ARSENIC: HISTORY, STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2025

The arsenic is a semimetal or semimetal belonging 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 (bottom image). It loses its shine when exposed to air, forming arsenous oxide (As ₂ O ₃), which when heated emits a garlic odor. On the other hand, its yellow and black allotropes are molecular and amorphous, respectively.

Metallic arsenic. Source: Hi-Res Images of Chemical Elements

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 (As ₄ S ₄), orpiment (As ₂ S ₃), loellingite (FeAs ₂) and enargite (Cu ₃ AsS ₄). Arsenic is also obtained as a by-product of smelting metals such as lead, copper, cobalt, and gold.

Arsenic compounds are toxic, especially arsine (AsH ₃). However, arsenic has many industrial applications, including alloying with lead, used in the manufacture of automotive batteries, and alloying with gallium with various uses in electronics.

History of its discovery

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 consisting of As ₂ S ₃. In the 11th century, three species of arsenic were recognized: white (As ₄ O ₄), yellow (As ₂ S ₃) and red (As ₄ S ₄).

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. Later, 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.

Structure of arsenic

Arsenic is isomorphic to antimony; that is, they are structurally identical, differing only in the size of their atoms. Each arsenic atom forms three As-As covalent bonds, in such a way that they originate “wrinkled or steep” hexagonal As ₆ units, since the hybridization of the As atoms is sp ³.

Then the As ₆ units connect giving rise to steep layers of arsenic, which interact weakly with each other. As a result of 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 pair of electrons of arsenic, the As ₆ units formed between parallel layers do not define a perfect but distorted octahedron:

Crystal structure of gray arsenic. Source: Gabriel Bolívar.

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 As ₆ unit 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 grayish arsenic becomes amorphous, it becomes a semiconductor.

Yellow arsenic

Yellow arsenic, the most toxic allotrope of this element, is a purely molecular solid. It consists of As molecules ₄ units due to weak dispersion forces, which do not prevent them from volatilizing.

Black arsenic

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 As ₆ unit planes have larger areas and different patterns of disorder.

Electronic configuration

3d ¹⁰ 4s ² 4p ³

It has all the orbitals of level 3 filled. It forms bonds using the 4s and 4p orbitals (as well as the 4d) through different chemical hybridizations.

Properties

Molecular weight

74.922 g / mol

Physical description

Gray arsenic is a greyish solid with a metallic appearance and a brittle consistency.

Color

Three allotropic forms, yellow (alpha), black (beta), and gray (gamma).

Odor

Toilet

Taste

Tasteless

Melting point

1,090 K at 35.8 atm (triple point of arsenic).

At normal pressure it has no melting point, since it sublimates to 887 K.

Density

-Gray arsenic: 5.73 g / cm ³.

-Yellow arsenic: 1.97 g / cm ³.

Water solubility

Insoluble

Atomic radio

139 pm

Atomic volume

13.1 cm ³ / mol

Covalent radius

120 pm

Specific heat

0.328 J / gmol at 20 ° C

Evaporation heat

32.4 kJ / mol

Electronegativity

2.18 on the Pauling scale

Ionization energy

First ionization energy 946.2 kJ / mol

Oxidation states

-3, +3, +5

Stability

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 (As ₂ O ₃).

Decomposition

When arsenic is heated to decomposition, it emits a white smoke of As ₂ O ₃. The procedure is dangerous because arsine, a very poisonous gas, can also be released.

Auto-ignition

180 ºC

Hardness

3.5 on the Mohs hardness scale.

Reactivity

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.

Applications

Alloys

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.

electronics

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. Besides 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.

Agriculture and wood conservation

Most applications have been scrapped due to their high toxicity and that of their compounds. As ₂ O ₃ has been used as a pesticide, while As ₂ O ₅ is an ingredient in herbicides and insecticides.

Arsenic acid (H ₃ AsO ₄) 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.

Medicinal

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 As ₂ O ₃, a highly toxic compound, was approved in the treatment of acute promyelocytic leukemia resistant to all-trans retinoic acid. Recently, the radioactive isotope ⁷⁴ As was used for tumor localization.

The isotope produces good images, clearer than those obtained with the ¹²⁴ I, because the iodine is carried to the thyroid and produces noise in the signal.

Other uses

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.

Where is it located?

Arsenic can be found in small amounts in an 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 (FeAs ₂), enargite (Cu ₃ AsS ₄), orpiment (As ₂ S ₃) and realgar (As ₄ S ₄).

How is it obtained?

Arsenopyrite is heated to 650-700ºC, in the absence of air. Arsenic evaporates, leaving iron sulfide (FeS) as a residue. During this process, arsenic binds to oxygen to form As ₄ O ₆, known as "white arsenic."

As ₄ O ₆ is modified to form As ₂ O ₃, whose vapors are collected and condensed in a set of brick chambers, purifying the arsenic by sublimation.

Most of the arsenic is produced by carbon reduction of the formed dust of As ₂ O ₃.

References

1. Stephen R. Marsden. (April 23, 2019). Chemistry of arsenic. Chemistry LibreTexts. Recovered from: chem.libretexts.org
2. Helmenstine, Anne Marie, Ph.D. (December 03, 2018). Interesting Facts About Arsenic. Recovered from: thoughtco.com
3. Wikipedia. (2019). Arsenic. Recovered from: en.wikipedia.org
4. Dr. Dough Stewart. (2019). Arsenic element facts. Chemicool. Recovered from: chemicool.com
5. Royal Society of Chemistry. (2019). Arsenic. Recovered from: rsc.or
6. The Editors of Encyclopaedia Britannica. (May 03, 2019). Arsenic. Encyclopædia Britannica. Recovered from: britannica.com