Arsenic Atomic Number

Notes on the properties of Arsenic: Boiling Point: Arsenic sublimates at this temperature. Specific Heat: Value given for solid alpha form. Vickers Hardness: converted from Brinell scale Up to date, curated data provided by Mathematica's ElementData function from Wolfram Research, Inc.

Chemical properties of arsenic - Health effects of arsenic - Environmental effects of arsenic


Atomic mass

74.9216 g.mol -1

Electronegativity according to Pauling



5.7 at 14°C

Melting point

814 °C (36 atm)

Boiling point

615 °C (sublimation)

Vanderwaals radius

0.139 nm

Ionic radius

0.222 nm (-2) 0,047 nm (+5) 0,058 (+3)



Electronic shell

[ Ar ] 3d10 4s2 4p3

Energy of first ionisation

947 kJ.mol -1

Energy of second ionisation

1798 kJ.mol -1

Energy of third ionisation

2736 kJ.mol -1

Standard potential

- 0.3 V (As3+/ As )

Discovered by

The ancients

  • As is the chemical symbol for the element arsenic. Its atomic number is 33, which is the number of protons in the nuclei of its atoms. In a neutral atom, the number of electrons equals the number of protons, which means that neutral arsenic atoms have 33 electrons.
  • Our arsenic page has over 190 facts that span 107 different quantities. Each entry has a full citation identifying its source. Areas covered include atomic structure, physical properties, atomic interaction, thermodynamics, identification, atomic size, crystal structure, history, abundances, and nomenclature.
  • Arsenic is a grey-appearing chemical element (atomic number 33, symbol As in the periodic table) also termed a metalloid. Arsenic can exist in a metallic state in three forms (yellow, black, and gray; with gray predominating) and in ionic forms.


Arsenic appears in three allotropic forms: yellow, black and grey; the stable form is a silver-gray, brittle crystalline solid. It tarnishes rapidly in air, and at high temperatures burns forming a white cloud of arsenic trioxide. Arsenic is a member of group Va of the periodic table, which combines readily with many elements.
The metallic form is brittle, tharnishes and when heated it rapidly oxidizes to arsenic trioxide, which has a garlic odor. The non metallic form is less reactive but will dissolve when heated with strong oxidizing acids and alkalis.


Arsenic compounds are used in making special types of glass, as a wood preservative and, lately, in the semiconductor gallium arsenade, which has the ability to convert electric current to laser light. Arsine gas AsH3, has become an important dopant gas in the microchip industry, although it requires strict guidelines regarding its use because it is extremely toxic.
During the 18th, 19th, and 20th centuries, a number of arsenic compounds have been used as medicines; copper acetoarsenite was used as a green pigment known under many different names.

Arsenic in the environment

Arsenic can be found naturally on earth in small concentrations. It occurs in soil and minerals and it may enter air, water and land through wind-blown dust and water run-off. Arsenic in the atmosphere comes from various sources: vulcanoes release about 3000 tonnes per year and microorganisms release volatile methylarsines to the extent of 20.000 tonnes per year, but human activity is responsible for much more: 80.000 tonnes of arsenic per year are released by the burning of fossil fuels.

Despite its notoriety as a deadly poison, arsenic is an essential trace element for some animals, and maybe even for humans, although the necessary intake may be as low as 0.01 mg/day.
Arsenic is a component that is extremely hard to convert to water-soluble or volatile products. The fact that arsenic is naturally a fairly a mobile component, basically means that large concentrations are not likely to appear on one specific site. This is a good thing, but the negative site to it is that arsenic pollution becomes a wider issue because it easily spreads. Arsenic cannot be mobilized easily when it is immobile. Due to human activities, mainly through mining and melting, naturally immobile arsenics have also mobilized and can now be found on many more places than where they existed naturally.

A little uncombined arsenic occurs naturally as microcrystalline masses, found in Siberia, Germany, France, Italy, Romania and in the USA. Most arsenic is found in conjuction with sulfur in minerals such as arsenopyrite (AsFeS), realgar, orpiment and enargite. Non is mined as such because it is produced as a by-product of refining the ores of other metals, such as copper and lead. World production of arsenic, in the form of its oxide, is around 50.000 tonnes per year, far in excess of that required by industry. China is the chief exporting country, followed by Chile and Mexico. World resources of arsenic in copper and lead ores exceed 10 million tonnes.

Health effects of arsenic

Arsenic is one of the most toxic elements that can be found. Despite their toxic effect, inorganic arsenic bonds occur on earth naturally in small amounts. Humans may be exposed to arsenic through food, water and air. Exposure may also occur through skin contact with soil or water that contains arsenic.
Levels of arsenic in food are fairly low, as it is not added due to its toxicity. But levels of arsenic in fish and seafood may be high, because fish absorb arsenic from the water they live in. Luckily this is mainly the fairly harmless organic form of arsenic, but fish that contain significant amounts of inorganic arsenic may be a danger to human health.
Arsenic exposure may be higher for people that work with arsenic, for people that live in houses that contain conserved wood of any kind and for those who live on farmlands where arsenic-containing pesticides have been applied in the past.
Exposure to inorganic arsenic can cause various health effects, such as irritation of the stomach and intestines, decreased production of red and white blood cells, skin changes and lung irritation. It is suggested that the uptake of significant amounts of inorganic arsenic can intensify the chances of cancer development, especially the chances of development of skin cancer, lung cancer, liver cancer and lymphatic cancer.
A very high exposure to inorganic arsenic can cause infertility and miscarriages with women, and it can cause skin disturbances, declined resistance to infections, heart disruptions and brain damage with both men and women.
Finally, inorganic arsenic can damage DNA.

A lethal dose of arsenic oxide is generally regarded as 100 mg.
Organic arsenic can cause neither cancer, nor DNA damage. But exposure to high doses may cause certain effects to human health, such as nerve injury and stomachaches.

Environmental effects of arsenic

The arsenic cycle has broadened as a consequence of human interference and due to this, large amounts of arsenic end up in the environment and in living organisms. Arsenic is mainly emitted by the copper producing industries, but also during lead and zinc production and in agriculture. It cannot be destroyed once it has entered the environment, so that the amounts that we add can spread and cause health effects to humans and animals on many locations on earth.

Plants absorb arsenic fairly easily, so that high-ranking concentrations may be present in food. The concentrations of the dangerous inorganic arsenics that are currently present in surface waters enhance the chances of alteration of genetic materials of fish. This is mainly caused by accumulation of arsenic in the bodies of plant-eating freshwater organisms. Birds eat the fish that already contain eminent amounts of arsenic and will die as a result of arsenic poisoning as the fish is decomposed in their bodies.

More information on properties of arsenic

Read more on arsenic in water

Information on arsenicosis

Back to the periodic table of elements

More from 'Elements'

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The Element Arsenic

[Click for Isotope Data]

Atomic Number: 33

Chemical Properties Of Arsenic

Atomic Weight: 74.921595

Melting Point: 1090 K (817°C or 1503°F)

Boiling Point: 887 K (614°C or 1137°F)

Density: 5.776 grams per cubic centimeter

Phase at Room Temperature: Solid

Element Classification: Semi-metal

Period Number: 4

Group Number: 15

Group Name: Pnictogen

What's in a name? From the Latin word arsenicum, the Greek word arsenikon and the Arabic word Az-zernikh.

Arsenic atomic number and mass

Say what? Arsenic is pronounced as AR-s'n-ik.

History and Uses:

Although arsenic compounds were mined by the early Chinese, Greek and Egyptian civilizations, it is believed that arsenic itself was first identified by Albertus Magnus, a German alchemist, in 1250. Civilization v - scenario pack: wonders of the ancient world cracker barrel. Arsenic occurs free in nature, but is most often found in the minerals arsenopyrite (FeAsS), realgar (AsS) and orpiment (As2S3). Today, most commercial arsenic is obtained by heating arsenopyrite.

Arsenic and its compounds are poisonous. They have been used to make rat poison and some insecticides. Small amounts of arsenic are added to germanium to make transistors. Gallium arsenide (GaAs) can produce laser light directly from electricity.

6.88 Grams Of Arsenic To Mass Percent

If you were paying careful attention to the physical data listed above, you may have noticed that arsenic's boiling point is lower than its melting point. This occurs because these two temperatures are measured at different atmospheric pressures. When heated at standard atmospheric pressure, arsenic changes directly from a solid to a gas, or sublimates, at a temperature of 887 K. In order to form liquid arsenic, the atmospheric pressure must be increased. At 28 times standard atmospheric pressure, arsenic melts at a temperature of 1090 K. If it were also measured at a pressure of 28 atmospheres, arsenic's boiling point would be higher than its melting point, as you would expect.

Estimated Crustal Abundance: 1.8 milligrams per kilogram

Estimated Oceanic Abundance: 3.7-3 milligrams per liter

Number of Stable Isotopes: 1 (View all isotope data)

Ionization Energy: 9.815 eV

Oxidation States: +5, +3, -3

Electron Shell Configuration:


2s2 2p6

3s2 3p6 3d10

4s2 4p3

For questions about this page, please contact Steve Gagnon.

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