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 luster, with a rhombohedral crystal lattice. When heated to 600 ° C, As sublimates. When the vapor is cooled, a new modification arises - yellow arsenic. Above 270 ° C, all forms of As become black arsenic.
Discovery story
Arsenic was known long before its recognition as a chemical element. In the 4th century BC e. Aristotle mentioned a substance called "sandarak", which is now believed to be realgar, or arsenic sulfide. And in the 1st century AD e. the writers Pliny the Elder and Pedanius Dioscorides described an auripigment - dye As 2 S 3 . In the XI century. n e. Three types of arsenic were distinguished: white (As 4 O 6 ), yellow (As 2 S 3 ) and red (As 4 S 4 ). The element itself was probably first identified in the 13th century by Albert the Great, who noted the appearance of a metal-like substance when arsenicum, another name As 2 S 3 , was heated with soap. But there is no certainty that this natural scientist received pure arsenic. The first genuine evidence of the release of a pure chemical element is dated 1649. German pharmacist Johann Schroeder prepared arsenic by heating its oxide in the presence of coal. Later, Nicola Lemery, a French doctor 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 is 1.5 ppm. It is found in soil and minerals and can get into the air, water and soil due to 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 80 thousand tons are released as a result of burning fossil fuels over the same period.
Despite the fact that As is a deadly poison, it is an important component of the nutrition of some animals and, possibly, humans, although the required dose does not exceed 0.01 mg / day.
Arsenic is extremely difficult to translate into a water-soluble or volatile state. The fact that it is quite mobile means that large concentrations of the substance cannot appear in any one place. On the one hand, this is good, but on the other, the ease with which it spreads is the reason that arsenic contamination is becoming an increasing problem. Due to human activity, mainly due to extraction and smelting, the usually immobile chemical element migrates, and now it can be found not only in places of its natural concentration.
The amount of arsenic in the earth's crust is about 5 g per ton. In space, its concentration is estimated as 4 atoms per million silicon atoms. This item is widespread. A small amount is present in a native state. As a rule, arsenic formulations with a purity of 90–98% are found together with metals such as antimony and silver. Most of it, however, is part of more than 150 different minerals - sulfides, arsenides, sulfoarsenides and arsenites. Arsenopyrite FeAsS is one of the most common As-containing minerals. Other common arsenic compounds are realgar minerals As 4 S 4, auripigment As 2 S 3, lellingite FeAs 2 and enargite Cu 3 AsS 4 . Arsenic oxide is also common. Most of this substance is a by-product of the smelting of copper, lead, cobalt and gold ores.
In nature, there is only one stable isotope of arsenic - 75 As. Among the artificial radioactive isotopes, 76 As stands out with a half-life of 26.4 hours. Arsenic-72, -74 and -76 are used in medical diagnostics.
Industrial production and application
Metallic arsenic is obtained by heating arsenopyrite to 650-700 ° C without air. If arsenopyrite and other metal ores are heated with oxygen, then As easily joins with it, forming easily sublimated As 4 O 6 , also known as “white arsenic”. The oxide vapors are collected and condensed, and later purified by repeated distillation. Most As is produced by its reduction with carbon 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 because of its metalloid properties. About 1% of 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 are improved in both thermal and mechanical characteristics when they contain about 3% arsenic. The presence of a small amount of this chemical element in lead alloys tempers them for use in rechargeable batteries and cable armor. Small arsenic impurities increase the corrosion resistance and thermal properties of copper and brass. In its pure form, chemical elemental As is used for applying a bronze coating and in pyrotechnics. Highly purified arsenic is used in semiconductor technology, where it is used with silicon and germanium, as well as in the form of gallium arsenide (GaAs) in diodes, lasers and transistors.
Compounds 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 types of compounds. The most important commercial value are its oxides, the main forms of which are As 4 O 6 and As 2 O 5 . 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, serves as a bleaching agent in the manufacture of glass and as a preservative for leathers. Arsenic pentoxide is formed by the action of an oxidizing agent (for example, nitric acid) on white arsenic. It is the main ingredient in insecticides, herbicides and metal adhesives.
Arsine (AsH 3 ), a colorless poisonous gas composed of arsenic and hydrogen, is another known substance. A substance, also called arsenic hydrogen, is obtained by hydrolysis of metal arsenides and reduction of metals from arsenic compounds in acid solutions. He found application as an alloying additive in semiconductors and combat poison gas. In agriculture, arsenic acid (H 3 AsO 4 ), lead arsenate (PbHAsO 4 ) and calcium arsenate [Ca 3 (AsO 4 ) 2 ], which are used for soil sterilization and pest control, are of great importance.
Arsenic is a chemical element that forms many organic compounds. Cacodyne (CH 3 ) 2 As − As (CH 3 ) 2 , for example, is used in the preparation of the widely used desiccant (drying agent) - cacodylate. 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 the most stable state, it is a brittle solid substance of steel gray color with low thermal and electrical conductivity. Although some forms of As are metal-like, assigning it to non-metals is a more accurate characterization of arsenic. There are other types of arsenic, but they are not very well studied, especially the yellow metastable form, consisting of As 4 molecules, like white phosphorus P 4 . Arsenic is sublimated at a temperature of 613 ° C, and in the form of vapor it exists as As 4 molecules that do not dissociate to a temperature of about 800 ° C. Complete dissociation into As 2 molecules occurs at 1700 ° C.
Atom structure and ability to form bonds
The electronic formula of arsenic - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 3 - resembles nitrogen and phosphorus in that there are five electrons in the outer shell, but it differs from them by the presence of 18 electrons in the penultimate shell instead of two or eight. The addition of 10 positive charges in the nucleus during the filling of five 3d orbitals often causes a general 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 is more electronegative than magnesium, and gallium is more than aluminum. However, in subsequent groups this difference decreases, and many disagree with the fact that germanium is electronegative of silicon, despite the abundance of chemical evidence. A similar transition from the 8- to 18-element membrane 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 unbound electron pair. The oxidation state should therefore be +3 or -3, depending on the relative mutual electronegativity. The structure of arsenic also suggests the possibility of using an external 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 (via electron donation) in the As atom appears much less than that of phosphorus and nitrogen.
Arsenic is stable in dry air, but in wet it is covered with black oxide. Its vapors easily burn, forming As 2 O 3 . What is free arsenic? It is practically not affected by water, alkalis and non-oxidizing acids, but it is oxidized by nitric acid to a state of +5. Halogens, sulfur react with arsenic, and many metals form arsenides.
Analytical chemistry
The arsenic substance can be qualitatively detected in the form of a yellow auripigment precipitating under the influence of a 25% hydrochloric acid solution. Traces of As are usually determined by converting it to arsine, which can be detected using the Marsh test. Arsine thermally decomposes, forming a black mirror of arsenic inside a narrow tube. According to the Gutzeit method, a probe impregnated with mercury chloride darkens under the action of arsine due to the release of mercury.
Toxicological characteristics of arsenic
The toxicity of the element and its derivatives varies widely, from extremely toxic arsine and its organic derivatives to just 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% cacodilic acid 26% of its sodium salt) testifies to what arsenic is.
In general, derivatives of this chemical element irritate the skin and cause dermatitis. Protection against inhalation of arsenic-containing dust is also recommended, but most poisoning occurs when ingested. The maximum permissible concentration of As in dust over an eight-hour working day is 0.5 mg / m 3 . For arsine, the dose is reduced to 0.05 ppm. In addition to using 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.
Health effects
Arsenic is one of the most toxic elements. Inorganic compounds of this chemical in vivo are found in small quantities. People can be exposed to arsenic through food, water, and air. Exposure may also occur when the skin comes in contact with contaminated soil or water.
The arsenic content in food is quite low. However, its levels in fish and seafood can be very high, as they absorb this chemical element from the water in which they live. A significant amount of inorganic arsenic in fish can be harmful to human health.
The people who work with it are also exposed to the substance, they live in houses built from the wood processed by it, and on agricultural lands where pesticides were used in the past.
Inorganic arsenic can cause various effects on human health, such as irritation of the stomach and intestines, decreased production of red and white blood cells, skin changes, and lung irritation. It is estimated that absorption of a significant amount of this substance may increase the chances of developing cancer, especially cancer of the skin, lungs, liver, and lymphatic system.
Very high concentrations of inorganic arsenic cause infertility and miscarriages in women, dermatitis, a decrease in the body's 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 can be harmful to human health, for example, cause nervous disorders or abdominal pain.
As Properties
The main chemical and physical properties of arsenic are as follows:
- The atomic number is 33.
- Atomic weight - 74.9216.
- The melting point of the gray form is 814 ° C at a pressure of 36 atmospheres.
- The density of the gray form is 5.73 g / cm 3 at 14 ° C.
- The density of the yellow form is 2.03 g / cm 3 at 18 ° C.
- The electronic formula of arsenic is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 3 .
- The oxidation state is -3, +3, +5.
- Arsenic valency - 3, 5.