Difference between revisions of "Arsenic"

Latest revision as of 15:37, 20 September 2018

Arsenic,  ₃₃As

General properties
Name, symbol	Arsenic, As
Allotropes	Grey (most common) Yellow Black
Appearance	Metallic grey
Arsenic in the periodic table
P ↑ As ↓ Sb Germanium ← Arsenic → Selenium
Atomic number	33
Standard atomic weight (Ar)	74.921595(6)
Group, block	(pnictogens); p-block
Period	period 4
Electron configuration	[Ar] 3d10 4s2 4p3
per shell	2, 8, 18, 5
Physical properties
Silvery-gray
Phase	Solid
Sublimation point	887 K ​(615 °C, ​1137 °F)
Density near r.t.	5.727 g/cm3
when liquid, at	5.22 g/cm3
Triple point	1090 K, ​3628 kPa
Critical point	1673 K,  MPa
Heat of fusion	24.44 kJ/mol (gray)
Heat of	34.76 kJ/mol
Molar heat capacity	24.64 J/(mol·K)
pressure
Atomic properties
Oxidation states	5, 4, 3, 2, 1, −1, −2, −3
Electronegativity	Pauling scale: 2.18
energies	1st: 947 kJ/mol 2nd: 1798 kJ/mol 3rd: 2735 kJ/mol
Atomic radius	empirical: 119 pm
Covalent radius	119±4 pm
Van der Waals radius	185 pm
Miscellanea
Crystal structure	​
Thermal expansion	5.6 µm/(m·K) (at r.t.)
Thermal conductivity	50.2 W/(m·K)
Electrical resistivity	333 Ω·m (at 20 °C)
Magnetic ordering	Diamagnetic
Young's modulus	8 GPa
Bulk modulus	22 GPa
Mohs hardness	3.5
Brinell hardness	1440 MPa
CAS Registry Number	7440-38-2
History
Discovery	c. 800 CE
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Arsenic is a metalloid with the symbol As and atomic number 33. It appears in pure form as a shiny, metal-like substance, though most samples are dark grey due to oxidation. Aside from its once-popular use in poisoning, arsenic was also used for preparing wood treatment chemicals.

Properties

Chemical

Arsenic has three important oxidation states, -3, +3 and +5. Notable compounds are arsine (AsH₃), arsenic trichloride (AsCl₃, "butter of arsenic"), and the two oxides As₄O₆ and As₄O₁₀.

Most arsenic compounds, as well as the element itself, are prone to sublimation at low temperatures (usually around 300 - 700 °C).

Physical

Arsenic occurs in three major allotropes: grey (or metallic), black, and yellow. The grey allotrope is by far the most commonly encountered form, and has semimetal properties. The yellow allotrope is encountered as a yellow amorphous solid, produced by very rapidly cooling arsenic vapor, and is not to be confused with orpiment, the sulfide mineral of arsenic. Yellow arsenic will slowly convert to the grey form over time. Black arsenic is brittle, glassy, and crystalline, forming from the cooling of arsenic vapor to more moderately high temperatures in the range of 100-200 °C.

Arsenic is unique among elements in that it typically does not occur in a liquid phase, but rather freely transfers between the solid and gas phases, often vaporizing even below its sublimation point, which only adds to the danger of this element.

Availability

Certain mineral ores of arsenic can be found at rock and gem fairs, such as skutterudite (CoAs₃) and arsenopyrite (FeAsS). It can also be bought online.

In some places, the sale of arsenic and arsenic compounds is regulated.

Preparation/Isolation

If you do not have any experience handling arsenic compounds, it is highly recommended you do not try to produce arsenic. Do not produce arsenic indoors without excellent ventilation. A glove box is also ideal.

Assuming that one has thoroughly researched the toxicity of arsenic and proper handling procedures from a reliable source, there are two major ways to prepare elemental arsenic. The first the wet method - involving dissolving the cation part of the ore (usually some metal) in nitric acid, leaving behind a residue of arsenic(III) oxide and reducing this to the element.

The second, and by far the most lethal, is to sublimate the crushed ore in a closed tube. It is absolutely required to weld the ends of the tube shut to prevent vapor of arsenic or arsenic oxides from escaping, and one should probably add some reducing agent such as carbon as well to prevent the arsenic vapor reacting with the air in the closed container. When sublimating, heating should be done so that all of the vapor collects at one spot - this will make collection of the sublimated product significantly easier.

Projects

• Extracting arsenic from skutterudite
• Element collecting
• Arsenic trioxide seynthesis
• Arsenic trichloride synthesis
• Organoarsenic compounds

Handling

Safety

Arsenic is notoriously one of the most toxic elements on the periodic table, as less than half a gram will cause death in a very painful fashion. When working with this, gloves, a lab coat, full facial protection, and a fume hood are a must. If volatilized and organic arsenic compounds are used, a glove box or gas mask is not optional - you must use one if you plan to live afterwards.

Toxicity of arsenic-bearing ores

The only arsenic ore that can be considered almost non-toxic (safe enough to handle by hand in powdered form) is skutterudite, due to its crystal structure effectively trapping the arsenic atoms in a lattice of cobalt, iron and sometimes nickel. Other ores such as arsenopyrite, realgar and orpiment are safe to handle by hand in bulk form, but not under any other conditions (and it's frankly recommended to not handle any of these by hand anyway).

Storage

Arsenic should be stored in closed bottles or ampoules.

Disposal

NEVER DUMP ARSENIC IN THE ENVIRONMENT! All arsenic compounds must be taken to hazardous chemicals disposal centers.

The only exception would be to redeposit it in the geological area from where you collected the arsenic containing mineral, as there is already arsenic in that area. Even so, the arsenic must be converted to an inert compound, to prevent it from infiltrating the groundwater.

References

Sciencemadness Library

• A TEXT-BOOK OF INORGANIC CHEMISTRY. VOLUME XI. ORGANOMETALLIC COMPOUNDS. PART II. DERIVATIVES OF ARSENIC.

Relevant Sciencemadness threads

• Arsenic from Skutterudite