Selenium

General:

Name:Selenium Symbol:Se
Type:Other Non-Metal, Chalcogen Atomic weight:78.96
Density @ 293 K:4.79 g/cm3 Atomic volume:16.45 cm3/mol
Discovered:

Selenium exists in only trace amounts around us. It was discovered by Jons J. Berzelius in 1817. Martin Klaproth, the discoverer of uranium and zirconium had concluded that a red colored byproduct of sulfuric acid production contained tellurium, an element whose compounds he had been studying. Berzelius analyzed the sample and decided it did not contain tellurium, but in fact contained a new element similar to tellurium. Since 'tellus' in Latin means earth goddess, Berzelius named the new element selenium from the Greek word 'selene', meaning moon goddess.

States

State (s, l, g):solid
Melting point: 493 K (220 °C) Boiling point:958 K (685 °C)

Energies

Specific heat capacity: 0.32 J g-1 K-1 Heat of atomization:227 kJ mol-1
Heat of fusion: 6.694 kJ mol-1 Heat of vaporization :26.32 kJ mol-1
1st ionization energy:940.9 kJ mol-1 2nd ionization energy:2044.5 kJ mol-1
3rd ionization energy: 2973.7 kJ mol-1 Electron affinity:194.97 kJ mol-1

Oxidation & Electrons

Shells:2,8,18,6 Electron configuration: [Ar] 3d10 4s-2 4p4
Minimum oxidation number:-2 Maximum oxidation number:6
Min. common oxidation no.:-2 Max. common oxidation no.:6
Electronegativity (Pauling Scale):2.55 Polarizability volume:3.8 Å3

Appearance & Characteristics

Structure:long, helical chains (crystalline hexagonal), Se8 rings (crystalline monoclinic) Color:gray or red (crystalline), black or red (amorphous
Hardness: 2.0 mohs
Harmful effects:

Selenium is considered to be toxic if taken in excess. It is carcinogenic and teratogenic. Selenates and selenites are very toxic, and hydrogen selenide (SeH2) is an extremely toxic, corrosive gas.

Characteristics:

Selenium exists in several allotropic forms. The most stable, crystalline hexagonal selenium, is metallic gray. Crystalline monoclinic selenium is a deep red color. Amorphous selenium is red in powder form and is black in vitreous form. Gray crystalline 'metallic' selenium conducts electricity better in the light than in the dark (photoconductive) and it can convert light directly into electricity (photovoltaic).

In the same way as sulfur forms sulfides, sulfates, and sulfites, selenium combines with metals and oxygen to form selenides (such as zinc selanide, ZnSe), selenates (such as calcium selenate, CaSeO4), and selenites (such as silver selenite, Ag2SeO3).


Uses:

Selenium is used in the glass industry to decolorize glass and to make red-colored glasses and enamels.

It is used as a catalyst in many chemical reactions.

Selenium is used in solar cells and photocells - in fact the first solar cell was made using selenium. It is also used as a photographic toner.

Selenium is used with bismuth in brasses and as an additive to stainless steel.

Despite its toxicity, selenium is also an essential trace element in the animal and human diet. It has a goldilocks-like quality that you must not be exposed to too much or too little of it, you must get the right amount.

Reactions

Reaction with air:vigorous, w/ht ⇒ SeO2 Reaction with 6 M HCl:none
Reaction with 15 M HNO3:mild , ⇒ H2SeO3, NOx Reaction with 6 M NaOH:

Compounds

Oxide(s):SeO2 Chloride(s):Se2Cl2, Se4Cl16
Hydride(s): SeH2

Radius

Atomic radius:119 pm Ionic radius (1+ ion):pm
Ionic radius (2+ ion):pm Ionic radius (3+ ion):pm
Ionic radius (2- ion):184 pm Ionic radius (1- ion):pm

Conductivity

Thermal conductivity: 0.52 W m-1 K-1 Electrical conductivity:8 x 106 S m-1

Abundance & Isotopes

Abundance earth's crust: 50 parts per billion by weight, 10 parts per billion by moles
Abundance solar system:parts per billion by weight, part per billion by moles
Cost, pure: $61 per 100g
Cost, bulk:$5.30 per 100g
Source:

Selenium occasionally occurs free in nature, but more often occurs as selenides of iron, lead, silver, or copper. Commercially, selenium is obtained mainly from anode mud waste produced in the electrolytic refining of copper. Brazil nuts are the richest known dietary source of selenium.

Isotopes:

Selenium has 24 isotopes whose half-lives are known, with mass numbers 67 to 91. Of these, five are stable: 74Se, 76Se, 77Se, 78Se and 80Se.

Other

Other:

 

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