.5 ppm. It occurs in soil and minerals and can be released into the air, water and soil through wind and water erosion. In addition, the element enters the atmosphere from other sources. As a result of volcanic eruptions, about 3 thousand tons of arsenic are released into the air per year, microorganisms form 20 thousand tons of volatile methylarsine per year, and as a result of burning fossil fuels, 80 thousand tons are released over the same period.
Despite the fact that As is a deadly poison, it is an important part of the diet of some animals and possibly humans, although the required dose does not exceed 0.01 mg / day.
Arsenic is extremely difficult to convert into a water-soluble or volatile state. The fact that it is quite mobile means that large concentrations of the substance in any one place cannot appear. On the one hand, this is good, but on the other hand, the ease with which it spreads is the reason why arsenic pollution is becoming an increasing problem. Due to human activities, mainly through mining and smelting, a normally immobile chemical element migrates, and now it can be found not only in placesits natural concentration.
The amount of arsenic in the earth's crust is about 5 g per ton. In space, its concentration is estimated at 4 atoms per million silicon atoms. This element is widespread. A small amount is present in the native state. As a rule, formations of arsenic with a purity of 90–98% are found together with metals such as antimony and silver. Most of it, however, is included in the composition of more than 150 different minerals - sulfides, arsenides, sulfoarsenides and arsenites. Arsenopyrite FeAsS is one of the most common As-bearing minerals. Other common arsenic compounds are realgar minerals As4S4, orpiment As2S 3, lellingite FeAs2 and enargite Cu3AsS4. Arsenic oxide is also common. Most of this substance is a by-product of smelting copper, lead, cob alt and gold ores.
In nature, there is only one stable isotope of arsenic - 75As. Among artificial radioactive isotopes, 76As with a half-life of 26.4 hours stands out. Arsenic-72, -74 and -76 are used in medical diagnostics.
Industrial production and application
Metal arsenic is obtained by heating arsenopyrite to 650-700 °C without air. If arsenopyrite and other metallic ores are heated with oxygen, then As easily enters into combination with it, forming easily sublimated As4O6, also known like "whitearsenic". The oxide vapor is collected and condensed, and later purified by resublimation. Most As is produced by carbon reduction from white arsenic thus obtained.
World consumption of metallic arsenic is relatively small - only a few hundred tons per year. Most of what is consumed comes from Sweden. It is used in metallurgy due to its metalloid properties. About 1% arsenic is used in the production of lead shot, as it improves the roundness of the molten drop. The properties of lead-based bearing alloys improve both thermally and mechanically when they contain about 3% arsenic. The presence of a small amount of this chemical element in lead alloys hardens them for use in batteries and cable armor. Small impurities of arsenic increase the corrosion resistance and thermal properties of copper and brass. In its pure form, chemical elemental As is used for bronze plating and in pyrotechnics. Highly purified arsenic finds use in semiconductor technology, where it is used with silicon and germanium, and in the form of gallium arsenide (GaAs) in diodes, lasers and transistors.
Connections As
Since the valency of arsenic is 3 and 5, and it has a number of oxidation states from -3 to +5, the element can form various kinds of compounds. Most important commercially are its oxides, the main forms of which are As4O6 andAs2O5. Arsenic oxide, commonly known as white arsenic, is a by-product of roasting ores of copper, lead, and some other metals, as well as arsenopyrite and sulfide ores. It is the starting material for most other compounds. In addition, it is used in pesticides, as a bleaching agent in glass manufacture, and as a preservative for leathers. Arsenic pentoxide is formed by the action of an oxidizing agent (eg, nitric acid) on white arsenic. It is the main ingredient in insecticides, herbicides and metal adhesives.
Arsine (AsH3), a colorless poisonous gas composed of arsenic and hydrogen, is another known substance. The substance, also called arsenic hydrogen, is obtained by the hydrolysis of metal arsenides and the reduction of metals from arsenic compounds in acid solutions. It has found use as a dopant in semiconductors and as a military poison gas. In agriculture, arsenic acid (H3AsO4), lead arsenate (PbHAsO444) and calcium arsenate [Ca3(AsO4)
2], which are used to sterilize soil and pest control.
Arsenic is a chemical element that forms many organic compounds. HowOne (CH3)2As−As(CH3)2 , for example, is used in the preparation of a widely used desiccant (desiccant) - cacodylic acid. Complex organic compounds of the element are used in the treatment of certain diseases, for example, amoebic dysentery,caused by microorganisms.
Physical properties
What is arsenic in terms of its physical properties? In its most stable state, it is a brittle, steely gray solid with low thermal and electrical conductivity. Although some forms of As are metal-like, classifying it as a non-metal is a more accurate characterization of arsenic. There are other types of arsenic, but they are not well studied, especially the yellow metastable form, consisting of As4 molecules, similar to white phosphorus P4. Arsenic sublimates at 613°C and exists as a vapor as As4 molecules that do not dissociate up to about 800°C. Complete dissociation into As2 molecules occurs at 1700 °C.
The structure of the atom and the ability to form bonds
The electronic formula of arsenic is 1s22s22p63s 23p63d104s24p3 - resembles nitrogen and phosphorus in that it has five electrons in the outer shell, but it differs from them by having 18 electrons in the penultimate shell instead of two or eight. The addition of 10 positive charges in the nucleus while filling five 3d orbitals often causes an overall decrease in the electron cloud and an increase in the electronegativity of the elements. Arsenic in the periodic table can be compared with other groups that clearly demonstrate this pattern. For example, it is generally accepted that zinc ismore electronegative than magnesium and gallium than aluminum. However, in subsequent groups, this difference narrows, and many disagree that germanium is more electronegative than silicon, despite the abundance of chemical evidence. A similar transition from 8- to 18-element shell from phosphorus to arsenic can increase electronegativity, but this remains controversial.
The similarity of the outer shell of As and P suggests that they can form 3 covalent bonds per atom in the presence of an additional unbonded electron pair. The oxidation state must therefore be +3 or -3, depending on the relative mutual electronegativity. The structure of arsenic also speaks to the possibility of using the outer d-orbital to expand the octet, which allows the element to form 5 bonds. It is realized only by reaction with fluorine. The presence of a free electron pair for the formation of complex compounds (through electron donation) in the As atom is much less pronounced than in phosphorus and nitrogen.
Arsenic is stable in dry air, but in wet air it becomes covered with black oxide. Its vapor burns easily, forming As2O3. What is free arsenic? It is practically unaffected by water, alkalis and non-oxidizing acids, but is oxidized by nitric acid to a state of +5. Halogens, sulfur react with arsenic, and many metals form arsenides.
Analytical chemistry
The substance arsenic can be qualitatively detected as a yellow orpiment that precipitates under the influence of 25%hydrochloric acid solution. Traces of As are generally determined by converting it to arsine, which can be detected using the Marsh test. Arsine thermally decomposes, forming a black arsenic mirror inside a narrow tube. According to the Gutzeit method, a probe impregnated with mercury chloride, under the influence of arsine, darkens due to the release of mercury.
Toxicological characteristics of arsenic
The toxicity of the element and its derivatives varies widely over a wide range, from the extremely poisonous arsine and its organic derivatives to simply As, which is relatively inert. The use of its organic compounds as chemical warfare agents (lewisite), vesicant and defoliant (Agent Blue based on an aqueous mixture of 5% cacodylic acid and 26% of its sodium s alt) tells us what arsenic is.
In general, derivatives of this chemical element irritate the skin and cause dermatitis. Inhalation protection against arsenic-containing dust is also recommended, but most poisoning occurs when it is ingested. The maximum permissible concentration of As in dust for an eight-hour working day is 0.5 mg/m3. For arsine, the dose is reduced to 0.05 ppm. In addition to the use of compounds of this chemical element as herbicides and pesticides, the use of arsenic in pharmacology made it possible to obtain salvarsan, the first successful drug against syphilis.
He alth effects
Arsenic is one of the most toxic elements. Inorganic compounds of a given chemicalSubstances naturally occur in small quantities. Humans can be exposed to arsenic through food, water, and air. Exposure can also occur through skin contact with contaminated soil or water.
The content of arsenic in food is quite low. However, levels in fish and seafood can be very high as they absorb the chemical from the water they live in. Significant amounts of inorganic arsenic in fish can pose a risk to human he alth.
People who work with the substance, live in houses built from wood treated with it, and on agricultural land where pesticides have been used in the past are also exposed to the substance.
Inorganic arsenic can cause various he alth effects in humans, such as stomach and intestinal irritation, reduced production of red and white blood cells, skin changes, and lung irritation. It is believed that ingesting significant amounts of this substance may increase the chances of developing cancer, especially skin, lung, liver, and lymphatic system cancers.
Very high concentrations of inorganic arsenic cause infertility and miscarriage in women, dermatitis, reduced resistance to infections, heart problems and brain damage. In addition, this chemical element can damage DNA.
The lethal dose of white arsenic is 100 mg.
Organic compounds of the element do not cause cancer or damage to the genetic code, but high doses cancause harm to human he alth, such as causing nervous disorders or abdominal pain.
Properties As
The main chemical and physical properties of arsenic are as follows:
- Atomic number - 33.
- Atomic weight is 74.9216.
- The melting point of the gray mold is 814 °C at a pressure of 36 atmospheres.
- Grey Density 5.73g/cm3 at 14°C.
- Yellow mold density 2.03 g/cm3 at 18°C.
- The electronic formula of arsenic is 1s22s22p63s 23p63d104s24p3 .
- Oxidation states – -3, +3, +5.
- Arsenic valency is 3, 5.
