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Element Fact
There are around 120 known elements. Of these, 91 elements occur naturally on Earth. The rest no longer exist or are found only in traces as intermediate decay products alongside with other radioisotopes.

Arsenic, As

arsenic sample

Atomic number: 33
Atomic mass: 74.92160(2)
Natural abundance: 1.8 ppm
Isotopes:
75As (74.92160) - 100%

History and Production

From Latin arsenicum or Greek arsenikon meaning yellow orpiment. Arsenic may perhaps be isolated by A. Magnus in the year 1250 by heating orpiment (As2S3) with soap. The element can now be obtained by smelting loellingite (FeAs2) at about 675C in the absence of air and condensing the sublimed element. Residual arsenic can further be extracted by roasting the sulfide residues in air to give white arsenic (As2O3), which is found widely used as herbicides for weed and pest control. Alternatively, the oxide can be reduced with charcoal to give more arsenic. Its main use is in alloys with lead and copper. Depends on the amount being added, the physical properties of lead can be altered significantly. For example, 0.75% improves the hardness and castability of lead, 0.5-2.0% improves the mechanics of lead ammunition. It is also used in making semiconductors such as gallium arsenide (GaAs) and indium arsenide (InAs) for light-emitting diodes (LEDs).

Physical Data

Arsenic can exist in several allotropic forms. The alpha-form (grey arsenic, shown above) is the most stable form in room temperature which is brittle, steel-gray 'metallic' in appearance. However, it lacks the usual metallic behavior such as ductility and it has high electrical resistivity. It does not occur in large amount in nature (less than 2ppm in crustal rocks). Its minerals, however, are widely distributed and realgar (As4S4) and orpiment being the most common. It also occurs as arsenide of iron, cobalt and nickel (e.g. nicolite, NiAs and safforlite, CoAs).

Interatomic distance: 249 pm
Melting point: 817C (triple point)
Boiling point: 614C (sublimes)
Thermal conductivity/Wm-1K-1: 50.0 (alpha) (27C)
Density/kgm-3: 5780 (20C, alpha), 4700 (20C, beta)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 302.5 kJ/mol
Gibbs free energy of formation: 261 kJ/mol
Entropy: 174.2 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ar] 3d10 4s2 4p3
Term symbol: 4S3/2
Electron affinity: 78.5391 kJ/mol   Electronegativity (Pauline): 2.18
Ionization energy (first, second, third): 944.457, 1797.81, 2735.46 kJ/mol

Chemical properties

In general, the chemical behavior of arsenic is quite similar to phosphorus but a few differences arise due to the more electropositive nature of arsenic. Grey arsenic is not oxidized in air but strong heating gives the oxide, As4O6. It is not attacked by most dilute acid except nitric acid. However, concentrated sulfuric acid oxidize it to arsenious acid (arsenic(III) acid), H3AsO3, while hot concentrated nitric acid oxidize it to arsenic(V) acid, H3AsO4.

Test for arsenic:
(1) Arsenic compounds give the yellow sulfide, As2S3, or the pentavalent arsenic as the yellow sulfide, As2S5, by hydrogen sulfide in acid solution.
(2) White precipitate of white magnesium ammonium arsenate, NH4MgAsO4.6H2O formed by addition of magnesium chloride and ammonium chloride to an ammoniacal solution of arsenate. On heating, the precipitate is converted to pyroarsenate, Mg2As2O7, which can be quantitatively weighed on cooling.

Test for arsenic(III) compounds:
A strip of polished copper is placed in a solution of an arsenic(III) compound in dilute hydrochloric acid and boiled. A grey film of copper arsenide, Cu5As2, is deposited on the copper. If the strip is taken out, dried and heated gently in a test tube, arsenic(III) oxide is formed as a white crystalline sublimate on the walls of the tube. This is called the Reinsch's test.


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