Arsenic (Ar)

Arsenic is a chemical element with an atomic number 33 in the periodic table of elements. There’s 1.5–2 ppm of this crystalline metalloid in the Earth’s crust. Being a member of the phosphorus family of periodic table elements, this chemical element has five valence electrons that help arsenic make numerous compounds with other substances.

Chemical and Physical Properties of Arsenic

Atomic Number33
Crystal StructureRhombohedral
Atomic Weight74.92159
Orbitals[Ar] 3d10 4s2 4p3
Covalent Radius1.20 Å
Ionic Radius.58 (+3) Å
Atomic Radius1.33 Å
Atomic Volume13.1 cm³/mol
Name OriginGreek: arsenikon; Latin: arsenicum
Discovered BySome mineral forms known to the ancients, but Albertus Magnus accredited with discovery
Year13th Century (Albertus Magnus)
Oxidation States(±3),5
Density5.72 g/cm³
UsesWeed killer, rat poison, semiconductors, paints, wallpapers, ceramics
DescriptionSoft metalloid, grey metallic to red glassy appearance
Physical Statemetalloid/semimetal (brittle crystalline solid)
Half-life12 days
Density5.7 at 14°C
Melting Point814 °C (36 atm)
Boiling Point615 °C (sublimation)
Van der Waals Radius0.139 nm
Most Characteristic Isotope
The Energy of the First Ionization947 kJ.mol -1
The Energy of the Second Ionization1798 kJ.mol -1
The Energy of the Third Ionization2736 kJ.mol -1

The chemical and physical properties of arsenic make this substance a semimetal since it displays both metal and non-metal characteristics. Having a place in the periodic table of elements under the symbol As, with an atomic number of 33, an atomic weight (mass) of 74.9216 g.mol -1, and electronic configuration [Ar] 3d¹⁰ 4s² 4p³, arsenic is ductile and reaches its boiling point at 615 °C (sublimation), which is lower than its melting point at 814 °C (36 atm). 

When arsenic is submitted to heat treatment, it changes from a solid aggregate state into a gas aggregate state. 

The fact sheet of this member of the boron family of elements in the periodic table also discloses that arsenic has an electronegativity of 2.0 according to Pauling. The atomic radius according to van der Waals is 0.139 nm.

How Was Arsenic Discovered?

Arsenic and its toxic properties have been known since ancient times. Even the early Egyptian, Chinese, and Greek civilizations made use of the arsenic compound. In the Middle Ages, Paracelsus was the most famous physician-alchemist who had special formulas for the preparation of metallic arsenic. During the rule of the Ming dynasty in China, the toxic arsenic compounds were used as pesticides in rice fields.  

The ancient civilizations were also familiar with As2O3   trioxide, known under the name of ‘white arsenic,” which has extremely high toxicity, which imposed an increased risk on human health. This arsenic compound was made by the burning of arsenic. The arsenic metal was produced by mixing arsenic trioxide with olive oil. 

In the 1920s, the discovery of arsenic as a periodic table element was attributed to the German Catholic Dominican friar, bishop, and alchemist Albertus Magnus (before 1200 – November 15, 1280). Namely, by heating the arsenic compound orpiment (As2S3) together with a piece of soap, he managed to produce the pure form of arsenic.

How Did Arsenic Get Its Name?

The name “arsenic” originates from the Greek expression “arsenikon”, which means “yellow orpiment”.

Where Can You Find Arsenic?

Rarely found in its free form, arsenic can mostly be smelted from compounds of minerals, such as arsenopyrite (FeAsS), realgar (AsS), and orpiment (As2S3). Arsenic commonly occurs in the form of arsenic sulfide, metal arsenates, or metal arsenides. 

How Is Arsenic Used in Everyday Life?

Arsenic has a wide application in our everyday life. It is used for bronzing, pyrotechnics, and the production of a special type of glass. In the United States, arsenic is mostly used as a wood preservative.

This member of the phosphorus family of chemical elements is also found in traditional Chinese pharmaceutical preparations for the treatment of various health problems, such as inflammation, asthma, joint pain, cough, and cancer.

How Dangerous Is Arsenic?

This substance is considered a sort of human carcinogen due to its extremely toxic effects. The most dangerous aspect of arsenic is the fact that it’s odourless and colorless, which enables the unaware long-term exposure and consumption of a certain amount of arsenic. This toxic substance can contaminate the air and water via volcanic activity, metal smelting, production, and use of pesticides, wood preservatives, and underground waters. 

Arsenic Poisoning

Arsenicosis, or poisoning after exposure to arsenic, mostly occurs when even low levels are inhaled or consumed via contaminated drinking water or high arsenic concentrations in groundwater. People who work with arsenic-based pesticides, or whose work involves contact with high levels of inorganic arsenic compounds may be exposed to higher levels of arsenic. 

Some of the negative health effects that appear as a result of arsenic contamination or exposure to both higher and lower levels of arsenic include abdominal pain, skin rash, and redness, nausea, vomiting, and dysfunction of the digestive system organs, etc. Long-term exposure to high levels of arsenic and arsenates (typically via contaminated drinking water) can lead to more severe medical conditions, such as lung cancer, skin cancer, or bladder cancer.

One such occurrence has been recorded in Bangladesh, where the water supply, being polluted by arsenic, imposed a great risk on the health of the population living in that area. 

Environmental Effects of Arsenic

Arsenic exposure hazard is most pronounced in industrial areas, but could also stem from the groundwater, private wells (contaminated well water), and other similar water sources contaminated with high levels of arsenic concentration. Bangladesh, Chile, the US, Argentina, Mexico, India, Taiwan, and China, comprise the list of countries with an increased risk of arsenic exposure and public health hazards related to arsenic. 

EPA or the U.S. Environmental Protection Agency is responsible for the protection of the environment and public health. Together with the Department of Health, the CDC – Center for Disease Control and Prevention, the Agency for Toxic Substances and Disease Registry, the Food and Drug Administration (FDA), and the U.S. Department of Health and Human Services (DHHS), EPA has classified inorganic arsenic as a human carcinogen. 

In collaboration with the International Agency for Research on Cancer (IARC), the World Health Organization published a scientific paper confirming the toxic effects of arsenic and supporting the claim that exposure to soluble inorganic arsenic presents a major public health concern. 

Additionally, the Occupational Safety & Health Administration provides all the needed information on arsenic health hazards, as well as means of precaution and protection after arsenic poisoning. 

Isotopes of Arsenic

This chemical element counts 33 known isotopes, among which there’s only one stable isotope (75As).

The following is a chart representation of the arsenic isotopes.


[n 1]

ZNIsotopic mass (Da)

[n 2][n 3]



[n 4]



[n 5][n 6]

Spin and


[n 7][n 8]

Natural abundance (mole fraction)
Excitation energy[n 8]Normal proportionRange of variation
60As332759.99313(64)# p59Ge5+#  
61As332860.98062(64)# p60Ge3/2−#  
62As332961.97320(32)# p61Ge1+#  
63As333062.96369(54)# p62Ge(3/2−)#  
64As333163.95757(38)#40(30) ms

[18(+43-7) ms]

65As333264.94956(32)#170(30) msβ+65Ge3/2−#  
66As333365.94471(73)95.77(23) msβ+66Ge(0+)  
66m1As1356.70(17) keV1.1(1) µs  (5+)  
66m2As3023.9(3) keV8.2(5) µs  (9+)  
67As333466.93919(11)42.5(12) sβ+67Ge(5/2−)  
68As333567.93677(5)151.6(8) sβ+68Ge3+  
68mAs425.21(16) keV111(20) ns

[?107(+23-16) ns]

69As333668.93227(3)15.2(2) minβ+69Ge5/2−  
70As333769.93092(5)52.6(3) minβ+70Ge4(+#)  
70mAs32.008(23) keV96(3) µs  2(+)  
71As333870.927112(5)65.28(15) hβ+71Ge5/2−  
72As333971.926752(5)26.0(1) hβ+72Ge2−  
73As334072.923825(4)80.30(6) dEC73Ge3/2−  
74As334173.9239287(25)17.77(2) dβ+ (66%)74Ge2−  
β (34%)74Se
75mAs303.9241(7) keV17.62(23) ms  9/2+  
76As334375.922394(2)1.0942(7) dβ (99.98%)76Se2−  
EC (.02%)76Ge
76mAs44.425(1) keV1.84(6) µs  (1)+  
77As334476.9206473(25)38.83(5) hβ77mSe3/2−  
77mAs475.443(16) keV114.0(25) µs  9/2+  
78As334577.921827(11)90.7(2) minβ78Se2−  
79As334678.920948(6)9.01(15) minβ79mSe3/2−  
79mAs772.81(6) keV1.21(1) µs  (9/2)+  
80As334779.922534(25)15.2(2) sβ80Se1+  
81As334880.922132(6)33.3(8) sβ81mSe3/2−  
82As334981.92450(21)19.1(5) sβ82Se(1+)  
82mAs250(200) keV13.6(4) sβ82Se(5-)  
83As335082.92498(24)13.4(3) sβ83mSe3/2−#  
84As335183.92906(32)#4.02(3) sβ (99.721%)84Se(3)(+#)  
β, n (.279%)83Se
84mAs0(100)# keV650(150) ms     
85As335284.93202(21)#2.021(10) sβ, n (59.4%)84Se(3/2−)#  
β (40.6%)85Se
86As335385.93650(32)#0.945(8) sβ (67%)86Se   
β, n (33%)85Se
87As335486.93990(32)#0.56(8) sβ (84.6%)87Se3/2−#  
β, n (15.4%)86Se
88As335587.94494(54)#300# ms

[>300 ns]

β, n87Se
89As335688.94939(54)#200# ms

[>300 ns]

90As335789.95550(86)#80# ms

[>300 ns]

91As335890.96043(97)#50# ms

[>300 ns]

92As335991.96680(97)#30# ms

[>300 ns]


Source: Wikipedia

Arsenic Compounds 

Arsenic compounds exhibit both metallic and nonmetallic properties. The most common oxidation states are the trivalent and pentavalent forms. The inorganic forms of arsenic are considered as the most toxic substances. After ingestion, the inorganic arsenic compounds are methylated to dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA). These two compounds of arsenic are less toxic, and can be easily excreted via urine. 

Forms of Arsenic Compounds

Regarding their biological and toxicological properties, the arsenic compounds can be classified into three major groups:

  • Inorganic arsenic compounds (arsenic trioxide, sodium arsenite, arsenic trichloride, arsenic pentoxide, arsenic acid, lead arsenate, and calcium arsenate);
  • Common organic arsenic compounds (arsanilic acid, methyl-arsonic acid, dimethylarsinic acid (cacodylic acid), and arsenobetaine);
  • Arsine gas (AsH3 )   This odorless, colorless, and non-irritating gas has extremely toxic properties that impose a great risk upon human health. Arsine gas poisoning can lead to fatal consequences if inhaled in high concentrations. 

5 Interesting Facts And Explanations 

  1. Arsenic has its own symbol in alchemy. This chemical element is represented by the symbol of a swan, as a reference of the transformational properties of the physical appearance of arsenic, just like the ugly duckling turns into a swan. 
  2. Orpiment is a mineral of bright yellow color made up of arsenic trisulphide As2S3.
  3. There are several allotropes of arsenic, which mostly appear in gray, yellow, and black form. 
  4. Legends say that Albertus Magnus discovered the philosopher’s stone, which he later gave to Tomas Aquinas. The philosopher’s stone refers to a substance that allegedly has alchemic powers to turn metal elements into pure gold
  5. Arsenic can be more easily absorbed and accumulated in rice and rice products than in any other crop.