Selenium compounds commonly exist in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s −2, +2, +4, and +6.
Chalcogen compounds
Selenium forms two
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s:
selenium dioxide (SeO
2) and
selenium trioxide
Selenium trioxide is the inorganic compound with the formula Se O3. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds.Egon Wiberg, Arnold Frederick Holl ...
(SeO
3). Selenium dioxide is formed by the reaction of elemental selenium with oxygen:
:
Se8 + 8 O2 -> 8 SeO2
It is a
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ic solid that forms monomeric SeO
2 molecules in the gas phase. It dissolves in water to form
selenous acid
Selenous acid (or selenious acid) is the chemical compound with the formula . Structurally, it is more accurately described by . It is the principal oxoacid of selenium; the other being selenic acid.
Formation and properties
Selenous acid is ana ...
, H
2SeO
3. Selenous acid can also be made directly by oxidizing elemental selenium with
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
:
:
3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO
Unlike sulfur, which forms a stable
trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C:
:
2 SeO3 -> 2 SeO2 + O2 (ΔH = −54 kJ/mol)
Selenium trioxide is produced in the laboratory by the reaction of
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
potassium selenate
Potassium selenate, , is an odorless, white solid that forms as the potassium salt of selenic acid.
Preparation
Potassium selenate is produced by the reaction of selenium trioxide and potassium hydroxide.
:
Alternatively, it can be made by tr ...
(K
2SeO
4) and sulfur trioxide (SO
3).
Salts of selenous acid are called selenites. These include
silver selenite
Silver selenite is an inorganic compound of formula Ag2SeO3.
Production
Silver selenite is formed during the recovery of selenium from copper anode slimes when they are subjected to oxidative roasting, causing some silver selenide to be converted ...
(Ag
2SeO
3) and
sodium selenite (Na
2SeO
3).
Hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
reacts with aqueous selenous acid to produce
selenium disulfide
Selenium disulfide, also known as selenium sulfide, is a chemical compound and medication used to treat seborrheic dermatitis, dandruff, and pityriasis versicolor. It is applied to the affected area as a lotion or shampoo. Symptoms frequently ret ...
:
:
H2SeO3 + 2 H2S -> SeS2 + 3 H2O
Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS
2, with individual rings varying in composition, such as Se
4S
4 and Se
2S
6. Selenium disulfide has been used in shampoo as an anti
dandruff
Dandruff is a skin condition that mainly affects the scalp. Symptoms include flaking and sometimes mild itchiness. It can result in social or self-esteem problems. A more severe form of the condition, which includes inflammation of the skin, ...
agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in
fireworks
Fireworks are a class of Explosive, low explosive Pyrotechnics, pyrotechnic devices used for aesthetic and entertainment purposes. They are most commonly used in fireworks displays (also called a fireworks show or pyrotechnics), combining a l ...
.
Selenium trioxide may be synthesized by dehydrating
selenic acid
Selenic acid is the inorganic compound with the formula . It is an oxoacid of selenium, and its structure is more accurately described as . It is a colorless compound. Although it has few uses, its derivative sodium selenate is used in the prod ...
, H
2SeO
4, which is itself produced by the oxidation of selenium dioxide with
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
:
:
SeO2 + H2O2 -> H2SeO4
Hot, concentrated selenic acid can react with gold to form gold(III) selenate.
Halogen compounds
Iodides of selenium are not well known. The only stable
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
is
selenium monochloride
Selenium monochloride is an inorganic compound with the formula Se2Cl2. Although it is called selenium monochloride, a more descriptive name might be diselenium dichloride. It is a reddish-brown, oily liquid that hydrolyses slowly. It exists in c ...
(Se
2Cl
2), which might be better known as selenium(I) chloride; the corresponding
bromide is also known. These species are structurally analogous to the corresponding
disulfur dichloride
Disulfur dichloride is the inorganic compound of sulfur and chlorine with the Chemical formula, formula S2Cl2.
Some alternative names for this compound are ''sulfur monochloride'' (the name implied by its empirical formula, SCl), ''disulphur dich ...
. Selenium dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation of Se
7). It is prepared by treating selenium with
sulfuryl chloride (SO
2Cl
2). Selenium reacts with
fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
to form
selenium hexafluoride
Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.
Structure, preparation, and reacti ...
:
:
Se8 + 24 F2 -> 8 SeF6
In comparison with its sulfur counterpart (
sulfur hexafluoride
Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...
),
selenium hexafluoride
Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.
Structure, preparation, and reacti ...
(SeF
6) is more reactive and is a toxic
pulmonary irritant.
Some of the selenium oxyhalides, such as
selenium oxyfluoride (SeOF
2) and
selenium oxychloride
Selenium oxydichloride is the inorganic compound with the formula SeOCl2. It is a colorless liquid. With a high dielectric constant (55) and high specific conductance, it is an attractive solvent. Structurally, it is a close chemical relative of ...
(SeOCl
2) have been used as specialty solvents.
Selenides
Analogous to the behavior of other chalcogens, selenium forms
hydrogen selenide
Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic sele ...
, H
2Se. It is a strongly
odiferous, toxic, and colorless gas. It is more acidic than H
2S. In solution it ionizes to HSe
−. The selenide dianion Se
2− forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include
mercury selenide
Mercury selenide (HgSe; sometimes mercury(II) selenide) is a chemical compound of mercury and selenium. It is a grey-black crystalline solid semi-metal with a sphalerite structure. The lattice constant is 0.608 nm.
HgSe occurs naturally as ...
(HgSe),
lead selenide
Lead selenide (PbSe), or lead(II) selenide, a selenide of lead, is a semiconductor material. It forms cubic crystals of the NaCl structure; it has a direct bandgap of 0.27 eV at room temperature. (Note that incorrectly identifies PbSe and o ...
(PbSe),
zinc selenide
Zinc selenide (ZnSe) is a light-yellow, solid compound comprising zinc (Zn) and selenium (Se). It is an intrinsic semiconductor with a band gap of about 2.70 eV at . ZnSe rarely occurs in nature, and is found in the mineral that was named af ...
(ZnSe), and
copper indium gallium diselenide
Copper indium gallium (di)selenide (CIGS) is a I- III- VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide ...
(Cu(Ga,In)Se
2). These materials are
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
s. With highly electropositive metals, such as
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, these selenides are prone to hydrolysis:
:
Al2Se3 + 3 H2O -> Al2O3 + 3 H2Se
Alkali metal selenides react with selenium to form polyselenides, , which exist as chains.
Other compounds
Tetraselenium tetranitride, Se
4N
4, is an explosive orange compound analogous to
tetrasulfur tetranitride (S
4N
4).
It can be synthesized by the reaction of
selenium tetrachloride
Selenium tetrachloride is the inorganic compound composed with the formula SeCl4. This compound exists as yellow to white volatile solid. It is one of two commonly available selenium chlorides, the other example being selenium monochloride, Se2 ...
(SeCl
4) with
.
Selenium reacts with
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
s to yield
selenocyanates:
:
8 KCN + Se8 -> 8 KSeCN
Organoselenium compounds
Selenium, especially in the II oxidation state, forms stable bonds to
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
, which are structurally analogous to the corresponding
organosulfur compounds
Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur ...
. Especially common are selenides (R
2Se, analogues of
thioethers), diselenides (R
2Se
2, analogues of
disulfides), and
selenols (RSeH, analogues of
thiol
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...
s). Representatives of selenides, diselenides, and selenols include respectively
selenomethionine,
diphenyldiselenide
Diphenyl diselenide is the chemical compound with the formula (C6H5)2Se2, abbreviated Ph2Se2. This orange-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis.
Preparation ...
, and
benzeneselenol. The
sulfoxide
In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
in sulfur chemistry is represented in selenium chemistry by the selenoxides (formula RSe(O)R), which are intermediates in organic synthesis, as illustrated by the
selenoxide elimination
Selenoxide elimination (also called α-selenation) is a method for the chemical synthesis of alkenes from selenoxides. It is most commonly used to synthesize α,β-unsaturated carbonyl compounds from the corresponding saturated analogues. It is me ...
reaction. Consistent with trends indicated by the
double bond rule
In chemistry, the double bond rule states that elements with a principal quantum number greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double bon ...
, selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.
See also
*
Arsenic compounds
Compounds of arsenic resemble in some respects those of phosphorus which occupies the same group (column) of the periodic table. The most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compound ...
*
Bromine compounds
References
{{Selenium compounds
Selenium
Selenium compounds
Chemical compounds by element