Before considering the degree of oxidation of arsenic, we reveal its position in the periodic table and structural features. In addition, we name the main physical and chemical properties of this chemical element.
The situation in the PS
The main degrees of oxidation of arsenic, chemical properties, application - all this can be found by looking at the location of the element in the periodic system of Mendeleev. Arsenic is located in the fifth group, the main subgroup, is a representative of the nitrogen family. It has a relative atomic mass of 74.9216. The main oxidation states of arsenic are associated precisely with the group in which it is located in the PS. This item has 33 serial numbers. The number of energy levels corresponds to the number of the period in which the element is located, it is equal to four.
Consider the arrangement of electrons on each atomic shell. At the first energy level there are only two paired electrons, the second shell is occupied by eight particles: 2s and 6p. At the third level, in addition to them, there are ten d-electrons, that is, only 18 particles. The external energy level represents 2s electrons, as well as three unpaired p-electrons. It is the number of valence electrons that determines the possible oxidation states of arsenic.
Pages of history
It is arsenic that belongs to the five "alchemical" elements known from the Middle Ages. An interesting point is the fact that four of them are in the fifth group of PS. In those days, nobody knew how to determine the degree of oxidation of arsenic, but its compounds were successfully used for the manufacture of medicines and the creation of paints.
After the change of the Stone Age by the Bronze Age, people learned how to make this alloy with special characteristics. It turns out that up to 7 percent of arsenic and only 3 percent of tin were in its composition. Scientists believe that during the first smelting of bronze, instead of green malachite, green sulfides of copper-arsenic minerals were mistakenly taken.
The amazing performance of the alloy made it popular among the ancient masters. They specifically searched in nature for minerals containing the substance.
The oxidation states of arsenic in compounds of this type are positive and correspond to its higher valency. To identify sulfides containing arsenic, the mineral is heated. The appearance of a specific garlic odor confirms the presence of arsenic in the compound. Gradually, smelting of arsenic bronze was abandoned. Among the reasons for the cessation of production, scientists call the constant poisoning of the masters during work.
Natural minerals
In the form of a mineral, the substance under consideration has been known since ancient times. For example, oxidation state 3 shows arsenic in a compound known in ancient China as “mine dust”. Aristotle described the mineral sandarak, which is arsenic sulfide. Translated from Latin, its name sounds like “gold paint”. Used it in those days as a yellow dye.
In the eleventh century, three different types of this substance were distinguished by alchemists. The oxidation states of arsenic in the compounds represented by these varieties correspond to the group number. Its hexavalent oxide was called white arsenic, sulfide was called yellow, and As4S4 (tetrasulfide tetrasulfide) was called red.
The white version was obtained by sublimating impurities during the roasting of copper ores, which included arsenic. As condensation occurs from the gaseous state, arsenic oxide settles as a white coating. Since ancient times, it has been used as a means to kill pests.
In the thirteenth century, Albert the Great obtained a metal-like substance. He heated yellow arsenic with soap. The substance obtained as a result of the interaction did not confirm the “mystical connection” of the seven metals with the planets. Perhaps, precisely because of the contradiction revealed by the ancient alchemists, arsenic was considered an “illegitimate” element. It was in those distant days that his ability to give copper a white color was revealed, due to which he began to be called a means of “bleaching Venus”.
As an individual substance, this chemical element was identified only in the middle of the seventeenth century. The German pharmacist Johann Schroeder managed to isolate it by conducting a chemical reduction of coal oxides. After some time, Nicola Lemery managed to isolate the metal by heating potash, soap, arsenic oxide. In the 18th century, this metal was known as an unusual "semimetal".
At the end of the 18th century, the Swedish chemist K.V. Scheele obtained arsenic acid, in which the highest degree of arsenic oxidation is manifested: +5. In the nineteenth century, organic substances that contain arsenic were identified.
Being in nature
The higher and lower oxidation states of arsenic are manifested in its natural compounds. In the earth's crust, the percentage concentration of this element does not exceed 5 grams per ton. In many minerals, it is contained simultaneously with nickel, cobalt, copper, and iron.
At present, about two hundred different natural minerals are known, which include the chemical element we are considering. Given that they manifest the highest and lowest oxidation state of arsenic, they have different uses. For example, in combination with antimony, arsenic exhibits a negative oxidation state. Given the fact that this metal has a low electronegativity, the lowest oxidation state of arsenic is -3. This indicator is characteristic for arsenides, as well as for the mineral alemontite.
Most of the compounds with arsenic metals, given the specific nature of the composition, are intermetallic compounds that differ in the variable composition of this chemical element.
Arsenide Characterization
Arsenides are characterized by the content of several metals having a similar structure of crystal lattices. These minerals are characterized by a metallic luster, they are opaque, have low hardness.
The following compounds can be considered as examples of natural arsenides:
- Lellingite, similar to pyrite;
- nickel, referred to as nickel red pyrite;
- langisitis;
- oregonite;
- sperrylitis.
This, of course, is not a complete list of such minerals - at the moment, about twenty-five such compounds are isolated. Among the most common in nature, mention may be made of arsenopyrite, called arsenic pyrite. It is a product obtained by replacing sulfur atoms in pyrite with arsenic. Compounds of a similar plan, in which the highest oxidation state of arsenic does not appear, are called sulfosalts.
Their analogues are cobalt shine, gersdorfite, enargite, and also proustite. The latter is an important silver ore located in the upper layer of precious veins. The composition of sulfosalts may include noble metals of the platinum group. Among them, irarsit is of interest, as well as orsite. They contain rare metals that are used as excellent catalysts in organic and inorganic syntheses.
The maximum oxidation state of arsenic is manifested in its natural sulfides. For example, in orange-yellow dimorphite, which is arsenic sulfide (5). In the thirties of the last century, natural deposits of auripigment, which included sulfide of trivalent arsenic, were discovered in the south of the Verkhoyansk Range. The size of the detected crystals reached 60 centimeters in length, and the weight was estimated at 30 kilograms.
Arsenate Characterization
Possible oxidation states of arsenic can be considered using salts as an example. So, the compounds of arsenic acid, called arsenates, show the maximum value for this metal: +5. As an example of such compounds, we give erythrin having a bright pink color. This salt is called cobalt color; it has the formula Co3 (AsO4) 2 * 8H 2 O. It is also possible to note a brown-red gasparite in the form of (Ce, La, Nd) ArO 4 .
In the center of Sweden, Langban iron-manganese quarries were discovered, where about fifty different arsenates were found and characterized. These compounds were formed by the interaction of arsenic acid with manganese hydroxide (2) at low temperatures.
What arsenic oxidation states are manifested in arsenates? The characterization of these salts confirms the presence of sulfur in them. Despite the lack of industrial application, their aesthetic appearance allows them to be used to create mineralogical collections.
An interesting story has cupric nickel, which corresponds to the mineral nickel. Medieval German miners called nickel the mountain evil spirit, and “fake copper” was called “kupfernickel”. The masters revealed the external similarity of copper-red crystals of this mineral with copper ores. They were used in glass making to give the resulting product a green tint. Only in the mid-eighteenth century did the mineralogist Axel Kronstedt manage to isolate nickel from this mineral.
Features of accommodation in nature
Arsenic is characterized by high inertness; therefore, it can also be found in a native state. Such a metal in the composition has from two to sixteen percent of impurities, mainly they will be silver, iron, cobalt, nickel. In our country, geologists discovered native arsenic in the Amur Region, Transbaikalia.
It can be found in rocks, and in minerals, and in plants, so it is often rightly called the omnipresent element.
What is the maximum and minimum oxidation state of arsenic? The highest value corresponds to the number of the group in which this element is located, and is +5. This is characteristic of compounds in which it exhibits reducing properties. More details of the compounds of this unique metal will be considered a little later.
Arsenic is distributed extremely unevenly across the globe. The reason is the formation of the lithosphere, as well as in desorption and sorption processes occurring in sedimentary rocks and soils.
Due to the excellent solubility of this metal in water, it easily migrates. For example, in a humid climate, it is washed out of the soil with subsequent movement along with groundwater and rivers.
Physiological effect
Arsenic is found in significant amounts in mineral waters. There are certain standards for the content of this metal. If the permissible values are exceeded, there is a serious threat of harm to the human body. In the course of chemical studies, it was found that arsenic can be contained in various forms in natural water. What should be the degree of oxidation of arsenic? The properties of compounds found in water confirm the presence of metal in the form of a solution of arsenic acid.
Arsenic in live form contains about 6 mg per 1 kilogram. Some algae can accumulate the substance in themselves to such an extent that it can be dangerous to the human body. Some of their species, located in Asian countries, are able to multiply in a pure acid solution. They are used as a means of combating rats. In the brain tissue of a person, as well as in his muscles, there is a sufficient amount of this metal. In addition, it is present in the nail plates, accumulates in the hair.
physical characteristics
Despite the fact that arsenic resembles metal in appearance, it also has non-metallic features. For example, it is not able to form salts with sulfuric acid, acting as an acid-forming element. Arsenic is able to exist in various allotropic modifications, resembling phosphorus. The most stable is gray arsenic, which when heated sublimates like iodine.
This modification is inferior to copper in electrical conductivity, but exceeds mercury. When cooling the vapors of arsenic, you can get a soft transparent substance of a yellow hue, similar to yellow phosphorus. During heating, it turns into another allotropic modification of this chemical element.
When arsenic vapor is deposited on glass, the appearance of a mirror film can be observed.
Arsenic compounds
The oxidation state of arsenic in higher oxide is +5, i.e., corresponds to its higher valency. But upon sublimation of the vapors of a given metal in moist air, a black film of its arsenic As2O 3 anhydride forms. It is in this form that the oxide of this element mainly exists. This oxide exhibits amphoteric chemical properties.
In the process of oxidation, it turns into a higher oxide, where arsenic demonstrates a value of oxidation state +5.
Pure metal is oxidized by diluted nitric acid to ortho-arsenic acid H 3 AsO 3 , in which it has valency 3. By its chemical capabilities, it is considered an acid of medium strength, similar to boric acid. Salts are considered arsenites, exhibiting bright reducing properties.
In chloride, a metal has an oxidation state of +3, acting as a typical metal element. Salt-like arsenides formed by interaction with active metals undergo water hydrolysis. For example, arsine (AsH 3 ) is a toxic, colorless, odorless gas.
Organometallic compounds
Arsenic is able to form various organometallic compounds. For example, at the end of the 18th century, during distillation with arsenic oxide (3) of potassium acetate, a smoking liquid was obtained that had an unpleasant odor. The resulting product was called Alarsin. Subsequent studies have revealed that arsenic is present in its composition.
At the end of the nineteenth century, aromatic arsines were synthesized. The reaction was carried out by exposure to a mixture of arsenic trichloride and aryl halide with sodium metal. Some of these products have demonstrated antimicrobial properties. Today, the synthesis of tens of thousands of organo-arsenic substances is carried out.
Arsenic use
More than half of the total metal mined is used in the form of various compounds. In its pure form, it is practically not used. In a small amount, it is introduced into bearing alloys. Such additives have a positive effect on the hardness of the finished product, so they are in demand in the manufacture of electric cables and batteries.
Minimum doses of arsenic contribute to increased corrosion resistance, improve the thermal performance of brass and copper. If this element does not contain additional impurities, then it is in demand in the manufacture of semiconductor devices. To create them, arsenic is fused with germanium or silicon.
In addition, it is an excellent alloying agent for steel. Arsenic is also needed as a valuable nozzle in non-ferrous metallurgy. Even with its low content in the alloy, it is possible to significantly increase the strength of the manufactured product. Arsenic increases the fluidity of copper during casting, facilitating the process of obtaining wire.
There is a definite negative effect of this transition metal on production. Its presence in ore turns the process into harmful production.
Among the various compounds of arsenic, trivalent oxide is of interest. It is currently used in glass making. Salts with an indicator of +5 are in demand as antiseptics. This transition element is one of the most popular in modern chemical production, as it exhibits dual properties depending on the process environment.