Selenium

Selenium, ₃₄Se

Selenium
Pronunciation	/sɪˈliːniəm/ (listenⓘ) ​(sə-LEE-nee-əm)
Appearance	grey metallic-looking, red, and vitreous black (not pictured) allotropes
Standard atomic weight Ar°(Se)
	78.971(8)
Selenium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson S ↑ Se ↓ Te arsenic ← selenium → bromine
Atomic number (Z)	34
Group	group 16 (chalcogens)
Period	period 4
Block	p-block
Electron configuration	[Ar] 3d10 4s2 4p4
Electrons per shell	2, 8, 18, 6
Physical properties
Phase at STP	solid
Melting point	494 K ​(221 °C, ​430 °F)
Boiling point	958 K ​(685 °C, ​1265 °F)
Density (near r.t.)	gray: 4.81 g/cm3 alpha: 4.39 g/cm3 vitreous: 4.28 g/cm3
when liquid (at m.p.)	3.99 g/cm3
Critical point	1766 K, 27.2 MPa
Heat of fusion	gray: 6.69 kJ/mol
Heat of vaporization	95.48 kJ/mol
Molar heat capacity	25.363 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 500 552 617 704 813 958
Atomic properties
Oxidation states	−2, −1, 0, +1, +2, +3, +4, +5, +6 (a strongly acidic oxide)
Electronegativity	Pauling scale: 2.55
Ionization 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
Spectral lines of selenium
Other properties
Natural occurrence	primordial
Crystal structure	​trigonal
Speed of sound thin rod	3350 m/s (at 20 °C)
Thermal expansion	amorphous: 37 µm/(m⋅K) (at 25 °C)
Thermal conductivity	amorphous: 0.519 W/(m⋅K)
Magnetic ordering	diamagnetic
Molar magnetic susceptibility	−25.0×10−6 cm3/mol (298 K)
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 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)
Isotopes of selenium
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 72Se synth 8.4 d ε 72As γ – 74Se 0.860% stable 75Se synth 119.8 d ε 75As γ – 76Se 9.23% stable 77Se 7.60% stable 78Se 23.7% stable 79Se trace 3.27×105 y β− 79Br 80Se 49.8% stable 82Se 8.82% 1.08×1020 y β−β− 82Kr

Black, gray, and red forms of selenium

Selenium is a chemical element. Its chemical symbol is Se and its atomic number is 34. It has 34 protons and 34 electrons and a mass number of 78.96.

Properties

Physical properties

Selenium (selenium atoms only) has several different forms. They are called allotropes. The most stable of these is a dense gray semimetal (an element that is partially a metal and a nonmetal). The way the atoms are put together is a named a trigonal polymer chain. It lets electricity pass through it better in the light than in the dark. This form is used in photocells.

Selenium also has many nonconductive forms: a black glass-like substance, as well as several red crystalline forms. In the red form the eight atoms of selenium form ring to make a molecule. These rings then stack together to make the solid red selenium. The way that red seleniums atoms are put together is similar to sulfur.

Chemical properties

Gallium selenide used in semiconductors

Mercury selenide as a mineral

Copper selenide as a mineral

Sodium selenite in a bottle

Selenium is not very reactive. It does not dissolve in acids except nitric acid. It also dissolves in alkalis like sodium hydroxide. It reacts with air when powdered to make selenium(IV) oxide. It does not react with many things, so selenium(IV) oxide is what most selenium compounds are made from.

Selenium forms several oxidation states; -2, +2, +4, and +6. The -2 state is in selenides. Selenides are strong reducing agents. They are stronger reducing agents than sulfides. Hydrogen selenide is the acid made from selenide ion. Selenium also reacts with reactive metals to make selenides, such as sodium selenide or aluminium selenide.

The +1 state is found in some selenium compounds, such as selenium(I) chloride. They are the most unreactive selenium compounds.

The +4 state is found in selenites. Selenites and selenous acid are moderate oxidizing agents. It is made by dissolving selenium dioxide in water and making selenous acid. Then selenous acid is reacted with bases to make selenites.

The +6 state is found in selenates. Selenates and selenic acid are powerful oxidizing agents. Selenic acid can even dissolve gold! They are made by reacting hydrogen peroxide with selenium(IV) oxide to get selenium(VI) oxide, which dissolves in water to make selenic acid. Selenates are more reactive than sulfates.

Most selenium compounds are colorless. Selenium compounds are not common.

Chemical compounds

-2 compounds

Selenides are the most common -2 compounds. Selenides are strong reducing agents.

• Selenide, the ion
• Aluminium selenide
• Hydrogen selenide
• Sodium selenide

+1 compounds

• Selenium monochloride

+4 compounds

+4 compounds can exist in anions or cations.

• Selenium(IV) chloride
• Selenium(IV) fluoride
• Selenium dioxide
• Selenium disulfide

Selenites

Selenites are weak oxidizing agents.

• Selenite, the ion
• Selenous acid
• Sodium selenite

+6 compounds

+6 compounds can exist in anions or cations, too.

• Selenium hexafluoride

Selenates

Selenates are powerful oxidizing agents.

• Selenate, the ion
• Selenic acid
• Sodium selenate

Occurrence

Selenium is very rarely found as an element in the ground, but it can be found in some soils. Most selenium in soil is in a very tiny amount though, and it is easily washed away. It is in very small amounts in the human body. Selenium is found most in sulfide ores like pyrite. This selenium is in selenides. Selenium is gotten as a byproduct. Sometimes selenium is concentrated in plants. It can also be leached into rivers when copper is mined. Too much selenium is bad for a river. Some coal has selenium in it.

Preparation

It is made as a byproduct when refining copper and certain other sulfide ores. Selenium is made by oxidizing selenide ores to selenium dioxide. The selenium dioxide is dissolved in acidic water to make selenous acid, which is reacted with sulfur dioxide to make selenium as an element. It is the red form that is made. To make the black form, the red form is heated and melted.

Uses

In materials

Selenium is used in photocells. The gray metallic form is used, as it changes its electrical conductivity when light shines on it. Selenium is also used as a catalyst. The largest use is to color glass red. It can be used in special brasses instead of lead. It is used in some rectifiers. Most use silicon instead. It is used to tone photographs. It can be used to remove dandruff from hair in the form of selenium sulfide.

In the human body

Selenium is a trace element in the human body. It is an essential nutrient in the body, but it needs only very small amounts, about 50-200 micrograms. The amount of selenium the body needs depends on the amount of other nutrients, like copper and zinc, and vitamin E is in the body. Selenium can be toxic if more than 400 micrograms are taken. A selenium deficiency may cause the damage to the muscles. Once there was coal that had a large amount of selenium in it. People were getting selenium poisoning from the coal.

Selenium deficiency is rare. Although selenium has some helpful effects on the human body, it also has some harmful effects and only a very little should be eaten.

Safety

It is toxic in large amounts. Some selenium compounds are very toxic and harmful to things that live in water. Consuming around 5 mg of selenium per day can kill a human being after a certain amount of time.

Related pages

• Selenium compounds