Selenium

Selenium,  ₃₄Se

Selenium powder - black
General properties
Name, symbol	Selenium, Se
Allotropes	Black, red, gray
Appearance	Black, red, gray solid
Selenium in the periodic table
S ↑ Se ↓ Te Arsenic ← Selenium → Bromine
Atomic number	34
Standard atomic weight (Ar)	78.971(8)
Group, block	, p-block
Period	period 4
Electron configuration	[Ar] 3d10 4s2 4p4
Physical properties
Black, gray, red
Phase	Solid
Melting point	494 K ​(221 °C, ​430 °F)
Boiling point	958 K ​(685 °C, ​1265 °F)
Density near r.t.	4.81 g/cm3 (gray) 4.39 g/cm3 (alpha) 4.28 g/cm3 (vitreous)
when liquid, at	3.99 g/cm3
Critical point	1766 K, 27.2 MPa
Heat of fusion	6.69 kJ/mol (gray)
Heat of	95.48 kJ/mol
Molar heat capacity	25.363 J/(mol·K)
pressure
Atomic properties
Oxidation states	6, 5, 4, 3, 2, 1, −1, −2 ​(a strongly acidic oxide)
Electronegativity	Pauling scale: 2.55
energies	1st: 941.0 kJ/mol 2nd: 2045 kJ/mol 3rd: 2973.7 kJ/mol
Atomic radius	empirical: 120 pm
Covalent radius	120±4 pm
Van der Waals radius	190 pm
Miscellanea
Crystal structure	​Hexagonal
Speed of sound thin rod	3350 m/s (at 20 °C)
Thermal expansion	37 µm/(m·K) (amorphous)
Thermal conductivity	0.519 W/(m·K) (amorphous)
Magnetic ordering	Diamagnetic
Young's modulus	10 GPa
Shear modulus	3.7 GPa
Bulk modulus	8.3 GPa
Poisson ratio	0.33
Mohs hardness	2.0
Brinell hardness	736 MPa
CAS Registry Number	7782-49-2
History
Naming	After Selene, Greek goddess of the moon
Discovery and first isolation	Jöns Jakob Berzelius and Johann Gottlieb Gahn (1817)

Selenium is a chemical element with symbol Se and atomic number 34. It is widely used in the semiconductor industry.

Properties

Chemical

Like sulfur, selenium will burn in the presence of oxygen to yield selenium dioxide.

Se + O₂ → SeO₂

Unlike sulfur dioxide, selenium dioxide is a solid.

Selenium will be oxidized by chlorine in water to give selenic acid and hydrochloric acid.

Se + 4 H₂O + 3 Cl₂ → H₂SeO₄ + 6 HCl

Selenium dissolves in concentrated nitric acid with evolution of oxides of nitrogen:

4 HNO₃ + Se → H₂SeO₃ + 4 NO₂ + H₂O

When selenous acid is reduced with sulfur dioxide or metabisulfite red selenium is formed.

In concentrated sulfuric acid selenium dissolves to give a deep green solution.

Se₈ + 3 H₂SO₄ → Se₈²⁺ + 2 HSO₄^- + SO₂ + 2 H₂O

Diluting this solution in water forms red selenium.

When selenium is dissolved in hot concentrated sodium or potassium hydroxide, deep red polyselenides form. Polyselenides form less readily than polysulfides. When strongly diluted, polyselenides hydrolyze into red selenium.

Physical

Selenium is a solid nonmetal at standard conditions. It has several allotrope forms:

• Amorphous or red: Has a brick-red color, as is the most common form of selenium obtained from chemical reactions.
• Black: Also known as vitreous selenium, this form is a dull grey solid, obtained by rapidly melting its amorphous form. The structure of black selenium is irregular and complex, consisting of polymeric rings. It is slightly soluble in carbon disulfide.
• Gray: Obtained by heating black selenium to 180°C. This is the most dense and stable form of selenium. It is not soluble in carbon disulfide or other organic solvents. Unlike the black variety, its structure is hexagonal crystal lattice consisting of helical polymeric chains. Gray selenium is a semiconductor, that also shows appreciable photoconductivity.

Availability

Selenium is sold by chemical suppliers. It can also be purchased from Ebay and Metallium.

Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.

Preparation

Elemental selenium can be made by reducing selenous acid with sulfur dioxide.

The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS₂).

A good tutorial to make selenium allotropes can be found here.

Projects

• Selenium halides
• Made cadmium selenide
• Make red glass
• Make red selenium
• Make the green Se₈²⁺
• Make polyselenides
• Make selenites and selenates

Handling

Safety

Selenium and most of its compounds tend to be toxic if ingested in high quantities.

Storage

In closed containers.

Disposal

As selenium is not cheap and not easy to come by, it's best to try to recycle it.

Gallery

• 

  The red allotrope of selenium

• 

  The green solution of Se₈²⁺ obtained from selenium powder in hot concentrated sulfuric acid

• 

  When selenium is dissolved in hot concentrated sodium hydroxide, a deep red solution of polyselenide forms

References

Relevant Sciencemadness threads

• Elemental Selenium
• Selenium toxicity