The arsenic It is a semimetal or metalloid that belongs to group 15 or VA of the periodic table. It is represented by the chemical symbol As, and its atomic number is 33. It can be found in three allotropic forms: yellow, black and gray; the latter being the only one with industrial importance.
Gray arsenic is a brittle, metallic-looking solid with a steely, crystalline color (image below). It loses its shine when exposed to air, forming arsenous oxide (AstwoOR3), which when heated emits a garlic smell. On the other hand, its yellow and black allotropes are molecular and amorphous, respectively..
Arsenic is found in the earth's crust associated with numerous minerals. Only a small proportion is found in the native state, however associated with antimony and silver.
Among the most common minerals in which arsenic is found are the following: realgar (As4S4), the orpiment (AstwoS3), loellingite (FeAstwo) and enargite (Cu3AsS4). Arsenic is also obtained as a by-product of the smelting of metals such as lead, copper, cobalt and gold..
Arsenic compounds are toxic, especially arsine (AsH3). However, arsenic has numerous industrial applications, including alloying with lead, used in the manufacture of car batteries, and alloying with gallium with various uses in electronics..
Article index
The name 'arsenic' comes from the Latin arsenicum and the Greek arsenikon, referring to yellow orpiment, which was the main form of use of arsenic by alchemists.
Arsenic, long before being recognized as a chemical element, was known and used in the form of its compounds. For example, Aristotle in the 4th century BC. wrote about sandarache, a substance now thought to be arsenic sulfide.
Pliny the Elder and Pedanius Discórides, in the 1st century AD, described orpiment, a mineral made up of AstwoS3. In the 11th century, three species of arsenic were recognized: white (As4OR4), the yellow (AstwoS3) and red (As4S4).
Arsenic as a pure element was first observed by Albertus Magnus (1250). Magnus heated the arsenic sulfide with soap, noting the appearance of a substance with a characteristic similar to the grayish allotrope in the image. However, the first authentic report of his isolation was published in 1649 by Johann Schroeder, a German pharmacist..
Schroeder prepared the arsenic by heating its oxide with charcoal. Subsequently, Nicolas Lémery managed to produce it by heating a mixture of arsenic oxide, soap and potash. In the 18th century, this element was finally recognized as a semi-metal..
Arsenic is isomorphic to antimony; that is, they are structurally identical, differing only in the size of their atoms. Each arsenic atom forms three covalent bonds As-As, in such a way that they originate hexagonal units As6 "Wrinkled or steep", since the hybridization of the As atoms is sp3.
Then the units As6 they connect giving rise to sheer layers of arsenic, which interact weakly with each other. Due to their intermolecular forces, dependent above all on their atomic masses, the rhombohedral gray arsenic crystals give the solid a fragile and brittle texture..
Possibly due to the repulsions of the free electron pair of arsenic, the As6 formed between parallel layers do not define a perfect but distorted octahedron:
Note that the black spheres draw the distorted plane in the space between two steep layers. Likewise, in the layer below there are bluish spheres which, together with the black sphere, make up the unit As6 mentioned at the beginning of the section.
The structure looks orderly, the rows go up and down, and therefore it is crystalline. However, it can become amorphous, with spheres squeezed in different ways. When greyish arsenic becomes amorphous, it transforms into a semiconductor.
Yellow arsenic, the most toxic allotrope of this element, is a purely molecular solid. It consists of molecules of As4 units by weak dispersal forces, which do not prevent them from volatilizing.
Black arsenic is amorphous; but not how the grayish allotrope can be. Its structure is slightly similar to the one just described, with the difference that its As6 have larger areas and different patterns of clutter.
[Ar] 3d104stwo4p3
It has all the level 3 orbitals filled. Forms bonds using the 4s and 4p orbitals (just like the 4d) through different chemical hybridizations.
74.922 g / mol
Gray arsenic is a grayish solid with a metallic appearance and a brittle consistency..
Three allotropic forms, yellow (alpha), black (beta), and gray (gamma).
Toilet
Tasteless
1,090 K at 35.8 atm (triple point of arsenic).
At normal pressure it has no melting point, as it sublimates to 887 K.
-Gray arsenic: 5.73 g / cm3.
-Yellow arsenic: 1.97 g / cm3.
Insoluble
139 pm
13.1 cm3/ mol
120 pm
0.328 J / gmol at 20 ° C
32.4 kJ / mol
2.18 on the Pauling scale
First ionization energy 946.2 kJ / mol
-3, +3, +5
Elemental arsenic is stable in dry air, but when exposed to humid air it becomes covered with a bronze-yellow layer that can become a black layer of arsenic oxide (AstwoOR3).
When arsenic is heated to decomposition, it emits white smoke of AstwoOR3. The procedure is dangerous because arsine, a very poisonous gas, can also be released.
180 ºC
3.5 on the Mohs hardness scale.
It is not attacked by cold sulfuric acid or concentrated hydrochloric acid. Reacts with hot nitric acid or sulfuric acid, forming arsenic acid and arsenic acid.
When gray arsenic is volatilized by heating, and the vapors are rapidly cooled, a yellow arsenic is formed. This returns to the grayish form, when subjected to ultraviolet light.
A small amount of arsenic added to lead, hardens its alloys enough to use them in the coating of cables, and in the manufacture of car batteries..
The addition of arsenic to brass, an alloy of copper and zinc, increases its resistance to corrosion. On the other hand, it corrects or reduces the loss of zinc in the brass, which causes an increase in its useful life..
Purified arsenic is used in semiconductor technology where it is used in conjunction with gallium and germanium, as well as in the form of gallium arsenide (GaAs) which is the second most widely used semiconductor.
GaAs have a direct band gap, which can be used in diode, laser, and LED manufacturing. In addition to gallium arsenide, there are other arsenides, such as indium arsenide and aluminum arsenide, which are also III-V semiconductors..
Meanwhile, cadmium arsenide is a type II-IV semiconductor. Arsine has been used in semiconductor doping.
Most of the applications have been scrapped due to their high toxicity and that of their compounds. The AcetwoOR3 has been used as a pesticide, while AstwoOR5 is an ingredient of herbicides and insecticides.
Arsenic acid (H3AsO4) and salts such as calcium arsenate and lead arsenate have been used to sterilize soils and control pests. This creates a risk of environmental contamination with arsenic..
Lead arsenate was used as an insecticide on fruit trees until the first half of the 20th century. But due to its toxicity, it was replaced by sodium methylarsenate, which stopped being used for the same reason since 2013.
Until the 20th century, several of its compounds were used as medicines. Arsphenamine and neolsalvarsan, for example, have been used in the treatment of syphilis and trypanosomiasis.
In 2000, the use of AstwoOR3, Very toxic compound, in the treatment of acute promyelocytic leukemia resistant to all-trans retinoic acid. Recently, the radioactive isotope was used 74Ace for tumor localization.
The isotope produces good images, clearer than those obtained with the 124I, because iodine is carried to the thyroid and produces noise in the signal.
Arsenic was used in the past as a feed additive in poultry and pig production.
It is used as a catalyst in the manufacture of ethylene oxide. It is also used in fireworks and tanning. Arsenous oxide is used as a decolorizer in the manufacture of glass.
Arsenic can be found in small amounts in the elemental state, with a high degree of purity. It is present in numerous compounds, such as: sulfides, arsenides and sulfoarseniides.
It is also found in several minerals, including: arsenopyrite (FeSAs), loellingite (FeAstwo), enargite (Cu3AsS4), the orpiment (AstwoS3) and realgar (As4S4).
Arsenopyrite is heated to 650-700ºC, in the absence of air. The arsenic evaporates, leaving iron sulfide (FeS) as a residue. During this process, arsenic joins oxygen to form As4OR6, known as "white arsenic".
The Ace4OR6 is modified to form the AstwoOR3, whose vapors are collected and condensed in a set of brick chambers, the arsenic being purified by sublimation.
Most of the arsenic is produced by reduction by carbon of the dust formed from AstwoOR3.
Yet No Comments