Arsenic

Arsenic is a chemical element which is placed in the 15th group and 4th period in the periodic table. Symbol of the arsenic element is As. It is a metalloid element with the atomic number 33. Phosphorus is placed above arsenic and antimony is placed below arsenic in the 15th group of the periodic table. Arsenic shows many similarities with phosphorus. For example, the structure of black arsenic is like black phosphorus and electronegativity and ionization energies of arsenic is similar to phosphorus. Another metalloid germanium is found at the left of arsenic and selenium is present at the right of it in the 4th period of the periodic table. As it is found in the 15th group so, it is a p – block element. As it is a metalloid, so it shows some properties of metals and some properties of nonmetals. 

Sulfides and oxides of arsenic have been used since ancient times. Arsenic was discovered by Arabic alchemists before AD 815. The word arsenic is derived from the Syriac word ‘zarniqa’ which means gold colored or yellow colored. Syriac word ‘zarniqa’ was adopted into Greek as arsenikon and then from Green into Latin as arsenicum. Arsenicum became arsenic in French and English.  It is rarely found as pure ore or in a pure elemental crystal form. It is found in many minerals with metals and sulfurs. It is also found as a pure elemental form. Primarily arsenic is obtained from its ore arsenopyrite (FeAsS). It is also obtained from the smelter dust of copper, lead and gold smelters. 

Arsenic is found as grey, yellow and black allotropes of it. Arsenic is a monoisotopic element as it has only one natural isotope which is 75As. Although almost 33 radioisotopes of arsenic have been synthesized. Out of synthetic isotopes of arsenic 73As is the most stable isotope with a half life period of 80.30 days.  

Arsenic Atomic Number and Electronic Configuration 

Atomic number of Arsenic is 33. Its electronic configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3 or it can be written as [Ar] 3d10 4s2 4p3. It has 2 electrons in K – shell, 8 electrons in L – shell, 18 electrons in M – shell and 5 electrons in its outermost shell N. 

Properties of Arsenic 

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Physical and chemical properties – Physical and chemical properties of arsenic are listed below –

• Atomic number of the element arsenic is 33. It means one atom of arsenic has 33 protons in its nucleus. 

• Atomic mass of arsenic is 74.92.

• It is found as solid at 20 ℃ temperature. 

• It has a metallic grey appearance. 

• Its melting point is 816.8 ℃.

• Boiling point of arsenic is 614 ℃.

• It sublimes at 616 ℃. 

• It has rhombohedral crystal structure.

• Its density is 5.727 g cm-3.

• It has grey, yellow and black allotropes. Its grey allotrope is the most common. 

• According to the Pauling scale, its electronegativity is 2.18

• It is a monoisotopic element. It is composed of only one isotope which is 75As.

• It is a metalloid. It shows some properties of metals and some of nonmetals. 

• Reaction with air – Arsenic reacts with moist air and forms arsenic oxide which forms a black covering on the surface of the element. Here you need to note that arsenic does not react with dry air and water in absence of air. Reaction of arsenic with oxygen is given below –

4As(s) + 5O2(g) → As4O10(s)

4As(s) + 3O2(g) → As4O6(s)

• Reaction with halogens – Arsenic reacts with fluorine and forms colorless arsenic pentafluoride. Reaction is given below –

2As(s) + 5F2(g) → 2AsF5(g) (colorless)

Arsenic reacts with other halogens as well under controlled conditions such as chlorine, bromine, iodine and forms arsenic trichloride (colorless), arsenic tribromide (pale yellow), arsenic triiodide (red) respectively. Reactions are given below –

2As(s) + 3Cl2(g) → 2AsCl3(l) 

2As(s) + 3Br2(g) → 2AsBr3(s) 

2As(s) + 3I2(g) → 2AsI3(s) 

• Formation of arsenic acid – Arsenic reacts with water and oxygen and forms arsenic acid. Reaction is given below –

2As + 3H2O + 5O3 → 2H3AsO4 + 5O2

Uses of Arsenic  

Being a metalloid arsenic is very useful in various fields. Few of its applications are listed below –

• Agricultural applications – Arsenic is used to kill rats, fungi, insects etc. which are harmful for the crop. It is used in a variety of insecticides and poisons used in farms. It is used in poultry and swine production as well. Organic arsenic compounds are used in the feed of chickens as they are less toxic than elemental arsenic and promote the growth of chickens. 

• Medicinal applications – Till the 20th century, arsenic was used in numerous medicines but slowly due to its toxic nature as its side effects came to light, its usage in the medicines has been limited. Medicine melarsoprol is still used for the treatment of trypanosomosis. Arsenic trioxide can be used for patients with acute promyelocytic leukaemia. Recently 74As has been used to detect tumours. Nanoparticles of As can be used to kill cancer cells. 

• As alloys – Many alloys can be made using arsenic and other metals and non-metals. Mainly in alloys of arsenic, it is mixed with lead. These alloys are used in car batteries etc. Alloy of gallium and arsenic is used as an important semiconductor material which is used in integrated circuits. Gallium arsenide can be used in laser diodes, LEDs etc. 

• In the military – Arsenic has been used during World War I for various military purposes. Although due to its toxic nature, soon its instruments were banned. 

• Other uses – Other uses of arsenic includes its uses in bronzing and pyrotechnics. In small quantities it is used in alpha brass. It is used as poison. It is used for taxonomic sample preservation. Various bacteria are able to obtain energy from arsenate. 

Disadvantages of Arsenic 

Arsenic is a toxic element. There is evidence that its high dosage in the form of formulations have killed athletes. Inorganic arsenic is carcinogen. We consume arsenic mostly from drinking water and agricultural products. High amounts of arsenic in the body causes arsenic poisoning which may affect a broad range of organs and organ systems. It may affect skin, cardiovascular system, immune system, endocrine system, nervous system and respiratory system etc. Elemental arsenic and inorganic arsenic compounds are more poisonous than organic arsenic compounds. 

Arsenic: Summary in Tabular Form  

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