Arsenic Element Facts

The chemical element arsenic is classed as a metalloid. It has been known since ancient times. Its discoverer and discovery date are unknown.

Arsenious acid – a poison.
Arsenious acid - a poison.

Data Zone

Classification: Arsenic is a metalloid
Color: gray
Atomic weight: 74.9216
State: solid
Melting point: 817 oC, 1090 K
Note: At normal atmospheric pressure arsenic does not melt when heated, it sublimes. i.e. when heated, arsenic undergoes a phase change directly from solid to gas.
The melting point quoted above is for gray arsenic under a pressure of 28 atmospheres.
Boiling point: 603 oC, 876 K
Electrons: 33
Protons: 33
Neutrons in most abundant isotope: 42
Electron shells: 2,8,18,5
Electron configuration: [Ar] 3d10 4s2 4p3
Density @ 20oC: 5.776 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 12.97 cm3/mol
Structure: rhombohedral; layers of 6-member rings
Hardness: 3.5 mohs
Specific heat capacity 0.33 J g-1 K-1
Heat of fusion 24.44 kJ mol-1
Heat of atomization 303 kJ mol-1
Heat of vaporization 32.4 kJ mol-1
1st ionization energy 946.5 kJ mol-1
2nd ionization energy 1797.8 kJ mol-1
3rd ionization energy 2735.4 kJ mol-1
Electron affinity 78 kJ mol-1
Minimum oxidation number -3
Min. common oxidation no. -3
Maximum oxidation number 5
Max. common oxidation no. 5
Electronegativity (Pauling Scale) 2.18
Polarizability volume 4.3 Å3
Reaction with air mild, w/ht ⇒ As4O6
Reaction with 15 M HNO3 mild, w/ht ⇒ H3AsO4, NOx
Reaction with 6 M HCl none
Reaction with 6 M NaOH none
Oxide(s) As2O3
Hydride(s) AsH3
Chloride(s) AsCl3 AsCl5
Atomic radius 115 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion) 72 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 50.2 W m-1 K-1
Electrical conductivity 3.85 x 106 S m-1
Freezing/Melting point: 817 oC, 1090 K

Discovery of Arsenic

Arsenic has been known since antiquity in its sulfide compound.

Greek philosopher Aristotle, in the fourth century BC, refers to “sandarach” renamed arhenicum by his student Theophrastus of Eresos.

Greek historian Olympiodorus of Thebes (5th century AD) roasted arsenic sulfide and obtained white arsenic (As2O3).

Albertus Magnus (1193-1280), a German philosopher and theologian, was the first to state that arsenic has a metal-like nature. In De Mineralibus he described how the metal could be obtained by heating orpiment (As2S3) with soap.

Two methods of preparing arsenic were published by German pharmacist Johann Schroeder in 1649.

The element name is believed to come from the Greek word ‘arsenikos’ meaning potent.

3d model of arsenic (III) oxide, As2O3. Sometimes called white arsenic, it is colorless, tasteless and was a common poison used by criminals before the development of forensic science.

An old government warning poster.

An old government warning poster.

Appearance and Characteristics

Harmful effects:

Arsenic is immediately dangerous to life or health at 5 mg m-3.

Our bodies do not readily absorb the element itself, hence pure arsenic is much less dangerous than As(III) compounds such as AsH3 and As2O3 which are absorbed easily and are carcinogenic with high toxicity.


Arsenic occurs in three distinct solid forms.

Gray arsenic is the most common. It has a metallic sheen and conducts electricity.

Yellow arsenic is metastable, is a poor electrical conductor and does not have a metallic sheen. It is prepared by cooling gray arsenic vapor in liquid air. It reverts to gray arsenic at room temperature.

Black arsenic can be prepared by cooling arsenic vapor at 100 oC – 200 oC. It is glassy, brittle and a poor electrical conductor.

Uses of Arsenic

As a result of its toxicity, arsenic compounds are used in wood preservation and insecticides.

Gallium arsenide (GaAs) is a semiconductor used in laser diodes and LEDs.

Small amounts of arsenic (less than two percent) can be used in lead alloys for ammunition.

Despite its potential toxicity, arsenic is also an essential element, necessary to our physiology. A level of 0.00001% is needed for growth and for a healthy nervous system.

Abundance and Isotopes

Abundance earth’s crust: 1.8 parts per million by weight, 0.5 parts per million by moles

Abundance solar system: 12 parts per billion by weight, 0.21 parts per billion by moles

Cost, pure: $320 per 100g

Cost, bulk: $ per 100g

Source: Most arsenic is obtained as a by-product of processing gold, silver, copper, and other metal ores.

Isotopes: Arsenic has 23 isotopes whose half-lives are known, with mass numbers 65 to 87. Of these, only one is stable: 75As.

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  1. Allyssa P says:

    Thank you soo much for the information on Arsenic! I had to do an element project for chemistry and this was perfect for the assignment.

  2. Summer d. says:

    Thanks i had to do a project on this element for my science class and i must admit this was great information thanks soooooo much!

  3. Thanks for the info I had to do a project. This is a good site!

  4. Heather P. says:

    Thanks a bunch, I needed the info for class. There was so much info found here that really helped in my research. ^w^

  5. can arsenic levels be lowered in borates if heated in a furnace at a high enough temperature?